A simple and reliable index for scoring rival stereotactic radiosurgery plans

Dosimetric comparison of ZAP-X, Gamma Knife, and CyberKnife stereotactic radiosurgery for single brain metastasis

PURPOSE

To evaluate the dosimetric characteristics of ZAP-X stereotactic radiosurgery (SRS) for single brain metastasis by comparing with two mature SRS platforms.

METHODS

Thirteen patients with single brain metastasis treated with CyberKnife (CK) G4 were selected retrospectively. The prescription dose for the planning target volume (PTV) was 18-24 Gy for 1-3 fractions. The PTV volume ranged from 0.44 to 11.52 cc.Treatment plans of thirteen patients were replanned using the ZAP-X plan system and the Gamma Knife (GK) ICON plan system with the same prescription dose and organs at risk (OARs) constraints. The prescription dose of PTV was normalized to 70% for both ZAP-X and CK, while it was 50% for GK. The dosimetric parameters of three groups included the plan characteristics (CI, GI, GSI, beams, MUs, treatment time), PTV (D2, D95, D98, Dmin, Dmean, Coverage), brain tissue (volume of 100%-10% prescription dose irradiation V100%-V10%, Dmean) and other OARs (Dmax, Dmean),all of these were compared and evaluated. All data were read and analyzed with MIM Maestro. One-way ANOVA or a multisample Friedman rank sum test was performed, where p < 0.05 indicated significant differences.

RESULTS

The CI of GK was significantly lower than that of ZAP-X and CK. Regarding the mean value, ZAP-X had a lower GI and higher GSI, but there was no significant difference among the three groups. The MUs of ZAP-X were significantly lower than those of CK, and the mean value of the treatment time of ZAP-X was significantly shorter than that of CK. For PTV, the D95, D98, and target coverage of CK were higher, while the mean of Dmin of GK was significantly lower than that of CK and ZAP-X. For brain tissue, ZAP-X showed a smaller volume from V100% to V20%; the statistical results of V60% and V50% showed a difference between ZAP-X and GK, while the V40% and V30% showed a significant difference between ZAP-X and the other two groups; V10% and Dmean indicated that GK was better. Excluding the Dmax of the brainstem, right optic nerve and optic chiasm, the mean value of all other OARs was less than 1 Gy. For the brainstem, GK and ZAP-X had better protection, especially at the maximum dose.

CONCLUSION

For the SRS treating single brain metastasis, all three treatment devices, ZAP-X system, CyberKnife G4 system, and GammaKnife system, could meet clinical treatment requirements. The newly platform ZAP-X could provide a high-quality plan equivalent to or even better than CyberKnife and Gamma Knife, with ZAP-X presenting a certain dose advantage, especially with a more conformal dose distribution and better protection for brain tissue. As the ZAP-X systems get continuous improvements and upgrades, they may become a new SRS platform for the treatment of brain metastasis.

Use of Zernike moments to characterize dose conformity for radiotherapy treatment plans

Dose conformity is an essential parameter used in radiotherapy and radiosurgery that measures the correspondence of the dose distribution derived from a Treatment Planning System (TPS) with the actual volume to be treated, the Planning Treatment Volume (PTV). The present work uses a method based on the expansion of dose distributions and PTVs by three-dimensional Zernike polynomials and further comparison of their moments to define a general criterion of dose conformity. To carry on this study, data coming from 20 patients comprising 80 datasets exported from the TPS, which included imaging data (PTVs) and dose distributions corresponding to different treatment modalities: three-dimensional conformal radiotherapy, intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT), were used. The expansions in Zernike polynomials were obtained up to order 6 and reconstructed dose distributions and PTVs were obtained and compared, and several definitions for a general dose conformity index were proposed. Results indicate agreement between the proposed dose conformity index and the Conformation Number CN. The proposed method allows for a systematic approach to the analysis of dose distributions with further extensions in AI applications.

Extracting the gradient component of the gamma index using the Lie derivative method

Objective. The gamma index (γ) has been extensively investigated in the medical physics and applied in clinical practice. However,γhas a significant limitation when used to evaluate the dose-gradient region, leading to inconveniences, particularly in stereotactic radiotherapy (SRT). This study proposes a novel evaluation method combined withγto extract clinically problematic dose-gradient regions caused by irradiation including certain errors.Approach. A flow-vector field in the dose distribution is obtained when the dose is considered a scalar potential. Using the Lie derivative from differential geometry, we definedL,S, andUto evaluate the intensity, vorticity, and flow amount of deviation between two dose distributions, respectively. These metrics multiplied byγ(γL,γS,γU), along with the threshold valueσ, were verified in the ideal SRT case and in a clinical case of irradiation near the brainstem region using radiochromic films. Moreover, Moran's gradient index (MGI), Bakai's χ factor, and the structural similarity index (SSIM) were investigated for comparisons.Main results. A highL-metric value mainly extracted high-dose-gradient induced deviations, which was supported by highSandUmetrics observed as a robust deviation and an influence of the dose-gradient, respectively. TheS-metric also denotes the measured similarity between the compared dose distributions. In theγdistribution,γLsensitively detected the dose-gradient region in the film measurement, despite the presence of noise. The thresholdσsuccessfully extracted the gradient-error region whereγ> 1 analysis underestimated, andσ= 0.1 (plan) andσ= 0.001 (film measurement) were obtained according to the compared resolutions. However, the MGI, χ, and SSIM failed to detect the clinically interested region.Significance. Although further studies are required to clarify the error details, this study demonstrated that the Lie derivative method provided a novel perspective for the identifying gradient-induced error regions and enabled enhanced and clinically significant evaluations ofγ.

Proposal of a New Conformal Factor and Normal Tissue Penalty Factor for Radiosurgery Treatments

The quality of a treatment plan is evaluated by the conformality of the prescribed isodose around the target and the homogeneity of dose distribution inside the target. Presently, to check the target volume conformality, a number of published conformity indices are in use. Most of these indices are based on the target volume coverage by prescribed isodose, with respect to the total volume of the target. Some take into account the normal tissue covered by the prescribed isodose and suitably weigh the target coverage to evaluate conformity. In this study, for the irradiation of normal tissue by the prescription isodose, a normal tissue penalty factor is proposed and for the target conformality, a new conformal factor is proposed by applying this normal tissue penalty factor to the target coverage. The proposed conformal factor is evaluated for a few sample analytical cases and the results are compared with those obtained using the published conformity indices.

Plan quality analysis of stereotactic ablative body radiotherapy treatment planning in liver tumor

PURPOSE

Stereotactic ablative body radiotherapy (SABR) in the liver, RTOG-1112 guides the treatment modalities including the dose constraints for this technique but not the plan parameters. This study is not only analyzing the plan quality by utilizing the plan parameters and indexes but also compares treatment modalities from the protocol implementation.

METHOD AND MATERIAL

Twenty-five patients treated in the period from February 2020 to September 2022 were recruited in this analysis. Two planners randomly selected the patients and modalities. The modalities employed were Volumetric-Modulated Arc Therapy (VMAT) and Helical Tomotherapy (HT). Various parameters and indexes were used to access not only the plan quality but also to compare each modality. The parameters and indexes studied were the homogeneity index (HI), conformity index (CI), gradient distance (GD), and the dose received by the organs at risk.

RESULT

The data reveals that the mean volume of PTV is 60.8 ± 53.9 cc where these targets exhibit no significant difference between each modality. The HI shows a consistent value for both modalities. Between each modality, the CI value shows less deviation, but the HT shows slightly higher performance than VMAT. The value of GD is 1.5 ± 0.3 cm where the HT provides a shorter distance compared to VMAT as well.

CONCLUSION

The parameters and indexes should be utilized for the plan evaluation although in the guidelines this was not required. Various modalities were employed for treatment. Both can achieve the treatment criteria with slightly low performance of VMAT.

STANDARD-DEVIATION BASED CONFORMITY INDEX FOR EVALUATING TREATMENT PLAN OF INTENSITY MODULATED RADIOTHERAPY IN LUNG CANCER

This paper attempts to find a new conformity index (CI) calculation method with slice and angle information for evaluating lung cancer radiation treatment plan. A total of 20 lung cancer patients in 2016-2019 were selected. Treatment plans were made for each patient. Parameters used in the process of making treatment plans were set the same. The CI and the standard-deviation based CI (SDCI) that contains angle and slice information were calculated. Comparison of results calculated with SDCI and CI were made. The results of the two methods for the patients showed the same trend. Different shapes of simulated dose distribution line shows SDCI can provide more detail information about the target area. Special shapes of simulated dose distribution line for SDCI showed inaccuracy in angle information. The parameter SDCI has more advantage towards the traditional CI for it can provide angle and slice information. However, more angles need to be calculated.

Automated Contouring and Planning in Radiation Therapy: What Is 'Clinically Acceptable'?

Developers and users of artificial-intelligence-based tools for automatic contouring and treatment planning in radiotherapy are expected to assess clinical acceptability of these tools. However, what is 'clinical acceptability'? Quantitative and qualitative approaches have been used to assess this ill-defined concept, all of which have advantages and disadvantages or limitations. The approach chosen may depend on the goal of the study as well as on available resources. In this paper, we discuss various aspects of 'clinical acceptability' and how they can move us toward a standard for defining clinical acceptability of new autocontouring and planning tools.

Stereotactic radiosurgery for limited brain metastasis using three different techniques: helical tomotherapy, volumetric modulated arc therapy, and cone-based LINAC radiosurgery

PURPOSE

Specific radiation delivered to tumors by stereotactic radiosurgery (SRS) has become widely used in the treatment of brain metastasis. This study aimed to compare radiation therapy planning and its parameters from SRS using three different modalities: helical tomotherapy (HT), volumetric modulated arc therapy (VMAT), and cone-based linac radiosurgery (Cone-based).

MATERIALS AND METHODS

Each contouring dataset of patents who experienced one to four brain metastasis received SRS in our center was re-planned to create radiation therapy planning in all three treatment systems (HT, VMAT, and Cone-based). The parameters of conformity index (CI), homogeneity index (HI), CI50, and gradient index (CGI) were analyzed to compare the effects of the three techniques. Decision score analysis was used to evaluate the performance on dosimetric and organs-at-risk parameters among the different techniques by applying the Cone-based technique as a benchmark.

RESULTS

A total of 21 patients with 39 lesions were included in this study. The results from the decision score analysis demonstrated statistically identical CI, CI50, and CGI values between Cone-based and VMAT for single lesions. For multiple lesions, VMAT also provided better CI when compared to Cone-based technique while HT exhibited the poorest dosimetric parameters. Moreover, VMAT exhibited the lowest BrainV5Gy value and displayed the shortest beam-on time calculation.

CONCLUSION

We have conducted a comprehensive comparison of SRS planning approaches. The Cone-based technique revealed the highest HI value, while VMAT provided the best estimated beam-on time value. HT displayed a feasible SRS modality for single lesions, but not for multiple lesions.

Integrated scoring approach to assess radiotherapy plan quality for breast cancer treatment

Background

Proposal of an integrated scoring approach assessing the quality of different treatment techniques in a radiotherapy planning comparison. This scoring method incorporates all dosimetric indices of planning target volumes (PTVs) as well as organs at risk (OARs) and provides a single quantitative measure to select an ideal plan.

Materials and methods

The radiotherapy planning techniques compared were field-in-field (FinF), intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), hybrid IMRT (H-IMRT), and hybrid VMAT (H-VMAT). These plans were generated for twenty-five locally advanced left-sided breast cancer patients. The PTVs were prescribed a hypofractionation dose of 40.5 Gy in 15 fractions. The integrated score for each planning technique was calculated using the proposed formula.

Results

An integrated score value that is close to zero indicates a superior plan. The integrated score that incorporates all dosimetric indices (PTVs and OARs) were 1.37, 1.64, 1.72, 1.18, and 1.24 for FinF, IMRT, VMAT, H-IMRT, and H-VMAT plans, respectively.

Conclusion

The proposed integrated scoring approach is scientific to select a better plan and flexible to incorporate the patient-specific clinical demands. This simple tool is useful to quantify the treatment techniques and able to differentiate the acceptable and unacceptable plans.

Rotational effect and dosimetric impact: HDMLC vs 5-mm MLC leaf width in single isocenter multiple metastases radiosurgery with Brainlab Elements™

Purpose:

To analyse the impact of multileaf collimator (MLC) leaf width in multiple metastases radiosurgery (SRS) considering the target distance to isocenter and rotational displacements.

Methods:

Ten plans were optimised. The plans were created with Elements Multiple Mets SRS v2·0 (Brainlab AG, Munchen, Germany). The mean number of metastases per plan was 5 ± 2 [min 3, max 9], and the mean volume of gross tumour volume (GTV) was 1·1 ± 1·3 cc [min 0·02, max 5·1]. Planning target volume margin criterion was based on GTV-isocenter distance and target dimensions. Plans were performed using 6 MV with high-definition MLC (HDMLC) and reoptimised using 5-mm MLC (MLC-5). Plans were compared using Paddick conformity index (PCI), gradient index, monitor units , volume receiving half of prescription isodose (PIV50), maximum dose to brainstem, optic chiasm and optic nerves, and V12Gy, V10Gy and V5Gy for healthy brain were analysed. The maximum displacement due to rotational combinations was optimised by a genetic algorithm for both plans. Plans were reoptimised and compared using optimised margin.

Results:

HDMLC plans had better conformity and higher dose falloff than MLC-5 plans. Dosimetric differences were statistically significant (p < 0·05). The smaller the lesion volume, the higher the dosimetric differences between both plans. The effect of rotational displacements produced for each target in SRS was not dependent on the MLC (p > 0·05).

Conclusions:

The finer HDMLC offers dosimetric advantages compared with the MLC-5 in terms of target conformity and dose to the surrounding organs at risk. However, only dose falloff differences due to rotations depend on MLC.

Investigating the number of radiation fields in intensity-modulated radiotherapy plans of optic nerve sheath meningioma patients using dose gradient index

Purpose:In optic nerve radiotherapy, vital organs are very close to the target volume, they are highly sensitive to radiation and have low dose tolerance. In this regard, evaluating dose fall-off steepness around the target volume is required to assess various intensity-modulated radiation therapy (IMRT) plans in the treatment of the optic nerve sheath meningioma (ONSM) patients.Materials and Methods:Thirteen ONSM patients were analyzed with three IMRT techniques, including three (IMRT-3F), five (IMRT-5F), and seven fields (IMRT-7F). These plans were studied using D_mean, D_max, D_2%, D_98%, V_100%, uniformity index (UI), homogeneity index (HI), conformity index (CI), and specifically the dose gradient indices (DGIs). Results: The values of D_maxand D_meanfor IMRT-3F, IMRT-5F and IMRT-7F were (5637.42 ± 57.08, 5322.84 ± 83.86), (5670.51 ± 67.87, 5383.00 ± 58.45), and (5692.99 ± 31.65, 5405.72 ± 51.73), respectively, which were increased with increment in the number of IMRT fields from 3 to 7. The UI and HI indices were significantly different between IMRT-3F and IMRT-7F (p=0.010 and p=0.005, respectively), and CI was close to the ideal value (0.99±0.01) in IMRT-7F. The significant findings of the dose gradient indices represented smaller values in IMRT-7F, which led to a faster dose fall-off, particularly at the 70%-85% isodose levels around the target. Conclusion: Increasing the number of radiation fields in IMRT treatment plans of ONSM patients had a considerable difference in both the dosimetric parameters of the target volume and at-risk organs, as well as the dose gradient indices. Overall, IMRT-7F could be considered as a preferred technique in the treatment of this meningioma.

A Volume-Independent Conformity Index for Stereotactic Radiosurgery

PURPOSE

To develop a volume-independent conformity metric called the Gaussian Weighted Conformity Index (GWCI) to evaluate stereotactic radiosurgery/radiotherapy (SRS/SRT) plans for small brain tumors.

METHODS

A signed bi-directional local distance (BLD) between the prescription isodose line and the target contour is determined for each point along the tumor contour (positive distance represents under-coverage). A similarity score function is derived from Gaussian function, penalizing under- and over-coverage at each point by assigning standard deviations of the Gaussian function. Each point along the dose line contour is scored with this score function. The average of the similarity scores determines the GWCI. A total of 40 targets from 18 patients who received Gamma-Knife SRS/SRT treatments were analyzed to determine appropriate penalty criteria. The resulting GWCIs for test cases already deemed clinically acceptable are presented and compared to the same cases scored with the New Conformity Index to determine the influence of tumor volumes on the two conformity indices.

RESULTS

A total of four penalty combinations were tested based on the signed BLDs from the 40 targets. A GWCI of 0.9 is proposed as a cutoff for plan acceptability. The GWCI exhibits no target volume dependency as designed.

CONCLUSION

A limitation of current conformity indices, volume dependency, becomes apparent when applied to SRS/SRT plans. The GWCI appears to be a more robust index which penalizes over- and under-coverage of tumors and is not skewed by the tumor volume. This article is protected by copyright. All rights reserved.

Planning benchmark study for SBRT of liver metastases: Results of the DEGRO/DGMP working group stereotactic radiotherapy and radiosurgery

PURPOSE

To investigate, if liver SBRT treatment planning can be harmonized across different treatment planning systems, delivery techniques and institutions by using a specific prescription method and to minimize the knowledge gap concerning inter-system and inter-user differences. To provide best practice guidelines for all used techniques.

METHODS

A multiparametric specification of target dose (GTV_D50%, GTV_D0.1cc, GTV_V90%, PTV_V70%) with a prescription dose of GTV_D50% = 3 × 20 Gy and OAR limits were distributed with CTs and structure sets from three liver metastases patients. Thirty-five institutions provided 132 treatment plans using different irradiation techniques. These plans were first analyzed for target and OAR doses. Four different renormalization methods were performed (PTV_Dmin, PTV_D98%, PTV_D2%, PTV_Dmax). The resulting 660 treatments plans were evaluated regarding target doses in order to study the effect of dose renormalization to different prescription methods. A relative scoring system was used for comparisons.

RESULTS

GTV_D50% prescription can be performed in all systems. Treatment plan harmonization was overall successful with standard deviations for D_max, PTV_D98%, GTV_D98% and PTV_Dmean of 1.6 Gy, 3.3 Gy, 1.9 Gy and 1.5 Gy, respectively. Primary analysis showed 55 major deviations from clinical goals in 132 plans, while in only <20% of deviations GTV/PTV dose was traded for meeting OAR limits. GTV_D50% prescription produced the smallest deviation from target planning objectives and between techniques, followed by the PTV_Dmax, PTV_D98%, PTV_D2% and PTV_Dmin prescription. Deviations were significant for all combinations but for the PTV_Dmax prescription compared with GTV_D50% and PTV_D98%. Based on the various dose prescription methods, all systems significantly differed from each other, while GTV_D50% and PTV_D98% prescription showed the least differences between the systems.

CONCLUSIONS

This study showed the feasibility of harmonizing liver SBRT treatment plans across different treatment planning systems and delivery techniques when a sufficient set of clinical goals is given.

A systematically compiled set of quantitative metrics to describe spatial characteristics of radiotherapy dose distributions and aid in treatment planning

PURPOSE

Many quantitative metrics have been proposed in literature for characterization of spatial dose properties. The aim of this study is to work towards much-needed consensus in the radiotherapy community on which of these metrics to use. We do this by comparing characteristics of the metrics and providing a systematically selected set of metrics to comprehensively quantify properties of the spatial dose distribution.

METHODS

We searched the literature for metrics to quantitatively evaluate dose conformity, homogeneity, gradient (overall and directional), and distribution and location of over- and under-dosed sub-volumes. For each spatial dose property, we compared the responses of its corresponding metrics to simulated dose variations in a virtual water phantom. Selection criteria were a metric's ability to describe simulated scenarios robustly and to be visualized in an intuitive way.

RESULTS

We saw substantial differences in the responses of metrics to the simulated dose variations. Some conformity and homogeneity metrics were unable to quantify certain types of changes (e.g. target under-coverage). Others showed a large dependency on the shape and volume of targets and isodoses. Metric values differed between calculations in a static plan and in simulated full treatment courses including setup errors, especially for metrics quantifying distribution and location of hot and cold spots. We provide an Eclipse plugin script to calculate and visualize selected metrics.

CONCLUSION

The selected set of metrics provides complementary and comprehensive quantitative information about the spatial dose distribution. This work serves as a step towards broader consensus on the use of spatial dose metrics.

Radiation Necrosis from Stereotactic Radiosurgery-How Do We Mitigate?

OPINION STATEMENT

Intracranial stereotactic radiosurgery (SRS) is an effective and convenient treatment for many brain conditions. Data regarding safety come mostly from retrospective single institutional studies and a small number of prospective studies. Variations in target delineation, treatment delivery, imaging follow-up protocols and dose prescription limit the interpretation of this data. There has been much clinical focus on radiation necrosis (RN) in particular, as it is being increasingly recognized on follow-up imaging. Symptomatic RN may be treated with medical therapy (such as corticosteroids and bevacizumab) with surgical resection being reserved for refractory patients. Nevertheless, RN remains a challenging condition to manage, and therefore upfront patient selection for SRS remains critical to provide complication-free control. Mitigation strategies need to be considered in situations where the baseline risk of RN is expected to be high-such as large target volume or re-irradiation. These may involve reduction in the prescribed dose or hypofractionated stereotactic radiation therapy (HSRT). Recently published guidelines and international meta-analysis report the benefit of HSRT in larger lesions, without compromising control rates. However, careful attention to planning parameters and SRS techniques still need to be adhered, even with HSRT. In cases where the risk is deemed to be high despite mitigation, a combination approach of surgery with or without post-operative radiation should be considered.

Use of genetic algorithm for PTV optimization in single isocenter multiple metastases radiosurgery treatments with Brainlab Elements™

PURPOSE

To optimize PTV margins for single isocenter multiple metastases stereotactic radiosurgery through a genetic algorithm (GA) that determines the maximum effective displacement of each target (GTV) due to rotations.

METHOD

10 plans were optimized. The plans were created with Elements Multiple Mets™ (Brainlab AG, Munchen, Germany) from a predefined template. The mean number of metastases per plan was 5 ± 2 [3,9] and the mean volume of GTV was 1.1 ± 1.3 cc [0.02, 5.1]. PTV margin criterion was based on GTV-isocenter distance and target dimensions. The effective displacement to perform specific rotational combination (roll, pitch, yaw) was optimized by GA. The original plans were re-calculated using the PTV optimized margin and new dosimetric variations were obtained. The D_mean, D₉₉, Paddick conformity index (PCI), gradient index (GI) and dose variations in healthy brain were studied.

RESULTS

Regarding targets located shorter than 50 mm from the isocenter, the maximum calculated displacement was 2.5 mm. The differences between both PTV margin criteria were statistically significant for D_mean (p = 0.0163), D₉₉ (p = 0.0439), PCI (p = 0.0242), GI (p = 0.0160) and for healthy brain V₁₂ (p = 0.0218) and V₁₀ (p = 0.0264).

CONCLUSION

The GA allows to determine an optimized PTV margin based on the maximum displacement. Optimized PTV margins reduce the detriment of dosimetric parameters. Greater PTV margins are associated with an increase in healthy brain volume.

Clinical evaluation of a real-time inverse planning for Gamma Knife radiosurgery by convex optimization: a prospective comparative trial in a series of vestibular schwannoma patients

BACKGROUND

Gamma Knife radiosurgery (GKRS) inverse dose planning is currently far from competing effectively with the quality of dose planning developed by experienced experts. A new inverse planning (IP) method based on « efficient convex optimization algorithms » is proposed, providing high-quality dose plans in real time.

MATERIALS AND METHODS

Eighty-six patients treated by GKRS for vestibular schwannomas (VS) were recruited. The treatment plans created by the first author, who has 27 years of experience and has developed and delivered more than 15,000 dose plans, served as reference. A first set of basic constraints determined by default led the IP for an initial real-time dose plan. Additional constraints were interactively proposed by the planner to take other parameters into account. A second optimized plan was then calculated by the IP. The primary endpoint was the Paddick Conformity Index (PCI). The statistical analysis was planned on a non-inferiority trial design. Coverage, selectivity, and gradient indexes, dose at the organ(s) at risk, and 12 Gy isodose line volume were compared.

RESULTS

After a single run of the IP, the PCI was shown to be non-inferior to that of the "expert." For the expert and the IP, respectively, the median coverage index was 0.99 and 0.98, the median selectivity index 0.92 and 0.90, the median gradient index 2.95 and 2.84, the median dose at the modiolus of the cochlea 2.83 Gy and 2.86 Gy, the median number of shots 14.31 and 24.13, and the median beam-on time 46.20 min and 26.77 min. In a few specific cases, advanced tools of the IP were used to generate a second run by adding new constraints either globally (for higher selectivity) or locally, in order to increase or decrease these constraints focally.

CONCLUSION

These preliminary results showed that this new IP method based on « efficient convex optimization algorithms », called IntuitivePlan®, provided high-quality dose plans in real time with excellent coverage, selectivity, and gradient indexes with optimized beam-on time. If the new IP evaluated here is able to compete in real time with the quality of the treatment plans of an expert with extensive radiosurgical experience, this could allow new planners/radiosurgeons with limited or no experience to immediately provide patients with high-quality GKRS for benign and malignant lesions.

The surface area effect: How the intermediate dose spill depends on the PTV surface area in SRS

PURPOSE

Stereotactic radiosurgery (SRS) is rapidly becoming the standard of care for many intracranial targets. The characteristics of the planning target volume (PTV) can affect the intermediate dose spill and thus normal brain volume dose which is correlated with brain toxicity. R50% (volume receiving 50% of prescription dose divided by PTV volume) is a useful metric to quantify the intermediate dose spill. We propose a novel understanding of how the PTV surface area (SAPTV ) affects the intermediate dose spill of SRS treatments.

METHODS

Using a phantom model provided by a computed tomography (CT) of the IROC Head Phantom® and Eclipse® Treatment Planning System, we investigate the relationship of R50% and SAPTV in single-target SRS treatments. The planning studies are conducted for SRS treatments on a Varian TrueBeam® linear accelerator with high-definition MLC and a 6 MVFFF beam mode. These data are analyzed to ascertain trends in R50% related to SAPTV . Since SAPTV is not available as a structure property in the Eclipse RTPS, we introduce an Eclipse script to extract PTV surface area of arbitrary-shaped PTVs. We compare a physically reasonable theoretical prediction of R50%, R50%Analytic , to the R50% achieved in treatment planning studies.

RESULTS

The SRS phantom study indicates good correlation between the plan R50% and SAPTV . A near-linear relationship of plan R50% vs SAPTV is observed as predicted by the R50%Analytic model. Agreement between plan R50% values and R50%Analytic predictions is good for all but the very smallest PTV volumes.

CONCLUSIONS

We demonstrate dependence of the intermediate dose spill measured by R50% on the SAPTV . We call that dependence the surface area effect. This dependence is explicit in the R50%Analytic prediction model. The predicted value of R50%Analytic for a given PTV could be used for guidance during SRS treatment plan optimization, and plan evaluation for that PTV.

An analytical expression for R50% dependent on PTV surface area and volume: A cranial SRS comparison

The intermediate dose spill for a stereotactic radiosurgery (SRS) plan can be quantified with the metric R50%, defined as the 50% isodose cloud volume (V_IDC50% ) divided by the volume of the planning target volume (PTV). By coupling sound physical principles with the basic definition of R50%, we derive an analytical expression for R50% for a spherical PTV. Our analytical expression depends on three quantities: the surface area of PTV (SA_PTV ), the volume of PTV (V_PTV ), and the distance of dose drop-off to 50% (Δr). The value of ∆r was obtained from a simple set of cranial phantom plan calculations. We generate values from our analytical expression for R50% (R50%_Analytic ) and compare the values to clinical R50% values (R50%_Clinical ) extracted from a previously published SRS data set that spans the V_PTV range from 0.15 to 50.1 cm³ . R50%_Analytic is smaller than R50%_Clinical in all cases by an average of 15% ± 7%, and the general trend of R50%_Clinical vs V_PTV is reflected in the same trend of R50%_Analytic . This comparison suggests that R50%_Analytic could represent a theoretical lower limit for the clinical SRS data; further investigation is required to confirm this. R50%_Analytic could provide useful guidance for what might be achievable in SRS planning.

Proton vs Hyperarc™ radiosurgery: A planning comparison

For many patients, stereotactic radiosurgery (SRS) offers a minimally invasive, curative option when surgical techniques are not possible. To date, the literature supporting the efficacy and safety of SRS treatment techniques uses photon beams. However, with the number of proton therapy facilities exponentially growing and the favorable physical properties of proton beam radiation therapy, there is an opportunity to develop proton therapy techniques for SRS. The goal of this paper is to determine the ability of clinical proton treatment planning systems to model small field dosimetry accurately and to compare various planning metrics used to evaluate photon SRS to determine the optimum beam configurations and settings for proton SRS (PSRS) treatment plans. Once established, these plan settings were used to perform a planning comparison on a variety of different SRS cases and compare SRS metrics between the PSRS plans and HyperArc™ (VMAT) SRS plans.

Dosimetric feasibility of brain stereotactic radiosurgery with a 0.35 T MRI-guided linac and comparison vs a C-arm-mounted linac

PURPOSE

MRI is the gold-standard imaging modality for brain tumor diagnosis and delineation. The purpose of this work was to investigate the feasibility of performing brain stereotactic radiosurgery (SRS) with a 0.35 T MRI-guided linear accelerator (MRL) equipped with a double-focused multileaf collimator (MLC). Dosimetric comparisons were made vs a conventional C-arm-mounted linac with a high-definition MLC.

METHODS

The quality of MRL single-isocenter brain SRS treatment plans was evaluated as a function of target size for a series of spherical targets with diameters from 0.6 cm to 2.5 cm in an anthropomorphic head phantom and six brain metastases (max linear dimension = 0.7-1.9 cm) previously treated at our clinic on a conventional linac. Each target was prescribed 20 Gy to 99% of the target volume. Step-and-shoot IMRT plans were generated for the MRL using 11 static coplanar beams equally spaced over 360° about an isocenter placed at the center of the target. Couch and collimator angles are fixed for the MRL. Two MRL planning strategies (VR1 and VR2) were investigated. VR1 minimized the 12 Gy isodose volume while constraining the maximum point dose to be within ±1 Gy of 25 Gy which corresponded to normalization to an 80% isodose volume. VR2 minimized the 12 Gy isodose volume without the maximum dose constraint. For the conventional linac, the TB1 method followed the same strategy as VR1 while TB2 used five noncoplanar dynamic conformal arcs. Plan quality was evaluated in terms of conformity index (CI), conformity/gradient index (CGI), homogeneity index (HI), and volume of normal brain receiving ≥12 Gy (V_12Gy ). Quality assurance measurements were performed with Gafchromic EBT-XD film following an absolute dose calibration protocol.

RESULTS

For the phantom study, the CI of MRL plans was not significantly different compared to a conventional linac (P > 0.05). The use of dynamic conformal arcs and noncoplanar beams with a conventional linac spared significantly more normal brain (P = 0.027) and maximized the CGI, as expected. The mean CGI was 95.9 ± 4.5 for TB2 vs 86.6 ± 3.7 (VR1), 88.2 ± 4.8 (VR2), and 88.5 ± 5.9 (TB1). Each method satisfied a normal brain V_12Gy  ≤ 10.0 cm³ planning goal for targets with diameter ≤2.25 cm. The mean V_12Gy was 3.1 cm³ for TB2 vs 5.5 cm³ , 5.0 cm³ and 4.3 cm³ , for VR1, VR2, and TB1, respectively. For a 2.5-cm diameter target, only TB2 met the V_12Gy planning objective. The MRL clinical brain plans were deemed acceptable for patient treatment. The normal brain V_12Gy was ≤6.0 cm³ for all clinical targets (maximum target volume = 3.51 cm³ ). CI and CGI ranged from 1.12-1.65 and 81.2-88.3, respectively. Gamma analysis pass rates (3%/1mm criteria) exceeded 97.6% for six clinical targets planned and delivered on the MRL. The mean measured vs computed absolute dose difference was -0.1%.

CONCLUSIONS

The MRL system can produce clinically acceptable brain SRS plans for spherical lesions with diameter ≤2.25 cm. Large lesions (>2.25 cm) should be treated with a linac capable of delivering noncoplanar beams.

Strategies to optimize stereotactic radiosurgery plans for brain tumors with volumetric-modulated arc therapy

Purpose

Prescription practice in SRS plans for brain tumors differs significantly for different modalities. In this retrospective study, the strategies to optimize SRS plans for brain tumors with volumetric arc therapy (VMAT) were presented.

Methods

Fifty clinically treated cases were stratified by the maximum target size into two groups (≥ 2 cm in 25 cases and <2 cm but ≥1 cm in 25 cases), which were optimized using traditional LINAC MLC‐based approaches with the average prescription isodose line (P‐IDL) of (91.4 ± 0.6)%. Four to five plans have been created for each case with variation of the P‐IDL from 65% to 90%. The optimization strategies to select an optimal P‐IDL, to use tuning structures within the target and beyond as well as to use NTO (normal tissue objectives), were applied to all new plans.

Results

The optimal P‐IDL was found to be around 75%. After applying the new optimization strategies with an average P‐IDL of 74.8%, the mean modified gradient index (mGI) and V12 were reduced by 25% and 35%, respectively for both groups. The Paddick conformity index (PCI) was averagely improved by 8%. The average homogeneity index (HI) and focal index (FI) were increased by 22% and 50%, respectively. The mGI was inversely proportional to the PTV volumes. The shape of the dose distribution in target was also changed from concave to convex. The comparison of PCI with historical data from other institutes and modalities shows that the plans in this study have the best conformity near the target.

Conclusions

With the new optimization strategies for VMAT SRS plan of brain tumor more conformal plans in both high and intermediate dose region (~50% of the PD) were created, in which the dose in the core of the target was notably increased while V12 and mGI were significantly decreased, and PCI was improved. The mGI was inversely proportional to the PTV volumes. The optimal P‐IDL is around 75%. The average PCI is the best in this study compared with the published historical data. These strategies are applicable to treatment planning for multiple brain and liver tumors where sparing the tissue peripheral to the target is critical.

The superficially averaged dose gradient at the target volume's boundary: A two-dimensional formulation and solution of anisotropic dose gradient problems

BACKGROUND AND PURPOSE

Dose conformity and steepness of dose fall-off at a target volume's boundary are important quality criteria in treatment planning to predict complication rates in normal tissue and critical structures. Several dose gradient measures are in use; ICRU Report 91 recommends one of two gradient metrics for reporting. All of the common gradient indices are one-dimensional, although dose gradient problems are at least two-dimensional and anisotropic. Four of ten investigated gradient indices show false characteristics on the mean value of the physical dose gradients. Anisotropic dose gradient measures can be the basis of clinical therapeutic decisions if tumours are surrounded by organs at risk with various tolerance dose values. To close this information gap, the author presents a mathematical description and solution of anisotropic dose gradient problems.

MATERIALS AND METHODS

The new two-dimensional dose gradient measure is called the superficially averaged dose gradient (SADG). The particular informative content of the ten common dose gradient indices was assessed by classification and analysing their properties. The correlations between all of the dose gradient measures were investigated for linac-based stereotactic radiosurgery of 13 brain metastases.

RESULTS

From all of the one-dimensional dose gradient indices, the approximated SADG* showed the best correlation on the SADG. Here, Pearson's correlation coefficient was 1.000 and the relative errors were in a range of -0.2 to 2.9%. Distributions of anisotropic dose gradients were graphically represented by dose gradient-solid angle histograms.

CONCLUSIONS

Two-dimensional dose gradient measures such as the SADG are urgently required for lesions that are located in non-homogeneous normal tissue. The quality of each present and future dose gradient measure concerning the description of anisotropic dose gradient problems is now verifiable by use of the SADG. Through the SADG, the influences of collimation types on the dose fall-off at the target volume's boundary can be investigated. The algorithm for determining the SADG should be implemented in treatment planning systems to utilise the formalism for all users. The dose gradient indices recommended in ICRU Report 91 overestimate and underestimate physical dose gradients, respectively.

Fixed-jaw technique to improve IMRT plan quality for the treatment of cervical and upper thoracic esophageal cancer

The purpose of this study was to investigate the potential advantages of the fixed-jaw technique (FJT) over the conventional split-field technique (SFT) for cervical and upper thoracic esophageal cancer (EC) patients treated with intensity-modulated radiotherapy. The SFT and FJT plans were generated for 15 patients with cervical and upper thoracic EC. Dosimetric parameters and delivery efficiency were compared. An area ratio (AR) of the jaw opening to multileaf collimator (MLC) aperture weighted by the number of monitor units (MUs) was defined to evaluate the impact of the transmission through the MLC on the dose gradient outside the PTV50.4, and the correlation between the gradient index (GI) and AR was analyzed. The FJT plans achieved a better GI and AR (P < 0.001). There was a positive correlation between the GI and AR in the FJT (r = 0.883, P < 0.001) and SFT plans (r = 0.836, P < 0.001), respectively. Moreover, the mean dose (D_mean ), V_5Gy -V_40Gy for the lungs and the D_mean , V_5Gy -V_50Gy for the body-PTV50.4 in the FJT plans were lower than those in the SFT plans (P < 0.05). The FJT plans demonstrated a reduction trend in the doses to the spinal cord PRV and heart, but only the difference in the heart D_mean reached statistical significance (P < 0.05). The FJT plans reduced the number of MUs and subfields by 5.5% and 17.9% and slightly shortened the delivery time by 0.23 min (P < 0.05). The gamma-index passing rates were above 95% for both plans. The FJT combined with target splitting can provide superior organs at risk sparing and similar target coverage without compromising delivery efficiency and should be a preferred intensity-modulated radiotherapy planning method for cervical and upper thoracic EC patients.

Multi-center evaluation of dose conformity in stereotactic body radiotherapy

BACKGROUND AND PURPOSE

Stereotactic body radiotherapy (SBRT) is an emerging technique for treating oligometastases, but limited data is available on what plan quality is achievable for a range of modalities and clinical sites.

METHODS

SBRT plans for lung, spine, bone, adrenal, liver and node sites from 17 participating centers were reviewed. Centers used various delivery techniques including static and rotational intensity-modulation and multiple non-coplanar beams. Plans were split into lung and other body sites and evaluated with different plan quality metrics, including two which are independent of target coverage; "prescription dose spillage" (PDS) and "modified gradient index" (MGI). These were compared to constraints from the ROSEL and RTOG 0813 clinical trials.

RESULTS

Planning target volume (PTV) coverage was compromised (PTV V100% < 90%) in 29% of patient plans in order to meet organ-at-risk (OAR) tolerances, supporting the use of plan quality metrics which are independent of target coverage. Both lung (n = 48) and other body (n = 99) site PDS values agreed well with ROSEL constraints on dose spillage, but RTOG 0813 values were too high to detect sub-optimal plans. MGI values for lung plans were mis-matched to both sets of previous constraints, with ROSEL values too high and RTOG 0813 values too low. MGI values were lower for other body plans as expected, though this was only statistically significant for PTV volumes <20 cm3.

CONCLUSIONS

Updated guidance for lung and other body site SBRT plan quality using the PDS and MGI metrics is presented.

Evaluation of plan optimisers in prostate VMAT using the dose distribution index

Purpose:

Dose distribution index (DDI) is a treatment planning evaluation parameter, reflecting dosimetric information of target coverage that can help to spare organs at risk (OARs) and remaining volume at risk (RVR). The index has been used to evaluate and compare prostate volumetric modulated arc therapy (VMAT) plans using two different plan optimisers, namely photon optimisation (PO) and its predecessor, progressive resolution optimisation (PRO).

Materials and methods:

Twenty prostate VMAT treatment plans were created using the PO and PRO in this retrospective study. The 6 MV photon beams and a dose prescription of 78 Gy/39 fractions were used in plans with the same dose–volume criteria for plan optimisation. Dose–volume histograms (DVHs) of the planning target volume (PTV), as well as of OARs such as the rectum, bladder, left and right femur were determined in each plan. DDIs were calculated and compared for plans created by the PO and PRO based on DVHs of the PTV and all OARs.

Results:

The mean DDI values were 0·784 and 0·810 for prostate VMAT plans created by the PO and PRO, respectively. It was found that the DDI of the PRO plan was about 3·3% larger than the PO plan, which means that the dose distribution of the target coverage and sparing of OARs in the PRO plan was slightly better. Changing the weighting factors in different OARs would vary the DDI value by ∼7%. However, for plan comparison based on the same set of dose–volume criteria, the effect of weighting factor can be neglected because they were the same in the PO and PRO.

Conclusions:

Based on the very similar DDI values calculated from the PO and PRO plans, with the DDI value in the PRO plan slightly larger than that of the PO, it may be concluded that the PRO can create a prostate VMAT plan with slightly better dose distribution regarding the target coverage and sparing of OARs. Moreover, we found that the DDI is a simple and comprehensive dose–volume parameter for plan evaluation considering the target, OARs and RVR.

Integration of rotational angiography enables better dose planning in Gamma Knife radiosurgery for brain arteriovenous malformations

OBJECTIVE

In Gamma Knife radiosurgery (GKS) for arteriovenous malformations (AVMs), CT angiography (CTA), MRI, and digital subtraction angiography (DSA) are generally used to define the nidus. Although the AVM angioarchitecture can be visualized with superior resolution using rotational angiography (RA), the efficacy of integrating RA into the GKS treatment planning process has not been elucidated.

METHODS

Using data collected from 25 consecutive patients with AVMs who were treated with GKS at the authors’ institution, two neurosurgeons independently created treatment plans for each patient before and after RA integration. For all patients, MR angiography, contrasted T1 imaging, CTA, DSA, and RA were performed before treatment. The prescription isodose volume before (PIVB) and after (PIVA) RA integration was measured. For reference purposes, a reference target volume (RTV) for each nidus was determined by two other physicians independent of the planning surgeons, and the RTV covered by the PIV (RTVPIV) was established. The undertreated volume ratio (UVR), overtreated volume ratio (OVR), and Paddick’s conformal index (CI), which were calculated as RTVPIV/RTV, RTVPIV/PIV, and (RTVPIV)2/(RTV × PIV), respectively, were measured by each neurosurgeon before and after RA integration, and the surgeons’ values at each point were averaged. Wilcoxon signed-rank tests were used to compare the values obtained before and after RA integration. The percentage change from before to after RA integration was calculated for the average UVR (%ΔUVRave), OVR (%ΔOVRave), and CI (%ΔCIave) in each patient, as ([value after RA integration]/[value before RA integration] − 1) × 100. The relationships between prior histories and these percentage change values were examined using Wilcoxon signed-rank tests.

RESULTS

The average values obtained by the two surgeons for the median UVR, OVR, and CI were 0.854, 0.445, and 0.367 before RA integration and 0.882, 0.478, and 0.463 after RA integration, respectively. All variables significantly improved after compared with before RA integration (UVR, p = 0.009; OVR, p < 0.001; CI, p < 0.001). Prior hemorrhage was significantly associated with larger %ΔOVRave (median 20.8% vs 7.2%; p = 0.023) and %ΔCIave (median 33.9% vs 13.8%; p = 0.014), but not %ΔUVRave (median 4.7% vs 4.0%; p = 0.449).

CONCLUSIONS

Integrating RA into GKS treatment planning may permit better dose planning owing to clearer visualization of the nidus and, as such, may reduce undertreatment and waste irradiation. Further studies examining whether the observed RA-related improvement in dose planning also improves the radiosurgical outcome are needed.

A novel index for assessing treatment plan quality in stereotactic radiosurgery

OBJECTIVE

Stereotactic radiosurgery (SRS) is characterized by high levels of conformity and steep dose gradients from the periphery of the target to surrounding tissue. Clinical studies have backed up the importance of these factors through evidence of symptomatic complications. Available data suggest that there are threshold doses above which the risk of symptomatic radionecrosis increases with the volume irradiated. Therefore, radiosurgical treatment plans should be optimized by minimizing dose to the surrounding tissue while maximizing dose to the target volume. Several metrics have been proposed to quantify radiosurgical plan quality, but all present certain weaknesses. To overcome limitations of the currently used metrics, a novel metric is proposed, the efficiency index (η50%), which is based on the principle of calculating integral doses: η50% = integral doseTV/integral dosePIV50%.

METHODS

The value of η50% can be easily calculated by dividing the integral dose (mean dose × volume) to the target volume (TV) by the integral dose to the volume of 50% of the prescription isodose (PIV50%). Alternatively, differential dose-volume histograms (DVHs) of the TV and PIV50% can be used. The resulting η50% value is effectively the proportion of energy within the PIV50% that falls into the target. This value has theoretical limits of 0 and 1, with 1 being perfect. The index combines conformity, gradient, and mean dose to the target into a single value. The value of η50% was retrospectively calculated for 100 clinical SRS plans.

RESULTS

The value of η50% for the 100 clinical SRS plans ranged from 37.7% to 58.0% with a mean value of 49.0%. This study also showed that the same principles used for the calculation of η50% can be adapted to produce an index suitable for multiple-target plans (Gη12Gy). Furthermore, the authors present another adaptation of the index that may play a role in plan optimization by calculating and minimizing the proportion of energy delivered to surrounding organs at risk (OARη50%).

CONCLUSIONS

The proposed efficiency index is a novel approach in quantifying plan quality by combining conformity, gradient, and mean dose into a single value. It quantifies the ratio of the dose “doing good” versus the dose “doing harm,” and its adaptations can be used for multiple-target plan optimization and OAR sparing.

Cyberknife Dosimetric Planning Using a Dose-Limiting Shell Method for Brain Metastases

Objective

We investigated the effect of optimization in dose-limiting shell method on the dosimetric quality of CyberKnife (CK) plans in treating brain metastases (BMs).

Methods

We selected 19 BMs previously treated using CK between 2014 and 2015. The original CK plans (CK_original) had been produced using 1 to 3 dose-limiting shells : one at the prescription isodose level (PIDL) for dose conformity and the others at lowisodose levels (10–30% of prescription dose) for dose spillage. In each case, a modified CK plan (CK_modified) was generated using 5 dose-limiting shells : one at the PIDL, another at intermediate isodose level (50% of prescription dose) for steeper dose fall-off, and the others at low-isodose levels, with an optimized shell-dilation size based on our experience. A Gamma Knife (GK) plan was also produced using the original contour set. Thus, three data sets of dosimetric parameters were generated and compared.

Results

There were no differences in the conformity indices among the CK_original, CK_modified, and GK plans (mean 1.22, 1.18, and 1.24, respectively; p=0.079) and tumor coverage (mean 99.5%, 99.5%, and 99.4%, respectively; p=0.177), whereas the CK_modified plans produced significantly smaller normal tissue volumes receiving 50% of prescription dose than those produced by the CK_original plans (p<0.001), with no statistical differences in those volumes compared with GK plans (p=0.345).

Conclusion

These results indicate that significantly steeper dose fall-off is able to be achieved in the CK system by optimizing the shell function while maintaining high conformity of dose to tumor.

Retrospective dosimetric analysis of brain lesions planned in Pinnacle 9.8 via a HDMLC linac

The University of Toledo Medical Center's Eleanor N. Dana Cancer Center located in northwest Ohio currently utilizes the Edge Radiosurgery System (Varian Medical Systems Inc., Palo Alto, CA) to deliver stereotactic radiosurgery for the treatment of brain lesions. The purpose of this study is to determine the quality of conformal arc radiotherapy in treating patients with brain lesions at The University of Toledo Medical Center and to provide more data for conformity and gradient indices (due to a lack of current data) to hopefully improve national standards by allowing centers to compare among each other. Treatment plans were assessed using the Pinnacle³ v9.8 Radiation Therapy Planning System (Philips Healthcare, Amsterdam, Netherlands). For patients (n = 41) presenting with small brain lesions (n = 82) and treated with conformal arc radiotherapy via the Edge Radiosurgery System, the RTOG conformity index, Paddick conformity index, conformity gradient index, gradient index, and dose gradient index were determined for each plan. This study additionally provides data to suggest the more accurate method of volume derivation provided by the Pinnacle³ v9.8 software. Using this method, average values for each of the following indices were calculated: RTOG conformity index = 1.36 ± 0.29; Paddick conformity index = 0.72 ± 0.12; conformity gradient index = 214.67 ± 12.35; gradient index = 3.64 ± 1.09; dose gradient index = -0.11 ± 0.16. Thus, The University of Toledo Medical Center provides favorable conformity of dose to intracranial target lesions.

Dose gradient curve: A new tool for evaluating dose gradient

Purpose

Stereotactic radiotherapy, which delivers an ablative high radiation dose to a target volume for maximum local tumor control, requires a rapid dose fall-off outside the target volume to prevent extensive damage to nearby normal tissue. Currently, there is no tool to comprehensively evaluate the dose gradient near the target volume. We propose the dose gradient curve (DGC) as a new tool to evaluate the quality of a treatment plan with respect to the dose fall-off characteristics.

Methods

The average distance between two isodose surfaces was represented by the dose gradient index (DGI) estimated by a simple equation using the volume and surface area of isodose levels. The surface area was calculated by mesh generation and surface triangulation. The DGC was defined as a plot of the DGI of each dose interval as a function of the dose. Two types of DGCs, differential and cumulative, were generated. The performance of the DGC was evaluated using stereotactic radiosurgery plans for virtual targets.

Results

Over the range of dose distributions, the dose gradient of each dose interval was well-characterized by the DGC in an easily understandable graph format. Significant changes in the DGC were observed reflecting the differences in planning situations and various prescription doses.

Conclusions

The DGC is a rational method for visualizing the dose gradient as the average distance between two isodose surfaces; the shorter the distance, the steeper the dose gradient. By combining the DGC with the dose-volume histogram (DVH) in a single plot, the DGC can be utilized to evaluate not only the dose gradient but also the target coverage in routine clinical practice.

Evaluating which plan quality metrics are appropriate for use in lung SBRT

OBJECTIVE

Several dose metrics in the categories-homogeneity, coverage, conformity and gradient have been proposed in literature for evaluating treatment plan quality. In this study, we applied these metrics to characterize and identify the plan quality metrics that would merit plan quality assessment in lung stereotactic body radiation therapy (SBRT) dose distributions.

METHODS

Treatment plans of 90 lung SBRT patients, comprising 91 targets, treated in our institution were retrospectively reviewed. Dose calculations were performed using anisotropic analytical algorithm (AAA) with heterogeneity correction. A literature review on published plan quality metrics in the categories-coverage, homogeneity, conformity and gradient was performed. For each patient, using dose-volume histogram data, plan quality metric values were quantified and analysed.

RESULTS

For the study, the radiation therapy oncology group (RTOG) defined plan quality metrics were: coverage (0.90 ± 0.08); homogeneity (1.27 ± 0.07); conformity (1.03 ± 0.07) and gradient (4.40 ± 0.80). Geometric conformity strongly correlated with conformity index (p < 0.0001). Gradient measures strongly correlated with target volume (p < 0.0001). The RTOG lung SBRT protocol advocated conformity guidelines for prescribed dose in all categories were met in ≥94% of cases. The proportion of total lung volume receiving doses of 20 Gy and 5 Gy (V20 and V5) were mean 4.8% (±3.2) and 16.4% (±9.2), respectively.

CONCLUSION

Based on our study analyses, we recommend the following metrics as appropriate surrogates for establishing SBRT lung plan quality guidelines-coverage % (ICRU 62), conformity (CN or CIPaddick) and gradient (R50%). Furthermore, we strongly recommend that RTOG lung SBRT protocols adopt either CN or CIPadddick in place of prescription isodose to target volume ratio for conformity index evaluation. Advances in knowledge: Our study metrics are valuable tools for establishing lung SBRT plan quality guidelines.

Dosimetric comparison of different treatment modalities for stereotactic radiotherapy

Background

The modalities for performing stereotactic radiotherapy (SRT) on the brain include the cone-based linear accelerator (linac), the flattening filter-free (FFF) volumetric modulated arc therapy (VMAT) linac, and tomotherapy. In this study, the cone-based linac, FFF-VMAT linac, and tomotherapy modalities were evaluated by measuring the differences in doses delivered during brain SRT and experimentally assessing the accuracy of the output radiation doses through clinical measurements.

Methods

We employed a homemade acrylic dosimetry phantom representing the head, within which a thermoluminescent dosimeter (TLD) and radiochromic EBT3 film were installed. Using the conformity/gradient index (CGI) and Paddick methods, the quality of the doses delivered by the various SRT modalities was evaluated. The quality indicators included the uniformity, conformity, and gradient indices. TLDs and EBT3 films were used to experimentally assess the accuracy of the SRT dose output.

Results

The dose homogeneity indices of all the treatment modalities were lower than 1.25. The cone-based linac had the best conformity for all tumors, regardless of the tumor location and size, followed by the FFF-VMAT linac; tomography was the worst-performing treatment modality in this regard. The cone-based linac had the best gradient, regardless of the tumor location and size, whereas the FFF-VMAT linac had a better gradient than tomotherapy for a large tumor diameter (28 mm). The TLD and EBT3 measurements of the dose at the center of tumors indicated that the average difference between the measurements and the calculated dose was generally less than 4%. When the 3% 3-mm gamma passing rate metric was used, the average passing rates of all three treatment modalities exceeded 98%.

Conclusions

Regarding the dose, the cone-based linac had the best conformity and steepest dose gradient for tumors of different sizes and distances from the brainstem. The results of this study suggest that SRT should be performed using the cone-based linac on tumors that require treatment plans with a steep dose gradient, even as the tumor is slightly irregular, we should also consider using a high dose gradient of the cone base to treat and protect the normal tissue. If normal tissues require special protection exist at positions that are superior or inferior to the tumor, we can consider using tomotherapy or Cone base with couch at 0° for treatment.

Planning benchmark study for SBRT of early stage NSCLC : Results of the DEGRO Working Group Stereotactic Radiotherapy

PURPOSE

The aim was to evaluate stereotactic body radiation therapy (SBRT) treatment planning variability for early stage nonsmall cell lung cancer (NSCLC) with respect to the published guidelines of the Stereotactic Radiotherapy Working Group of the German Society for Radiation Oncology (DEGRO).

MATERIALS AND METHODS

Planning computed tomography (CT) scan and the structure sets (planning target volume, PTV; organs at risk, OARs) of 3 patients with early stage NSCLC were sent to 22 radiotherapy departments with SBRT experience: each department was asked to prepare a treatment plan according to the DEGRO guidelines. The prescription dose was 3 fractions of 15 Gy to the 65% isodose.

RESULTS

In all, 87 plans were generated: 36 used intensity-modulated arc therapy (IMAT), 21 used three-dimensional conformal radiation therapy (3DCRT), 6 used static field intensity-modulated radiation therapy (SF-IMRT), 9 used helical radiotherapy and 15 used robotic radiosurgery. PTV dose coverage and simultaneously kept OARs doses were within the clinical limits published in the DEGRO guidelines. However, mean PTV dose (mean 58.0 Gy, range 52.8-66.4 Gy) and dose conformity indices (mean 0.75, range 0.60-1.00) varied between institutions and techniques (p ≤ 0.02). OARs doses varied substantially between institutions, but appeared to be technique independent (p = 0.21).

CONCLUSION

All studied treatment techniques are well suited for SBRT of early stage NSCLC according to the DEGRO guidelines. Homogenization of SBRT practice in Germany is possible through the guidelines; however, detailed treatment plan characteristics varied between techniques and institutions and further homogenization is warranted in future studies and recommendations. Optimized treatment planning should always follow the ALARA (as low as reasonably achievable) principle.

Tuning of AcurosXB source size setting for small intracranial targets

This study details a method to evaluate the source size selection for small field intracranial stereotactic radiosurgery (SRS) deliveries in Eclipse treatment planning system (TPS) for AcurosXB dose calculation algorithm. Our method uses end‐to‐end dosimetric data to evaluate a total of five source size selections (0.50 mm, 0.75 mm, 1.00 mm, 1.25 mm, and 1.50 mm). The dosimetric leaf gap (DLG) was varied in this analysis (three DLG values were tested for each scenario). We also tested two MLC leaf designs (standard and high‐definition MLC) and two delivery types for intracranial SRS (volumetric modulated arc therapy [VMAT] and dynamic conformal arc [DCA]). Thus, a total of 10 VMAT plans and 10 DCA plans were tested for each machine type (TrueBeam [standard MLC] and Edge [high‐definition MLC]). Each plan was mapped to a solid water phantom and dose was calculated with each iteration of source size and DLG value (15 total dose calculations for each plan). To measure the dose, Gafchromic film was placed in the coronal plane of the solid water phantom at isocenter. The phantom was localized via on‐board CBCT and the plans were delivered at planned gantry, collimator, and couch angles. The planned and measured film dose was compared using Gamma (3.0%, 0.3 mm) criteria. The vendor‐recommended 1.00 mm source size was suitable for TrueBeam planning (both VMAT and DCA planning) and Edge DCA planning. However, for Edge VMAT planning, the 0.50 mm source size yielded the highest passing rates. The difference in dose calculation among the source size variations manifested primarily in two regions of the dose calculation: (1) the shoulder of the high‐dose region, and (2) for small targets (volume ≤ 0.30 cc), in the central portion of the high‐dose region. Selection of a larger than optimal source size can result in increased blurring of the shoulder for all target volume sizes tested, and can result in central axis dose discrepancies in excess of 10% for target volumes sizes ≤ 0.30 cc. Our results indicate a need for evaluation of the source size when AcurosXB is used to model intracranial SRS delivery, and our methods represent a feasible process for many clinics to perform tuning of the AcurosXB source size parameter.

Comparison of Rapid Arc and Intensity-modulated Radiotherapy Plans Using Unified Dosimetry Index and the Impact of Conformity Index on Unified Dosimetry Index Evaluation

The aim of this study was to evaluate the impact of conformity index in the unified dosimetry index (UDI) score for two different planning techniques namely intensity-modulated radiotherapy (IMRT) and Rapid Arc. Rapid Arc and IMRT plans of 57 patients were evaluated and compared using UDI score which incorporates four indices. To determine the impact of conformity index on the IMRT and Rapid Arc plans, UDI at conformity index one of all plan (UDI_{unit_CI}) score was calculated by assuming conformity index is equal to one. Mean and standard deviations of all indices were calculated. Rapid Arc technique plans of different treatment sites of all patients scored lesser UDI than IMRT plans, and the conformity index of Rapid Arc plan was significantly better than IMRT plan. The average dose gradient, homogeneity, coverage, and conformity index of all sites with Rapid Arc plans were 0.212 ± 0.05, 1.123 ± 0.03, 0.959 ± 0.03, and 1.056 ± 0.09; with IMRT plans were 0.190 ± 0.05, 1.113 ± 0.04, 0.950 ± 0.04, and 1.172 ± 0.16, respectively. UDI score value with actual conformity index of Rapid Arc and IMRT plans differed significantly (P < 0.001). However, UDI_{unit_CI} score values with assumed conformity index equal to one did not differ significantly (P = 0.528). In the comparison of IMRT and Rapid Arc plans using the UDI score, the impact of conformity index was significant.

SPIDERplan: A tool to support decision-making in radiation therapy treatment plan assessment

AIM

In this work, a graphical method for radiotherapy treatment plan assessment and comparison, named SPIDERplan, is proposed. It aims to support plan approval allowing independent and consistent comparisons of different treatment techniques, algorithms or treatment planning systems.

BACKGROUND

Optimized plans from modern radiotherapy are not easy to evaluate and compare because of their inherent multicriterial nature. The clinical decision on the best treatment plan is mostly based on subjective options.

MATERIALS AND METHODS

SPIDERplan combines a graphical analysis with a scoring index. Customized radar plots based on the categorization of structures into groups and on the determination of individual structures scores are generated. To each group and structure, an angular amplitude is assigned expressing the clinical importance defined by the radiation oncologist. Completing the graphical evaluation, a global plan score, based on the structures score and their clinical weights, is determined. After a necessary clinical validation of the group weights, SPIDERplan efficacy, to compare and rank different plans, was tested through a planning exercise where plans had been generated for a nasal cavity case using different treatment planning systems.

RESULTS

SPIDERplan method was applied to the dose metrics achieved by the nasal cavity test plans. The generated diagrams and scores successfully ranked the plans according to the prescribed dose objectives and constraints and the radiation oncologist priorities, after a necessary clinical validation process.

CONCLUSIONS

SPIDERplan enables a fast and consistent evaluation of plan quality considering all targets and organs at risk.

Optimal technique of linear accelerator-based stereotactic radiosurgery for tumors adjacent to brainstem

Stereotactic radiosurgery (SRS) is a well-established technique that is replacing whole-brain irradiation in the treatment of intracranial lesions, which leads to better preservation of brain functions, and therefore a better quality of life for the patient. There are several available forms of linear accelerator (LINAC)-based SRS, and the goal of the present study is to identify which of these techniques is best (as evaluated by dosimetric outcomes statistically) when the target is located adjacent to brainstem. We collected the records of 17 patients with lesions close to the brainstem who had previously been treated with single-fraction radiosurgery. In all, 5 different lesion catalogs were collected, and the patients were divided into 2 distance groups-1 consisting of 7 patients with a target-to-brainstem distance of less than 0.5cm, and the other of 10 patients with a target-to-brainstem distance of ≥ 0.5 and < 1cm. Comparison was then made among the following 3 types of LINAC-based radiosurgery: dynamic conformal arcs (DCA), intensity-modulated radiosurgery (IMRS), and volumetric modulated arc radiotherapy (VMAT). All techniques included multiple noncoplanar beams or arcs with or without intensity-modulated delivery. The volume of gross tumor volume (GTV) ranged from 0.2cm(3) to 21.9cm(3). Regarding the dose homogeneity index (HIICRU) and conformity index (CIICRU) were without significant difference between techniques statistically. However, the average CIICRU = 1.09 ± 0.56 achieved by VMAT was the best of the 3 techniques. Moreover, notable improvement in gradient index (GI) was observed when VMAT was used (0.74 ± 0.13), and this result was significantly better than those achieved by the 2 other techniques (p < 0.05). For V4Gy of brainstem, both VMAT (2.5%) and IMRS (2.7%) were significantly lower than DCA (4.9%), both at the p < 0.05 level. Regarding V2Gy of normal brain, VMAT plans had attained 6.4 ± 5%; this was significantly better (p < 0.05) than either DCA or IMRS plans, at 9.2 ± 7% and 8.2 ± 6%, respectively. Owing to the multiple arc or beam planning designs of IMRS and VMAT, both of these techniques required higher MU delivery than DCA, with the averages being twice as high (p < 0.05). If linear accelerator is only 1 modality can to establish for SRS treatment. Based on statistical evidence retrospectively, we recommend VMAT as the optimal technique for delivering treatment to tumors adjacent to brainstem.

Inverse treatment planning for spinal robotic radiosurgery: an international multi-institutional benchmark trial

Stereotactic radiosurgery (SRS) is the accurate, conformal delivery of high‐dose radiation to well‐defined targets while minimizing normal structure doses via steep dose gradients. While inverse treatment planning (ITP) with computerized optimization algorithms are routine, many aspects of the planning process remain user‐dependent. We performed an international, multi‐institutional benchmark trial to study planning variability and to analyze preferable ITP practice for spinal robotic radiosurgery. 10 SRS treatment plans were generated for a complex‐shaped spinal metastasis with 21 Gy in 3 fractions and tight constraints for spinal cord (V14Gy<2 cc, V18Gy<0.1 cc) and target (coverage >95%). The resulting plans were rated on a scale from 1 to 4 (excellent‐poor) in five categories (constraint compliance, optimization goals, low‐dose regions, ITP complexity, and clinical acceptability) by a blinded review panel. Additionally, the plans were mathematically rated based on plan indices (critical structure and target doses, conformity, monitor units, normal tissue complication probability, and treatment time) and compared to the human rankings. The treatment plans and the reviewers' rankings varied substantially among the participating centers. The average mean overall rank was 2.4 (1.2‐4.0) and 8/10 plans were rated excellent in at least one category by at least one reviewer. The mathematical rankings agreed with the mean overall human rankings in 9/10 cases pointing toward the possibility for sole mathematical plan quality comparison. The final rankings revealed that a plan with a well‐balanced trade‐off among all planning objectives was preferred for treatment by most participants, reviewers, and the mathematical ranking system. Furthermore, this plan was generated with simple planning techniques. Our multi‐institutional planning study found wide variability in ITP approaches for spinal robotic radiosurgery. The participants', reviewers', and mathematical match on preferable treatment plans and ITP techniques indicate that agreement on treatment planning and plan quality can be reached for spinal robotic radiosurgery.

PACS number(s): 87.55.de

A dose-volume histogram based decision-support system for dosimetric comparison of radiotherapy treatment plans

Background

The choice of any radiotherapy treatment plan is usually made after the evaluation of a few preliminary isodose distributions obtained from different beam configurations. Despite considerable advances in planning techniques, such final decision remains a challenging task that would greatly benefit from efficient and reliable assessment tools.

Methods

For any dosimetric plan considered, data on dose-volume histograms supplied by treatment planning systems are used to provide estimates on planning target coverage as well as on sparing of organs at risk and the remaining healthy tissue. These partial metrics are then combined into a dose distribution index (DDI), which provides a unified, easy-to-read score for each competing radiotherapy plan. To assess the performance of the proposed scoring system, DDI figures for fifty brain cancer patients were retrospectively evaluated. Patients were divided in three groups depending on tumor location and malignancy. For each patient, three tentative plans were designed and recorded during planning, one of which was eventually selected for treatment. We thus were able to compare the plans with better DDI scores and those actually delivered.

Results

When planning target coverage and organs at risk sparing are considered as equally important, the tentative plan with the highest DDI score is shown to coincide with that actually delivered in 32 of the 50 patients considered. In 15 (respectively 3) of the remaining 18 cases, the plan with highest DDI value still coincides with that actually selected, provided that organs at risk sparing is given higher priority (respectively, lower priority) than target coverage.

Conclusions

DDI provides a straightforward and non-subjective tool for dosimetric comparison of tentative radiotherapy plans. In particular, DDI readily quantifies differences among competing plans with similar-looking dose-volume histograms and can be easily implemented for any tumor type and localization, irrespective of the planning system and irradiation technique considered. Moreover, DDI permits to estimate the dosimetry impact of different priorities being assigned to sparing of organs at risk or to better target coverage.

A method to improve dose gradient for robotic radiosurgery

For targets with substantial volume, collimators of relatively large size are usually selected to minimize the treatment time in robotic radiosurgery. Their large penumbrae may adversely affect the dose gradient around the target. In this study, we implement and evaluate an inner‐shell planning method to increase the dose gradient and reduce dose to normal tissues. Ten patients previously treated with CyberKnife M6 system were randomly selected with the only criterion being that PTV be larger than 2 cm3. A new plan was generated for each patient in which the PTV was split into two regions: a 5 mm inner shell and a core, and a 7.5 mm Iris collimator was exclusively applied to the shell, with other appropriate collimators applied to the core depending on its size. The optimization objective, functions, and constraints were the same as in the corresponding clinical plan. The results were analyzed for V12 Gy, V9 Gy, V5 Gy, and gradient index (GI). Volume reduction was found for the inner‐shell method at all studied dose levels as compared to the clinical plans. The absolute dose‐volume reduction ranged from 0.05 cm3 to 18.5 cm3 with a mean of 5.6 cm3 for 12 Gy, from 0.2 cm3 to 38.1 cm3 with a mean of 9.8 cm3 for 9 Gy, and from 1.5 cm3 to 115.7 cm3 with a mean of 24.8 cm3 for 5 Gy, respectively. The GI reduction ranged from 3.2% to 23.6%, with a mean of 12.6%. Paired t‐test for GI has a p‐value of 0.0014. The range for treatment time increase is from ‐3 min to 20 min, with a mean of 7.0 min. We conclude that irradiating the PTV periphery exclusively with the 7.5 mm Iris collimator, rather than applying mixed collimators to the whole PTV, can substantially improve the dose gradient, while maintaining good coverage, conformity, and reasonable treatment time.

PACS number: 87.55.de

Comparison of radiation dose spillage from the Gamma Knife Perfexion with that from volumetric modulated arc radiosurgery during treatment of multiple brain metastases in a single fraction

Object

The objective of this study was to examine radiation dose distributions created by 2 competing radiosurgery modalities for treating multiple brain metastases: single-isocenter volumetric modulated arc radiosurgery (VMAS) and Gamma Knife Perfexion (GKP). In addition, the effectiveness of multiple radiosurgery quality metrics was evaluated and compared between these advanced treatment modalities.

Methods

Seven anonymized MRI data sets, each showing 2–5 metastases, were used to create plans on each system. The GammaPlan (version 10.1) program was used for planning of GKP. A neurosurgeon contoured the volumes to be treated, and no planning target volume expansion was used. A prescription dose coverage of ≥ 99% was achieved for each tumor volume. The Philips Pinnacle (version 9.2) program was used for planning of VMAS, using the SmartArc optimization algorithm for delivery on a Varian iX linear accelerator. Contours were transferred from GammaPlan, and again no planning target volume expansion was used. Between 2 and 5 arcs with table angles of 90°–270° were used. Again, a V100% of ≥ 99% was achieved for each tumor volume. After planning, the MRI scans, tumor volumes, and dose information from each plan were exported according to the Digital Imaging and Communications in Medicine standard to the VelocityAI program for analysis. Brain dose-volume histograms (DVHs) for normal brain tissues were generated, and the volume of these tissues receiving 20%–90% of the prescription dose was tabulated. Finally, the prescription isodose to tumor volume ratio (PITV; Shaw et al., 1993), conformity index (CI; Paddick, 2000), gradient index (GI, Paddick and Lippitz, 2006), and conformity/gradient index (CGI, Wagner et al. 2003) were calculated for each plan. Both the PITV and CI have ideal values of 1, while the GI and CGI have ideal values of lowest and highest achievable, respectively.

Results

The DVHs consistently showed that with VMAS a higher amount of normal brain tissues received each dose level than with GKP. These increases were largest for lower isodose levels, with the volumes of normal brain that received 20%–50% and 60%–90% of the prescription dose showing average increases of 403% and 227%, respectively. Prescription isodose conformality showed only minor differences between the 2 modalities. Radiosurgery quality metrics including measures of the dose gradient (GI and CGI) indicated that the GKP plan was superior in each case, with respective average GI and CGI values of 3.04 and 57.75 for GKP and of 10.22 and 10.85 for VMAS. Metrics evaluating prescription isodose conformality alone differed only slightly between the modalities. Average respective PITV and CI values were 2.13 and 0.53 for GKP and 2.27 and 0.51 for VMAS.

Conclusions

Stereotactic radiosurgery plans for the treatment of multiple metastases with VMAS delivered significantly more dose to the normal brain tissues than plans for GKP. Radiosurgery quality metrics including a measure of the dose gradient are better suited to providing contrast between modern radiosurgery treatment platforms.

New conformity indices based on the calculation of distances between the target volume and the volume of reference isodose

OBJECTIVE

To present conformity indices (CIs) based on the distance differences between the target volume (TV) and the volume of reference isodose (VRI).

METHODS

The points on the three-dimensional surfaces of the TV and the VRI were generated. Then, the averaged distances between the points on the TV and the VRI were calculated (CIdistance). The performance of the presented CIs were evaluated by analysing six situations, which were a perfect match, an expansion and a reduction of the distance from the centroid to the VRI compared with the distance from the centroid to the TV by 10%, a lateral shift of the VRI by 3 cm, a rotation of the VRI by 45° and a spherical-shaped VRI having the same volume as the TV. The presented CIs were applied to the clinical prostate and head and neck (H&N) plans.

RESULTS

For the perfect match, CIdistance was 0 with 0 as the standard deviation (SD). When expanding and reducing, CIdistance was 10 and -10 with SDs <1.3, respectively. With shifting and rotating of the VRI, the CIdistance was almost 0 with SDs >11. The average value of the CIdistance in the prostate and H&N plans was 0.13 ± 7.44 and 6.04 ± 23.27, respectively.

CONCLUSION

The performance of the CIdistance was equal or better than those of the conventional CIs.

ADVANCES IN KNOWLEDGE

The evaluation of target conformity by the distances between the surface of the TV and the VRI could be more accurate than evaluation with volume information.

Selecting radiotherapy dose distributions by means of constrained optimization problems

The main steps in planning radiotherapy consist in selecting for any patient diagnosed with a solid tumor (i) a prescribed radiation dose on the tumor, (ii) bounds on the radiation side effects on nearby organs at risk and (iii) a fractionation scheme specifying the number and frequency of therapeutic sessions during treatment. The goal of any radiotherapy treatment is to deliver on the tumor a radiation dose as close as possible to that selected in (i), while at the same time conforming to the constraints prescribed in (ii). To this day, considerable uncertainties remain concerning the best manner in which such issues should be addressed. In particular, the choice of a prescription radiation dose is mostly based on clinical experience accumulated on the particular type of tumor considered, without any direct reference to quantitative radiobiological assessment. Interestingly, mathematical models for the effect of radiation on biological matter have existed for quite some time, and are widely acknowledged by clinicians. However, the difficulty to obtain accurate in vivo measurements of the radiobiological parameters involved has severely restricted their direct application in current clinical practice.In this work, we first propose a mathematical model to select radiation dose distributions as solutions (minimizers) of suitable variational problems, under the assumption that key radiobiological parameters for tumors and organs at risk involved are known. Second, by analyzing the dependence of such solutions on the parameters involved, we then discuss the manner in which the use of those minimizers can improve current decision-making processes to select clinical dosimetries when (as is generally the case) only partial information on model radiosensitivity parameters is available. A comparison of the proposed radiation dose distributions with those actually delivered in a number of clinical cases strongly suggests that solutions of our mathematical model can be instrumental in deriving good quality tests to select radiotherapy treatment plans in rather general situations.