Stereotactic radiotherapy of intracranial tumors: a comparison of intensity-modulated radiotherapy and dynamic conformal arc

SN, Tiwari R, Narayanan GS, Vishwanathan B. The Optimal Choice of Technique for Stereotactic Radiosurgery—A LINAC-Based Dosimetric Study between DCA, DCA-SSO, DCA-SSO-VDR, and VMAT

Introduction Advanced radiation therapy delivery techniques require greater understanding of various planning sequences and methods. The aim of this study is to determine a class solution that finds the best possible technique to deliver for stereotactic radiosurgery between dynamic conformal arc (DCA) techniques using various options such as DCA, DCA + SSO (segment shape optimization), and DCA + SSO + VDR (variable dose rate) using noncoplanar beam arrangement and volumetric modulated arc therapy (VMAT) using coplanar beams.

Materials and Methods In this dosimetric study, 11 brain cases were retrospectively planned for various techniques and analyzed for the Paddick conformity index (CI), Radiation Therapy Oncology Group homogeneity index (HI), Paddick gradient index (GI), treatment time in terms of monitor units (MU) and normal brain dose (V12Gy). The paired t-test was performed to know the statistical significance between the techniques.

Results In terms of CI, GI, and control of the normal brain dose, the VMAT plan was superior to other techniques. But, HI was found to be better with DCA. Above all, VMAT delivered higher MU than any other technique. The p-values between DCA + SSO and DCA, DCA + SSO + VDR and DCA + SSO, and VMAT and DCA + SSO + VDR are as follows: CI: 0.0004, 0.015, and 0.03; GI: 0.03, 0.33, and 0.29; HI: 0.008, 0.04, and 0.06; V12 Gy of normal brain: 0.1, 0.01, 0.38. VMAT requires approximately 41 ± 17% more MU than DCA + SSO + VDR.

Conclusion VMAT using coplanar beams is preferable among all the techniques, considering the dosimetric parameters studied. If VMAT is not available in the facility, DCA + SSO + VDR technique using non coplanar beams can be used to deliver SRS treatment.

Radiotherapy for the treatment of pituitary adenomas: A dosimetric comparison of three planning techniques

AIM

Our goal was to compare conformal 3D (C3D) radiotherapy (RT), modulated intensity RT (IMRT), and volumetric modulated arc therapy (VMAT) planning techniques in treating pituitary adenomas.

BACKGROUND

RT is important for managing pituitary adenomas. Treatment planning advances allow for higher radiation dosing with less risk of affecting organs at risk (OAR).

MATERIALS AND METHODS

We conducted a 5-year retrospective review of patients with pituitary adenoma treated with external beam radiation therapy (C3D with flattening filter, flattening filter-free [FFF], IMRT, and VMAT). We compared dose-volume histogram data. For OARs, we recorded D2%, maximum, and mean doses. For planning target volume (PTV), we registered V95%, V107%, D95%, D98%, D50%, D2%, minimum dose, conformity index (CI), and homogeneity index (HI).

RESULTS

Fifty-eight patients with pituitary adenoma were included. Target-volume coverage was acceptable for all techniques. The HI values were 0.06, IMRT; 0.07, VMAT; 0.08, C3D; and 0.09, C3D FFF (p < 0.0001). VMAT and IMRT provided the best target volume conformity (CI, 0.64 and 0.74, respectively; p < 0.0001). VMAT yielded the lowest doses to the optic pathway, lens, and cochlea. The position of the neck in extreme flexion showed that it helps in planning mainly with VMAT by allowing only one arc to be used and achieving the desired conformity, decreasing the treatment time, while allowing greater protection to the organs of risk using C3D, C3DFFF.

CONCLUSIONS

Our results confirmed that EBRT in pituitary adenomas using IMRT, VMAT, C3D, C3FFF provide adequate coverage to the target. VMAT with a single arc or incomplete arc had a better compliance with desired dosimetric goals, such as target coverage and normal structures dose constraints, as well as shorter treatment time. Neck extreme flexion may have benefits in treatment planning for better preservation of organs at risk. C3D with extreme neck flexion is an appropriate treatment option when other treatment techniques are not available.

A dosimetric comparison of linac-based stereotactic fractionated radiotherapy techniques for pituitary adenoma and craniopharyngioma

Aim

To compare the dosimetric outcomes of linear accelerator-based stereotactic radiotherapy (SRT) techniques—static conformal field (SCF), static conformal arc (SCA) and dynamic conformal arc (DCA), for treating pituitary adenoma and craniopharyngioma.

Materials and methods

Computer image sets of 20 patients with pituitary adenoma or craniopharyngioma and treated with post-operative SRT were selected for this study. For each dataset, three SRT plans, with SCF, SCA and DCA techniques were generated using Brain LAB, iPlan RT V.4.5.3, TPS software. The conformity index (CI), homogeneity index (HI), quality of coverage of the target, dose–volume histograms for the target and organs at risk (OARs) and the time taken to deliver treatment was compared across three sets of plan.

Results

There were 12 patients with pituitary adenoma and eight with craniopharyngioma. The CI and HI were comparable across three techniques. The quality of coverage was superior in DCA technique. OARs were better spared in SCF and DCA techniques. Time taken to deliver treatment was least in SCF technique.

Conclusions

The linac-based SRT techniques SCF, SCA and DCA are efficient in delivering highly conformal and homogenous dose to the target in pituitary adenoma and craniopharyngioma. Among these three techniques, SCF and DCA had acceptable quality of coverage. The dose received by OARs was least in the SCF technique.

Evaluation of a magnetic resonance guided linear accelerator for stereotactic radiosurgery treatment

INTRODUCTION

The purpose of this study was to investigate the systematic localization accuracy, treatment planning capability, and delivery accuracy of an integrated magnetic resonance imaging guided Linear Accelerator (MR-Linac) platform for stereotactic radiosurgery.

MATERIALS AND METHODS

The phantom for the end-to-end test comprises three different compartments: a rectangular MR/CT target phantom, a Winston-Lutz cube, and a rectangular MR/CT isocenter phantom. Hidden target tests were performed at gantry angles of 0, 90, 180, and 270 degrees to quantify the systematic accuracy. Five patient plans with a total of eleven lesions were used to evaluate the dosimetric accuracy. Single-isocenter IMRT treatment plans using 10-15 coplanar beams were generated to treat the multiple metastases.

RESULTS

The end-to-end localization accuracy of the system was 1.0 ± 0.1 mm. The conformity index, homogeneity index and gradient index of the plans were 1.26 ± 0.22, 1.22 ± 0.10, and 5.38 ± 1.44, respectively. The average absolute point dose difference between measured and calculated dose was 1.64 ± 1.90%, and the mean percentage of points passing the 3%/1 mm gamma criteria was 96.87%.

CONCLUSIONS

Our experience demonstrates that excellent plan quality and delivery accuracy was achievable on the MR-Linac for treating multiple brain metastases with a single isocenter.

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.

Non-coplanar volumetric-modulated arc therapy (VMAT) for craniopharyngiomas reduces radiation doses to the bilateral hippocampus: a planning study comparing dynamic conformal arc therapy, coplanar VMAT, and non-coplanar VMAT

Background

Recent studies suggest that radiation-induced injuries to the hippocampus play important roles in compromising neurocognitive functioning for patients with brain tumors and it could be important to spare the hippocampus using modern planning methods for patients with craniopharyngiomas. As bilateral hippocampus are located on the same level as the planning target volume (PTV) in patients with craniopharyngioma, it seems possible to reduce doses to hippocampus using non-coplanar beams. While the use of non-coplanar beams in volumetric-modulated arc therapy (VMAT) of malignant intracranial tumors has recently been reported, no dosimetric comparison has yet been made between VMAT using non-coplanar arcs (ncVMAT) and VMAT employing only coplanar arcs (coVMAT) among patients with craniopharyngiomas. We performed a planning study comparing dose distributions to the PTV, hippocampus, and other organs at risk (OAR) of dynamic conformal arc therapy (DCAT), coVMAT, and ncVMAT.

Methods

DCAT, coVMAT, and ncVMAT plans were created for 10 patients with craniopharyngiomas. The prescription dose was 52.2 Gy in 29 fractions, and 99 % of each PTV was covered by 90 % of the prescribed dose. The maximum dose was held below 107 % of the prescribed dose. CoVMAT and ncVMAT plans were formulated to satisfy the following criteria: the doses to the hippocampus were minimized, and the doses to the OAR were similar to or lower than those of DCAT.

Results

The mean equivalent doses in 2-Gy fractions to 40 % of the volumes of the bilateral hippocampus [EQD₂(40%_hippos)] were 15.4/10.8/6.5 Gy for DCAT/coVMAT/ncVMAT, respectively. The EQD₂(40%_hippos) for ncVMAT were <7.3 Gy, which is the threshold predicting cognitive impairment, as defined by Gondi et al.. The mean doses to normal brain tissue and the conformity indices were similar for the three plans, and the homogeneity indices were significantly better for coVMAT and ncVMAT compared with DCAT.

Conclusions

NcVMAT is more appropriate than DCAT and coVMAT for patients with craniopharyngiomas. NcVMAT significantly reduces radiation doses to the bilateral hippocampus (to 50 % that of the DCAT) without increasing the doses to normal brain tissue and other OAR.

Electronic supplementary material

The online version of this article (doi:10.1186/s13014-016-0659-x) contains supplementary material, which is available to authorized users.

Fractionated stereotactic radiotherapy of benign skull-base tumors: a dosimetric comparison of volumetric modulated arc therapy with Rapidarc® versus non-coplanar dynamic arcs

Background

Benign tumors of the skull base are a challenge when delivering radiotherapy. An appropriate choice of radiation technique may significantly improve the patient’s outcomes. Our study aimed to compare the dosimetric results of fractionated stereotactic radiotherapy between non-coplanar dynamic arcs and coplanar volumetric modulated arctherapy (Rapidarc®).

Methods

Thirteen patients treated with Novalis TX® were analysed: six vestibular schwannomas, four pituitary adenomas and three meningioma. Two treatment plans were created for each case: dynamic arcs (4–5 non coplanar arcs) and Rapidarc® (2 coplanar arcs). All tumors were >3 cm and accessible to both techniques. Patients had a stereotactic facemask (Brainlab) and were daily repositioned by Exactrac®. GTV and CTV were contoured according to tumor type. A 1-mm margin was added to the CTV to obtain PTV. Radiation doses were 52.2–54 Gy, using 1.8 Gy per fraction. Treatment time was faster with Rapidarc®.

Results

The mean PTV V95 % was 98.8 for Rapidarc® and 95.9 % for DA (p = 0.09). Homogeneity index was better with Rapidarc®: 0.06 vs. 0.09 (p = 0.01). Higher conformity index values were obtained with Rapidarc®: 75.2 vs. 67.9 % (p = 0.04). The volume of healthy brain that received a high dose (V90 %) was 0.7 % using Rapidarc® vs. 1.4 % with dynamic arcs (p = 0.05). Rapidarc® and dynamic arcs gave, respectively, a mean D40 % of 10.5 vs. 18.1 Gy (p = 0.005) for the hippocampus and a Dmean of 25.4 vs. 35.3 Gy (p = 0.008) for the ipsilateral cochlea. Low-dose delivery with Rapidarc® and dynamic arcs were, respectively, 184 vs. 166 cm³ for V20 Gy (p = 0.14) and 1265 vs. 1056 cm³ for V5 Gy (p = 0.67).

Conclusions

Fractionated stereotactic radiotherapy using Rapidarc® for large benign tumors of the skull base provided target volume coverage that was at least equal to that of dynamics arcs, with better conformity and homogeneity and faster treatment time. Rapidarc® also offered better sparing of the ipsilateral cochlea and hippocampus. Low-dose delivery were similar between both techniques.

Estimation of patient setup uncertainty using BrainLAB Exatrac X-Ray 6D system in image-guided radiotherapy

The purpose of this study was to evaluate setup uncertainties for brain sites with ExacTrac X‐Ray 6D system and to provide optimal margin guidelines. Fifteen patients with brain tumor were included in this study. Two X‐ray images with ExacTrac X‐Ray 6D system were used to verify patient position and tumor target localization before each treatment. The 6D fusion software first generates various sets of DRRs with position variations in both three translational and three rotational directions (six degrees of freedom) for the CT images. Setup variations (translation and rotation) after correction were recorded and corrected before treatment. The 3D deviations are expressed as mean±standard deviation. The random error (Σ(σi)), systematic error (μi), and group systematic error (M(μi)) for the different X‐ray were calculated using the definitions of van Herk.⁽¹⁾ Mean setup errors were calculated from X‐ray images acquired after all fractions. There is moderate patient‐to‐patient variation in the vertical direction and small variations in systematic errors and magnitudes of random errors are smaller. The global systematic errors were measured to be less than 2.0 mm in each direction. Random component of all patients are smaller ranging from 0.1–0.3 mm small. The safety margin (SM) to the lateral, is 0.5 mm and 2.6 mm for van Herk⁽¹⁾ and Stroom et al.,⁽²⁾ respectively, craniocaudal axis is 1.5 mm and 3.4 mm, respectively, and with respect to the antero–posterior axis, 2.3 mm and 3.9 mm. Daily X‐ray imaging is essential to compare and assess the accuracy of treatment delivery to different anatomical locations.

PACS number: 87.55.D

Optimal set of grid size and angular increment for practical dose calculation using the dynamic conformal arc technique: a systematic evaluation of the dosimetric effects in lung stereotactic body radiation therapy

PURPOSE

To recommend the optimal plan parameter set of grid size and angular increment for dose calculations in treatment planning for lung stereotactic body radiation therapy (SBRT) using dynamic conformal arc therapy (DCAT) considering both accuracy and computational efficiency.

MATERIALS AND METHODS

Dose variations with varying grid sizes (2, 3, and 4 mm) and angular increments (2°, 4°, 6°, and 10°) were analyzed in a thorax phantom for 3 spherical target volumes and in 9 patient cases. A 2-mm grid size and 2° angular increment are assumed sufficient to serve as reference values. The dosimetric effect was evaluated using dose-volume histograms, monitor units (MUs), and dose to organs at risk (OARs) for a definite volume corresponding to the dose-volume constraint in lung SBRT. The times required for dose calculations using each parameter set were compared for clinical practicality.

RESULTS

Larger grid sizes caused a dose increase to the structures and required higher MUs to achieve the target coverage. The discrete beam arrangements at each angular increment led to over- and under-estimated OARs doses due to the undulating dose distribution. When a 2° angular increment was used in both studies, a 4-mm grid size changed the dose variation by up to 3-4% (50 cGy) for the heart and the spinal cord, while a 3-mm grid size produced a dose difference of <1% (12 cGy) in all tested OARs. When a 3-mm grid size was employed, angular increments of 6° and 10° caused maximum dose variations of 3% (23 cGy) and 10% (61 cGy) in the spinal cord, respectively, while a 4° increment resulted in a dose difference of <1% (8 cGy) in all cases except for that of one patient. The 3-mm grid size and 4° angular increment enabled a 78% savings in computation time without making any critical sacrifices to dose accuracy.

CONCLUSIONS

A parameter set with a 3-mm grid size and a 4° angular increment is found to be appropriate for predicting patient dose distributions with a dose difference below 1% while reducing the computation time by more than half for lung SBRT using DCAT.

Negative margin technique - a novel planning strategy to improve dose conformation in SBRT using dynamic conformal arc delivery

The purpose of this study was to introduce a planning strategy for dynamic conformal arc therapy (DCAT), named negative margin technique (NMT), and evaluate its dosimetric gain in lung stereotactic body radiation therapy (SBRT). In DCAT, the field aperture is continuously conformed to the planning target volume (PTV) with an aperture margin (AM) to compensate for the penumbra effect with gantry rotation. It is a common belief the AM should be positive (or at least 'zero'). However, the radial penumbra width becomes significantly wider because of continuously overlapped beams in arc delivery. Therefore, we hypothesize if the 'negative margin' is applied in the radial direction, it would improve the PTV dose conformation while reducing normal tissue dose. For verification, trial plans were made using the NMT and compared with 'zero margin (ZM)' plans for five lung SBRT cases representing different situations depending on the location of the PTV and organs at risk. All plans met 95% PTV coverage with the prescription dose and spared the spinal cord below the tolerance. Two conventional conformation indices (the ratio of prescription isodose volume to the PTV (CI100) and the ratio of 50% prescription isodose volume to the PTV (CI50)) and a modified conformation index were investigated. The maximum dose at 2 cm from the PTV (Dmax-2cm) and the percent of lung volume receiving 20 Gy (V20) were also evaluated. Another planning simulation was performed with a total of ten randomly selected lung SBRT cases to mimic actual practice. In this simulation, optimization with ZM was first performed and further optimization using the NMT was processed for cases that could not meet a goal of CI100 = 1.2 with the ZM optimization. In all cases, both the CI100 and CI50 values were significantly reduced (overall, 9.4% ± 4.1% and 5.9%± 3.1% for CI100 and CI50, respectively). The modified conformation index values also showed similar improvement (overall, 10.1% ± 5.7% increase). Reduction of Dmax-2cm was also observed in all cases (4.5% ± 2.2%). V20 values decreased in all cases but one (5.7% ± 3.9%, excluding the increased case). In the random group simulation, it was possible to achieve the goal with just one NMT trial for five out of six cases that did not meet the goal in the ZM optimization. Interestingly, however, one case needed as many as six iterations to get the CI100 = 1.2 goal. The NMT turned out to be an effective planning strategy that could bring significant improvement of dose conformation. The NMT can be easily implemented in most clinics with no prerequisite.

Robotic radiosurgery versus micro-multileaf collimator: a dosimetric comparison for large or critically located arteriovenous malformations

Background

Stereotactic irradiation of large or critically located arteriovenous malformations (AVMs) is a special challenge for clinicians and radiation physicists. To date, no comprehensive comparison of two linac-based radiosurgery systems used for hypofractionated radiotherapy of large AVMs was published. The aim of the study was to compare dose distributions between CyberKnife (CK) system and linac with a micro-multileaf collimator (L-mMLC) in high-grade or critically located cerebral AVMs.

Methods

Two sets of plans made for 15 different patients with at least 95% target coverage were selected for comparisons. Conformity (CI), homogeneity (HI) and gradient score (GSI) indices, conformity index proposed by Lomax (CIL), conformation number (CN), quality of coverage (Q), volumes of brain receiving 12,10,8,6,4, and 2 Gy, minimum and maximum doses for critical structures in both treatment planning systems (TPS) were compared. Finally, the number of monitor units needed to deliver the prescribed dose was compared.

Results

The mean minimum doses in the target volume were 93.3% (CK) and 90.7% (L-mMLC),p=n.s, maximum: 119.7 and 110%, respectively (p=0.004). The mean CI was 1.46 and 1.86, HI: 1.2, and 1.11, CIL 0.7, and 0.6, CN: 0.68 and 0.58 for CK and mMLC, respectively (p<0.05). The values of GSI and Q were not significantly different. The volumes of the brain receiving low doses (4 Gy and 2 Gy) were significantly lower in the CK system. The number of monitor units necessary to deliver the prescribed dose was significantly greater in case of the CK system.

Conclusions

Better conformity can favor the CK system for treatment of large AVMs at the cost of higher maximum doses and worse homogeneity. L-mMLC is superior when shorter treatment time is required. Neither system can assure satisfying dose gradients outside large targets surrounded by numerous critical structures.

Coplanar versus noncoplanar intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) treatment planning for fronto-temporal high-grade glioma

The purpose of this study was to compare dosimetric and radiobiological parameters of treatment plans using coplanar and noncoplanar beam arrangements in patients with fronto‐temporal high‐grade glioma (HGG) generated for intensity‐modulated radiotherapy (IMRT) or volumetric‐modulated arc therapy (VMAT). Ten cases of HGG overlapping the optic apparatus were selected. Four separate plans were created for each case: coplanar IMRT, noncoplanar IMRT (ncIMRT), VMAT, and noncoplanar VMAT (ncVMAT). The prescription dose was 60 Gy in 30 fractions. Dose‐volume histograms and equivalent uniform doses (EUD) for planning target volumes (PTVs) and organs at risk (OARs) were generated. The four techniques resulted in comparable mean, minimum, maximum PTV doses, and PTV EUDs (p≥0.33). The mean PTV dose and EUD averaged for all techniques were 59.98 Gy (Standard Deviation (SD)±0.15) and 59.86 Gy (SD±0.27). Noncoplanar IMRT significantly reduced contralateral anterior globe EUDs (6.7 Gy versus 8.2 Gy, p=0.05), while both ncIMRT and ncVMAT reduced contralateral retina EUDs (16 Gy versus 18.8 Gy, p=0.03). Noncoplanar techniques resulted in lower contralateral temporal lobe dose (22.2 Gy versus 24.7 Gy). Compared to IMRT, VMAT techniques required fewer monitor units (755 vs. 478, p≤0.001) but longer optimization times. Treatment delivery times were 6.1 and 10.5 minutes for coplanar and ncIMRT versus 2.9 and 5.0 minutes for coplanar and ncVMAT. In this study, all techniques achieved comparable target coverage. Superior sparing of contralateral optic structures was seen with ncIMRT. The VMAT techniques reduced treatment delivery duration but prolonged plan optimization times, compared to IMRT techniques. Technique selection should be individualized, based on patient‐specific clinical and dosimetric parameters.

PACS number: 87

Dosimetric comparison of 2.5 mm vs. 3.0 mm leaf width micro-multileaf collimator-based treatment systems for intracranial stereotactic radiosurgery using dynamic conformal arcs: implications for treatment planning

PURPOSE

The objective of our study was to explore any significant dosimetric differences between different leaf width (3.0 mm vs. 2.5 mm) micro-multileaf collimator (mMLC)-based treatment systems for intracranial stereotactic radiosurgery using dynamic conformal arcs (DCAs).

MATERIALS AND METHODS

The systems included a 3 mm leaf width mMLC (m3) mounted on a nondedicated linac, and the Novalis Tx system with an integrated 2.5 mm width mMLC (HD120). Thirty plans for brain metastases were replanned for both systems using a uniform method for target definition and treatment planning for baseline comparison.

RESULTS

The target coverage values for the 80% isodose surface (IDS) and the D95 values in the HD120 plans were significantly lower than those for the m3 plans. The ratios of lower isodose volumes to the target for the HD120 were smaller than those for the m3. When a 1 mm leaf margin was added to the HD120 plans, these differences were reversed, but statistically significant differences were still observed.

CONCLUSION

Significant dosimetric differences were observed between these systems. Different planning methods are required for the two systems to attain similar target coverage values with selected IDS, which can be achieved by adjusting the leaf margin with 0.1 mm increments or isocenter dose settings.

The relation between various conformity indices and the influence of the target coverage difference in prescription isodose surface on these values in intracranial stereotactic radiosurgery

OBJECTIVE

The purpose of this study was to describe the relation between various frequently used conformity indices (CIs) and to examine the influence of the target coverage (TC) difference in prescription isodose surface (IDS) on these CI values in dynamic conformal arc (DCA) plans.

METHOD

73 plans for simple-shaped brain metastases that were previously characterised for dose distribution with regard to the effect of the target volume (TV) and the depth from the skin surface were reviewed. Three different-definition CI values for each TV were calculated at the 80% IDS, and at D99, D95, D90 and D85, considering the interplanner variability in the TC values for the prescription IDS.

RESULTS

The CI used as the Radiation Therapy Oncology Group criterion showed nearly perfect values at D90. The CI defined in the BrainSCAN (BrainLAB AG, Feldkirchen, Germany) treatment planning system (CI(BS)) denoted lower (superior) values as the TC of the reference IDS decreased. Nakamura's CI (NCI) had lower variability but demonstrated lower (superior) values at D95. NCI showed the most stringent (higher) values at an 80% IDS, but the differences between the plans were less distinct with NCI.

CONCLUSION

The TC difference in IDS chosen for dose prescription or evaluation significantly led to CI value variability in a definition-dependent manner, even when NCI was applied. Definition of the reference IDS at a specific TC value according to clinical situation would reduce the CI value variability to a minimum and would make the CI(BS) sufficient for the objective metric with a perfect value of 1.

A hybrid algorithm for instant optimization of beam weights in anatomy-based intensity modulated radiotherapy: A performance evaluation study

The study aims to introduce a hybrid optimization algorithm for anatomy-based intensity modulated radiotherapy (AB-IMRT). Our proposal is that by integrating an exact optimization algorithm with a heuristic optimization algorithm, the advantages of both the algorithms can be combined, which will lead to an efficient global optimizer solving the problem at a very fast rate. Our hybrid approach combines Gaussian elimination algorithm (exact optimizer) with fast simulated annealing algorithm (a heuristic global optimizer) for the optimization of beam weights in AB-IMRT. The algorithm has been implemented using MATLAB software. The optimization efficiency of the hybrid algorithm is clarified by (i) analysis of the numerical characteristics of the algorithm and (ii) analysis of the clinical capabilities of the algorithm. The numerical and clinical characteristics of the hybrid algorithm are compared with Gaussian elimination method (GEM) and fast simulated annealing (FSA). The numerical characteristics include convergence, consistency, number of iterations and overall optimization speed, which were analyzed for the respective cases of 8 patients. The clinical capabilities of the hybrid algorithm are demonstrated in cases of (a) prostate and (b) brain. The analyses reveal that (i) the convergence speed of the hybrid algorithm is approximately three times higher than that of FSA algorithm; (ii) the convergence (percentage reduction in the cost function) in hybrid algorithm is about 20% improved as compared to that in GEM algorithm; (iii) the hybrid algorithm is capable of producing relatively better treatment plans in terms of Conformity Index (CI) [~ 2% - 5% improvement] and Homogeneity Index (HI) [~ 4% - 10% improvement] as compared to GEM and FSA algorithms; (iv) the sparing of organs at risk in hybrid algorithm-based plans is better than that in GEM-based plans and comparable to that in FSA-based plans; and (v) the beam weights resulting from the hybrid algorithm are about 20% smoother than those obtained in GEM and FSA algorithms. In summary, the study demonstrates that hybrid algorithms can be effectively used for fast optimization of beam weights in AB-IMRT.

Intensity-modulated arc therapy for pediatric posterior fossa tumors

PURPOSE

To compare intensity-modulated arc therapy (IMAT) to noncoplanar intensity-modulated radiation therapy (IMRT) in the treatment of pediatric posterior fossa tumors.

METHODS AND MATERIALS

Nine pediatric patients with posterior fossa tumors, mean age 9 years (range, 6-15 years), treated using IMRT were chosen for this comparative planning study because of their tumor location. Each patient's treatment was replanned to receive 54 Gy to the planning target volume (PTV) using five different methods: eight-field noncoplanar IMRT, single coplanar IMAT, double coplanar IMAT, single noncoplanar IMAT, and double noncoplanar IMAT. For each method, the dose to 95% of the PTV was held constant, and the doses to surrounding critical structures were minimized. The different plans were compared based on conformity, total linear accelerator dose monitor units, and dose to surrounding normal tissues, including the entire body, whole brain, temporal lobes, brainstem, and cochleae.

RESULTS

The doses to the target and critical structures for the various IMAT methods were not statistically different in comparison with the noncoplanar IMRT plan, with the following exceptions: the cochlear doses were higher and whole brain dose was lower for coplanar IMAT plans; the cochleae and temporal lobe doses were lower and conformity increased for noncoplanar IMAT plans. The advantage of the noncoplanar IMAT plan was enhanced by doubling the treatment arc.

CONCLUSION

Noncoplanar IMAT results in superior treatment plans when compared to noncoplanar IMRT for the treatment of posterior fossa tumors. IMAT should be considered alongside IMRT when treatment of this site is indicated.

Characterisation of dose distribution in linear accelerator-based intracranial stereotactic radiosurgery with the dynamic conformal arc technique: consideration of the optimal method for dose prescription and evaluation

OBJECTIVES

The purpose of this study was to characterise dose distribution in linear accelerator-based intracranial stereotactic radiosurgery using the dynamic conformal arc technique, and to validate the pertinence of dose prescription to the specific percentage isodose surface (IDS).

METHODS

73 plans for brain metastases were reviewed and replanned with a uniform method for target definition and treatment planning.

RESULTS

In all cases except 1 the dose prescription to the 80% IDS satisfied the criteria of the standardised prescription IDS as previously proposed. However, both of the planning target volume (PTV) coverage values for the 80% and 90% IDSs and the PTV D99 and D95 (IDS receiving at least 99% or 95% of the PTV) were inconsistent and significantly increased as a function of the PTV size. The 80% IDS for a PTV of more than 5 cm(3) achieved adequate PTV coverage without a leaf margin. The dose conformity for 80% IDS gradually worsened as the PTV increased, whereas that for the PTV D99 or D95 improved as a function of the PTV size. The addition of a leaf margin attained 100% PTV coverage for 80% IDS, while leading to a poorer dose conformity.

CONCLUSION

The dose prescription to the specific percentage IDS does not necessarily guarantee consistent target coverage, D99 and D95, and desirable dose conformity in proportion to the target volume. The dose prescription and evaluation at the specific target coverage would therefore be preferable as an objective method in order to report the "marginal dose" and to clearly compare the planning parameters with those from other modalities.

Dose gradient analyses in Linac-based intracranial stereotactic radiosurgery using Paddick's gradient index: consideration of the optimal method for plan evaluation

The objective of our study was to describe the dose gradient characteristics of Linac-based stereotactic radiosurgery using Paddick's gradient index (GI) and to elucidate the factors influencing the GI value. Seventy-three plans for brain metastases using the dynamic conformal arcs were reviewed. The GI values were calculated at the 80% and 90% isodose surfaces (IDSs) and at the different target coverage IDSs (D99, D95, D90, and D85). The GI values significantly decreased as the target coverage of the reference IDS increased (the percentage of the IDS decreased). There was a significant inverse correlation between the GI values and target volume. The plans generated with the addition of a 1-mm leaf margin had worse GI values both at the D99 and D95 relative to those without leaf margin. The number and arrangement of arcs also affected the GI value. The GI values are highly sensitive to (1) the IDS selection variability for dose prescription or evaluation, (2) the target volume, and (3) the planning method. To objectively compare the quality of dose gradient between rival plans, it would be preferable to employ the GI defined at the reference IDS indicating the specific target coverage (e.g., D95), irrespective of the intended marginal dose. The modified GI (mGI), defined in this study, substituting the denominator of the original GI with the target volume, would be useful to compensate for the false superior GI value in cases of target over-coverage with the reference IDS and to objectively evaluate the dose gradient outside the target boundary.

Hypofractionated stereotactic radiotherapy for brain metastases: a dosimetric and treatment efficiency comparison between volumetric modulated arc therapy and intensity modulated radiotherapy

A treatment planning comparison study was performed to evaluate the dosimetric characteristic and treatment efficiency of volumetric modulated arc therapy with step-and-shoot intensity modulated radiotherapy (IMRT) for the hypofractionated stereotactic radiotherapy (HFSRT) in patients with multiple brain metastases. CT datasets of 10 patients with two to four brain metastases were selected for the comparison. Three plans were generated for each case: seven-field step-and-shoot IMRT, single (RA1) and double (RA2) arcs with RapidArc technique (RA, Varian Medical System). The prescribed dose was 50 Gy in 10 fractions and plans were all normalized to the mean dose to the PTV. For PTV, plans aim to achieve at least 98% of PTV was covered with the 95% of prescription dose, at least 95% of PTV was encompassed by the prescription dose, and an over-dosage of 110% of the prescription dose was allowed to 5% volume of the PTV. The plans generated using three techniques were clinically acceptable. The target conformity and homogeneity were improved slightly with RA2 compared to IMRT and RA1. The Paddick CI was 0.868 (IMRT), 0.863 (RA1) and 0.895 (RA2), and HI was 7.7 (IMRT), 7.5 (RA1) and 6.5 (RA2), respectively. Compared with IMRT, the maximum dose in RA2 plans to the brainstem, left and right optic nerves, left and right lens was reduced by 1.6 Gy, 6 Gy, 3 Gy, 1.5 Gy, 1.3 Gy, respectively. The percentage of healthy tissue volume receiving 5 Gy was larger with RA1 (56.7%) and RA2 (57.1%) than with IMRT (52.9%), while the percentages of volume receiving 15 Gy and 20 Gy were smaller with RA1 (27.1%, 18.7%) and RA2 (25%, 16.3%) than with IMRT (28.8%, 19.1%). No significant difference was observed between RA1 and RA2. The mean number of MU per fraction of 5 Gy was 1944 +/- 374 (IMRT), 1199 +/- 173 (RA1) and 1387 +/- 186 (RA2), respectively. Compared with IMRT, the MUs were reduced by 36.8% and 27.2% with RA1 and RA2. The pure beam-on time needed per fraction was 6.5 +/- 1.2 min (IMRT), 1.25 min (RA1) and 2.5 min (RA2), respectively. The beam-on time for RA1 and RA2 was approximately 80% and 40% less compared to IMRT. In conclusion, RA, single arc or double arcs, is a feasible technique with highly conformal dose distribution for the HFSRT in patients with oligo brain metastases. Compared with IMRT, RA1 provides similar plan quality, while RA2 achieves slight improvements in PTV coverage and sparing of organs at risk. The treatment efficiency, using less monitor units and shorter treatment delivery time, is the most obvious advantage.

Impact of the high-definition multileaf collimator on linear accelerator-based intracranial stereotactic radiosurgery

OBJECTIVES

The impact of two multileaf collimator (MLC) systems for linear accelerator-based intracranial stereotactic radiosurgery (SRS) was assessed.

METHODS

68 lesions formed the basis of this study. 2.5 mm leaf width plans served as reference. Comparative plans, with identical planning parameters, were based on a 5 mm leaf width MLC system. Two collimation strategies, with collimation fixed at 0° or 90° and optimised per arc or beam, were also assessed. Dose computation was based on the pencil beam algorithm with allowance for tissue heterogeneity. Plan normalisation was such that 100% of the prescription dose covered 95% of the planning target volume. Plan evaluation was based on target coverage and normal tissue avoidance criteria.

RESULTS

The median conformity index difference between the MLC systems ranged between 0.8% and 14.2%; the 2.5 mm MLC exhibited better dose conformation. The median reduction of normal tissue exposed to ≥100%, ≥50% and ≥25% of the prescription dose ranged from 13.4% to 29.7%, favouring the 2.5 mm MLC system. Dose fall-off was steeper for the 2.5 mm MLC system with an overall median absolute difference ranging from 0.4 to 1.2 mm. The use of collimation optimisation resulted in a decrease in differences between the MLC systems. The results demonstrated the dosimetric merit of the 2.5 mm leaf width MLC system over the 5 mm leaf width system, albeit small, for the investigated range of intracranial SRS targets.

CONCLUSION

The clinical significance of these results warrants further investigation to determine whether the observed dosimetric advantages translate into outcome improvements.

Dosimetric comparison of helical tomotherapy and dynamic conformal arc therapy in stereotactic radiosurgery for vestibular schwannomas

The dosimetric results of stereotactic radiosurgery (SRS) for vestibular schwannoma (VS) performed using dynamic conformal arc therapy (DCAT) with the Novalis system and helical TomoTherapy (HT) were compared using plan quality indices. The HT plans were created for 10 consecutive patients with VS previously treated with SRS using the Novalis system. The dosimetric indices used to compare the techniques included the conformity index (CI) and homogeneity index (HI) for the planned target volume (PTV), the comprehensive quality index (CQI) for nine organs at risk (OARs), gradient score index (GSI) for the dose drop-off outside the PTV, and plan quality index (PQI), which was verified using the plan quality discerning power (PQDP) to incorporate 3 plan indices, to evaluate the rival plans. The PTV ranged from 0.27-19.99 cm(3) (median 3.39 cm(3)), with minimum required PTV prescribed doses of 10-16 Gy (median 12 Gy). Both systems satisfied the minimum required PTV prescription doses. HT conformed better to the PTV (CI: 1.51 ± 0.23 vs. 1.94 ± 0.34; p < 0.01), but had a worse drop-off outside the PTV (GSI: 40.3 ± 10.9 vs. 64.9 ± 13.6; p < 0.01) compared with DCAT. No significant difference in PTV homogeneity was observed (HI: 1.08 ± 0.03 vs. 1.09 ± 0.02; p = 0.20). HT had a significantly lower maximum dose in 4 OARs and significant lower mean dose in 1 OAR; by contrast, DCAT had a significantly lower maximum dose in 1 OAR and significant lower mean dose in 2 OARs, with the CQI of the 9 OARs = 0.92 ± 0.45. Plan analysis using PQI (HT 0.37 ± 0.12 vs. DCAT 0.65 ± 0.08; p < 0.01), and verified using the PQDP, confirmed the dosimetric advantage of HT. However, the HT system had a longer beam-on time (33.2 ± 7.4 vs. 4.6 ± 0.9 min; p < 0.01) and consumed more monitor units (16772 ± 3803 vs. 1776 ± 356.3; p < 0.01). HT had a better dose conformity and similar dose homogeneity but worse dose gradient than DCAT. Plan analysis confirmed the dosimetric advantage of HT, although not all indices revealed a better outcome for HT. Whether this dosimetric advantage translates into a clinical benefit deserves further investigation.