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

Non-coplanar Arc-Involved Beam Arrangement With Sufficient Arc Rotations Is Suitable for Volumetric-Modulated Arc-Based Radiosurgery for Single Brain Metastasis

Introduction In linac-based stereotactic radiosurgery (SRS) leveraging a multileaf collimator (MLC) for brain metastasis (BM), volumetric-modulated arcs (VMAs) enable the generation of a suitable dose distribution with efficient planning and delivery. However, the arc arrangement, including the number of arcs, allocation, and rotation ranges, varies substantially among devices and facilities. Some modalities allow coplanar arc(s) (CA(s)) or beam(s) alone, and some facilities only use them intentionally despite the availability of non-coplanar arcs (NCAs). The study was conducted to examine the significance of NCAs and the optimal arc rotation ranges in VMA-based SRS for a single BM. Materials and methods This was a planning study for the clinical scenario of a single BM, including 20 clinical cases with a gross tumor volume (GTV) of 0.72-44.30 cc. Three different arc arrangements were compared: 1) reciprocating double CA alone of each 360º rotation with different collimator angles of 0 and 90º, 2) one CA and two NCAs of each 120º rotation with the shortest beam path lengths to the irradiation isocenter (NCA_L), and 3) one CA of 360º rotation and two NCAs of each 180º rotation (NCA_F). The three arcs were allocated similarly to equally divide the cranial hemisphere with different collimator angles of 0, 45, and 90º. Three VMA-based SRS plans were generated for each GTV using a 5 mm leaf-width MLC with the identical optimization method that prioritized the steepness of dose gradient outside the GTV boundary without any constraints to the GTV internal dose. A prescribed dose was uniformly assigned to the GTV D V-0.01 cc, the minimum dose of GTV minus 0.01 cc. The GTV dose conformity, the steepness of dose gradients both outside and inside the GTV boundary, the degree of concentric lamellarity of the dose gradients, and the appropriateness of the dose attenuation margin outside the GTV boundary were evaluated using metrics appropriate for each. Results The arc arrangements including NCAs showed significantly steeper dose gradients both outside and inside the GTV boundary with smaller dose attenuation margins than the CAs alone, while NCAs showed no significant advantage on the GTV dose conformity. In the NCA-involved arc arrangements, the NCA_F was significantly superior to the NCA_L in terms of the GTV dose conformity, the steepness of dose gradient outside the GTV, the degree of concentric lamellarity of the dose gradients outside and inside the GTV boundary, and the appropriateness of dose attenuation margin. However, the NCA_F showed no significant advantage on the steepness of dose increase inside the GTV boundary over the NCA_L. The dose increase just inside the prescribed isodose surface to the GTV boundary was significantly steeper with the NCA_L than the NCA_F. Conclusions In VMA-based SRS for a single BM, an arc arrangement including NCAs is indispensable, and sufficient arc rotations are suitable for achieving a dose distribution that maximizes therapeutic efficacy and safety in comparison to limited ones which are appropriate for dynamic conformal arcs. Although VMA with CAs alone can provide a non-inferior GTV dose conformity to NCAs, CA(s) alone should be applied only to situations where shorter irradiation time is prioritized over efficacy and safety.

Consideration of Optimal Evaluation Metrics for Internal Gross Tumor Dose Relevant to Tumor Response in Multi-fraction Stereotactic Radiosurgery of Brain Metastasis

Introduction In stereotactic radiosurgery (SRS) for brain metastasis (BM), the target dose inhomogeneity remains highly variable among modalities, irradiation techniques, and facilities, which can affect tumor response during and after multi-fraction SRS. Volumetric-modulated arcs (VMAs) can provide a concentrically-layered steep dose increase inside a gross tumor volume (GTV) boundary compared to dynamic conformal arcs. This study was conducted to review the optimal evaluation method for the internal GTV doses relevant to maximal response and local control, specifically to examine the significance of the doses 2 mm and 4 mm inside the GTV boundary in VMA-based SRS. Materials and methods This was a planning study for the clinical scenario of a single BM and targeted 25 GTVs of >0.50 cc, including eight spherical models with diameters of 10-45 mm and 17 clinical BMs (GTV: 0.72-44.33 cc). SRS plans were generated for each GTV using VMA with a 5-mm leaf-width multileaf collimator and the optimization that prioritized the steepness of the dose gradient outside the GTV boundary without any internal dose constraints. The dose prescription and evaluation were based on the GTV D V-0.01 cc, a minimum dose of GTV minus 0.01 cc. Two planning systems were compared for the GTV - 2 mm and GTV - 4 mm structures that were generated by equally reducing 2 mm and 4 mm from the GTV surface. The D eIIVs, a minimum dose of the irradiated isodose volume equivalent to the GTV - 2 mm and GTV - 4 mm, were compared to other common metrics. Results The GTV - 2 mm and GTV - 4 mm volumes differed significantly between the systems. In the spherical GTVs, the irradiated isodose surfaces of GTV D 80% and D 50% corresponded to 0.4-1.6 mm (<2 mm) and 1.0-4.6 mm inside the GTV boundary, respectively. In the 25 GTVs, the GTV - 2 mm coverage with the D eIIV varied from 83.7% to 98.2% (95-98% in 68% of the cases), while the GTV coverage with the GTV - 2 mm D eIIV was 20.2-75.9%. In the 23 GTVs of ≥1.26 cc, the GTV coverage with the GTV - 4 mm D eIIV varied from 1.9% to 55.6% (<50% in 87% of the cases). No significant difference was observed between the GTV D 50% and the GTV - 2 mm D eIIV, while the GTV - 4 mm D eIIV was significantly higher than the GTV D 50%. No significant correlations were observed between the GTV D 50% and the D eIIVs of the GTV - 2 mm and GTV - 4 mm. Conclusions The doses 2 mm and 4 mm inside a GTV have low correlations with the GTV D 50% and may be more relevant to maximal response and local control for SRS of BM. The D eIIV instead of the minimum dose of a fixed % coverage (e.g. D 98%) is suitable for reporting the doses 2 mm and 4 mm inside the GTV boundary in terms of avoiding the over- or under-coverage, with consideration to substantial variability in minus margin addition functions among planning systems. In VMA-based SRS with a steep dose gradient, the doses 2-4 mm inside a GTV decrease significantly as the GTV increases, which can attenuate the excessive dose exposure to the surrounding brain in a large BM due to the GTV shrinkage during multi-fraction SRS.

Proposal of an Alternative Near-Minimum Isodose Surface DV-0.01 cc Equally Minimizing Gross Tumor Volume Below the Relevant Dose as the Basis for Dose Prescription and Evaluation of Stereotactic Radiosurgery for Brain Metastases

Introduction In stereotactic radiosurgery (SRS) for brain metastasis (BM), the prescribed dose is generally reported as a minimum dose to cover a specific percentage (e.g. D98%) of the gross tumor volume (GTV) with or without a margin or an unspecified intended marginal dose to the GTV boundary. In dose prescription to a margin-added planning target volume (PTV), the GTV marginal dose is likely variable and unclear. This study aimed to reveal major flaws of dose prescription to a fixed % coverage of a target volume (TV), such as GTV D98% or PTV D95%, and to propose an alternative. Materials and methods Seven quasi-spherical models with volumes ranging from 1.00 to 15.00 cc were assumed as GTVs. The GTVs and the volumes generated by adding isotropic 1- and 2-mm margins to the GTV boundaries (GTV + 1 and 2 mm) were used for SRS planning, dose prescription, and evaluation. Volumetric-modulated arcs with a 5-mm leaf-width multileaf collimator were used to optimize each SRS plan to ensure the steepest dose gradient outside each TV boundary. In dose prescription to the GTV D98%, 0.02-0.3 cc of the GTV is below the prescribed dose, and the volume increases with larger GTVs. The volume below the prescribed dose should be less than the equivalent of a 3-mm-diameter lesion, i.e. 0.01 cc. Therefore, DV-0.01 cc was defined as an alternative near-minimum dose for which the TV below a relevant dose is less than 0.01 cc. Four different dose prescriptions, including the GTV DV-0.01 cc, were compared using specific doses in 1, 3, and 5 fractions, equivalent to 80, 60, and 50 Gy, respectively, as biologically effective doses (BEDs) to the boundaries of GTV, GTV + 1 mm, and GTV + 2 mm, respectively. Results Dose prescription to the GTV DV-0.01 cc corresponds to 95.0, 98.0, and 99.0-99.93% coverages for the GTV of 0.20, 0.50, and 1.00-15.00 cc, respectively. The GTV DV-0.01 cc varied substantially and decreased significantly as the GTV increased in dose prescriptions to the GTV D98%, GTV + 1 mm D95%, and GTV + 2 mm D95%. The GTV + 2 mm DV-0.01 cc increased significantly as the GTV increased, except for the dose prescription to the GTV + 2 mm D95% with a decreasing tendency. When comparing BED-based specific dose prescriptions, dose prescription to the GTV DV-0.01 cc was optimal in terms of the following: 1) consistency of the near-minimum dose of GTV; 2) the highest BED at 2 mm outside the GTV, except for 1.00 cc GTV, and the rational increase with increasing GTV; and 3) the highest BED at 2 mm inside the GTV. In dose prescription with the BED of 80 Gy in 1 fraction and 5 fractions to the GTV DV-0.01 cc, the GTV limits were ≤1.40 and ≤8.46 cc, respectively, in order for the irradiated isodose volume not to exceed the proposed thresholds for minimizing the risk of brain radionecrosis. Conclusions Dose prescription to a fixed % coverage of a GTV with or without a margin leads to the substantially varied near-minimum dose at the GTV boundary, which significantly decreases with increasing GTV. Alternatively, GTV DV-0.01 cc with a variable coverage (D>95%) for >0.20 cc GTV and fixed D95% for ≤0.20 cc GTV is recommended as the basis for dose prescription and evaluation, along with supplemental evaluation of the marginal dose of the GTV plus a margin (e.g. GTV + 2 mm) to demonstrate the appropriateness of dose attenuation outside the GTV boundary.

An Extremely Inhomogeneous Gross Tumor Dose is Suitable for Volumetric Modulated Arc-Based Radiosurgery with a 5-mm Leaf-Width Multileaf Collimator for Single Brain Metastasis

Introduction Single or multi-fraction (mf) stereotactic radiosurgery (SRS) is an indispensable treatment option for brain metastases (BMs). The integration of volumetric modulated arc therapy (VMAT) into linac-based SRS is expected to further enhance efficacy and safety and to expand the indications for the challenging type of BMs. However, the optimal treatment design and relevant optimization method for volumetric modulated arc-based radiosurgery (VMARS) remain unestablished with substantial inter-institutional differences. Therefore, this study was conducted to determine the optimal dose distribution suitable for VMARS of BMs, especially regarding dose inhomogeneity of the gross tumor volume (GTV). The GTV boundary, not margin-added planning target volume, was regarded as a basis for planning optimization and dose prescription. Materials and methods This was a planning study for the clinical scenario of a single BM. Eight sphere-shaped objects with diameters of 5-40 mm in 5-mm increments were assumed as GTVs. The treatment system included a 5-mm leaf width multileaf collimator (MLC) Agility® (Elekta AB, Stockholm, Sweden) and a dedicated planning system Monaco® (Elekta AB). The prescribed dose (PD) was uniformly assigned to just cover 98% of the GTV (D_98%). Three VMARS plans with different dose inhomogeneities of the GTV were generated for each GTV: the % isodose surfaces (IDSs) of GTV D_98%, normalized to 100% at the maximum dose (D_max), were ≤70% (extremely inhomogeneous dose, EIH); ≈80% (inhomogeneous dose, IH); and ≈90% (rather homogeneous dose, RH). VMARS plans were optimized using simple and similar cost functions. In particular, no dose constraint to the GTV D_max was assigned to the EIH plans. Results Intended VMARS plans fulfilling the prerequisites were generated without problems for all GTVs of ≥10 mm, whereas 86.4% was the lowest IDS for the D_98% for 5-mm GTV. Therefore, additional plans for 9- and 8-mm GTVs were generated, which resulted in 68.6% and 75.1% being the lowest IDSs for the D_98% values of 9- and 8-mm GTVs, respectively. The EIH plans were the best in terms of the following: 1) dose conformity, i.e., minimum spillage of PD outside the GTV; 2) moderate, not too excessive, dose attenuation outside the GTV, i.e., appropriate marginal dose 2-mm outside the GTV boundary as a function of GTV size; and 3) lowest dose of the surrounding normal tissue outside the GTV. In contrast, the RH plans were the worst based on all of the aforementioned measures. Conclusions On the assumption of uniform dose assignment to the GTV margin, a very inhomogeneous GTV dose is basically the most suitable for SRS of BMs in terms of 1) superior dose conformity; 2) minimizing the dose of the surrounding normal tissue outside the GTV; and 3) moderate dose spillage margin outside the GTV with a tumor volume-dependent rational increase, i.e., appropriate dose of the common PTV boundary. The concentrically laminated steep dose increase inside the GTV boundary for the EIH plan may also be advantageous for achieving superior tumor response, although early and excessive GTV shrinkage caused by the EIH plan during mfSRS can lead to surrounding brain injury.

Modified Dynamic Conformal Arcs With Forward Planning for Radiosurgery of Small Brain Metastasis: Each Double Arc and Different To-and-Fro Leaf Margins to Optimize Dose Gradient Inside and Outside the Gross Tumor Boundary

Dynamic conformal arcs (DCA) are a widely used technique for stereotactic radiosurgery (SRS) of brain metastases (BM) using a micro-multileaf collimator (mMLC), while the planning design and method considerably vary among institutions. In the usual forward planning of DCA, the steepness of the dose gradient outside and inside the gross tumor volume (GTV) boundary is simply defined by the leaf margin (LM) setting to the target volume edge. The dose fall-off outside the small GTV tends to be excessively precipitous, especially with an MLC of 2.5-mm leaf width, which is predisposed to the insufficient coverage of microscopic brain invasion and other inherent inaccuracies. Meanwhile, insufficient dose increase inside the GTV boundary, i.e., less inhomogeneous GTV dose, likely leads to inferior and less sustainable tumor response. The more inhomogeneous GTV dose is prone to the steeper dose gradient outside the GTV and vice versa. Herein, we describe an alternative simply modified DCA (mDCA) planning that was uniquely devised to optimize the dose gradient outside and inside the GTV boundary for further enhancing and consolidating local control of small BM. For a succinct exemplification, a 10-mm spherical target was assumed as a GTV for DCA planning using a 2.5-mm mMLC. The benchmark plan was generated by adding a 0-mm LM to the GTV edge by assigning a single fraction of 30 Gy to the isocenter, in which the GTV coverage by 24 Gy with 80% isodose surface (IDS) was 96%, i.e., D_96%, while the coverage of GTV + isotropic 2 mm volume by 18 Gy with 60% IDS was 70%, with the D_98% being 12 Gy with 40% IDS, viz., too steep dose fall-off outside the GTV boundary. Alternatively, the increase of LM with or without decreasing the isocenter dose enables the increase of the GTV + 2 mm coverage by 18 Gy while resulting in an inadequate GTV dose with either a less inhomogeneous dose or an excessive marginal dose. Meanwhile, in the newly devised mDCA planning, every single arc was converted to a double to-and-fro arc with different LM settings under the same spatial arrangement, which enabled GTV + 2 mm volume coverage with 18 Gy while preserving the GTV marginal dose and inhomogeneity similar to those for the benchmark plan. Additionally, the different collimator angle (CA) setting for the to-and-fro arcs led to further trimming of the dose conformity. The limitations of general forward planning with only adjusting the LM for every single arc were demonstrated, which can be a contributing factor for local tumor progression of small BM. Alternatively, the mDCA with each double to-and-fro arc and different LM and CA settings enables optimization of the dose gradient both outside and inside the GTV boundary according to the planners' intent, e.g., moderate dose spillage margin outside the GTV and steep dose increase inside the GTV boundary.

Ten-Fraction Stereotactic Radiosurgery With Different Gross Tumor Doses and Inhomogeneities for Brain Metastasis of >10 cc: Treatment Responses Suggesting Suitable Biological Effective Dose Formula for Single and 10 Fractions

Stereotactic radiosurgery (SRS) with >5 fractions (fr) has been increasingly adopted to improve local control and safety for brain metastases (BM) of >10 cm³, given the limited brain tolerance of SRS with ≤5 fr. However, the optimal indication and treatment design, including the prescribed dose and distribution for 10 fr SRS, remains uncertain. A single fr of 24 Gy provides approximately 95% of the one-year local tumor control probability. The potential SRS doses in 10 fr that is clinically equivalent to a single fr of 24 Gy regarding anti-tumor effect range from 48.4 to 81.6 Gy as biological effective doses (BED) as a function of the BED model formulas along with the alpha/beta ratios. The most appropriate BED formula in conjunction with an alpha/beta ratio to estimate similar anti-BM effects for single and 10 fr remains controversial. Herein, we describe four cases of symptomatic radiation-naïve BM >10 cm³ (range, 11 to 26 cm³), treated with 10 fr SRS with a standard prescribed dose of 42 Gy, for which modified dynamic conformal arcs were used with forward planning to improve dose conformity. In the first two cases with gross tumor volumes (GTV) of 15.3 and 10.9 cm³, 42 Gy was prescribed to 70%-80% isodose, normalized to 100% at the isocenter, which encompasses the boundary of the planning target volume: GTV + isotropic 1 mm margin. The tumor responses were initially marked regression followed by regrowth within three months in case 1 and no shrinkage with subsequent progression within three months in case 2. In the remaining two cases with larger GTVs of 19.1 and 26.2 cm³, the GTV boundary and 2-3 mm margin-added object volume was covered by 80% and 56% isodoses with 53 Gy and 37 Gy, respectively, to further increase the marginal and internal doses of GTV and to ensure moderate dose spillage outside the GTV, while >1-1.5 mm outside the GTV was covered by 42 Gy with 63% isodose. According to the BED based on the linear-quadratic (LQ) model with an alpha/beta ratio of 10 (BED₁₀), 53 Gy corresponds to approximately 81 Gy in BED₁₀ and 24 Gy in a single fr. Excellent initial maximum tumor response and subsequently sustained tumor regression (STR) were achieved in both cases. Subsequently, enlarging nodules that could not exclude the possibility of tumor regrowth were disclosed within two years, while late adverse radiation effects remained moderate. These dose-effect relationships suggest that a GTV marginal dose of ≥53 Gy with ≤80% isodose would be preferred to effect ≥1-year STR and that further dose escalation of both marginal and internal GTV may be necessary to achieve ≥2-year STR, while GTV of >25 cm³ may be unsuitable for 10 fr SRS in terms of long-term brain tolerance. Among LQ, LQ-cubic, and LQ-linear model formulas and alpha/beta ratios of 10-20, BED₁₀ may be clinically most suitable to estimate a 10 fr SRS dose that provides anti-BM efficacy similar to that for a single fr.

The effect of setup uncertainty on optimal dosimetric margin in LINAC-based stereotactic radiosurgery with dynamic conformal arc technique

PURPOSE

To estimate the combined effect of setup uncertainty on optimal dosimetric margin by analyzing the dose distribution and biological effect in LINAC-based stereotactic radiosurgery (SRS) with dynamic conformal arc (DCA) technique.

METHODS

SRS treatment plans were generated from CT scans of the Rando head phantom using four non-coplanar DCA's with total 480-degrees of arc. A single spherical planning target volume (PTV) of 4 different diameters was placed at the center of the phantom to simulate brain lesions. For each PTV, 5 treatment plans were created using identical dose calculation parameters, each with 5 different dosimetric margins. To simulate the effect of setup uncertainty, the isocenter for each plan was shifted to 13 different positions. A marginal dose of 20Gy in a single fraction with 6MV photon beam was prescribed to 49 different percentage isodose surfaces (%IDS). The plan quality was evaluated using Conformity Index (CI), Gradient Index (GI), EUD-based Tumor Control Probability (TCP), Normal Tissue Complication Probability (NTCP), and uncomplicated biological objective function (TCP x (1-NTCP) =p+).

RESULTS

A +1mm dosimetric margin could result in a much higher p+ compared to 0mm and 1mm dosimetric margins and a smaller GI while achieving an equivalent p+ in a certain range of %IDS compared to +2mm and +3mm dosimetric margins. With 2mm setup error and +1mm dosimetric margin, the %IDS range optimized for each PTV is: around 80%IDS (10mm diameter); 63~70%IDS (20mm diameter); 66~79%IDS (30mm diameter).

CONCLUSION

This simulation study identified the preferred prescription %IDS for a given setup error and dosimetric margin to achieve an optimal dose distribution and favorable biological effect.

Target volume geometric change and/or deviation from the cranium during fractionated stereotactic radiotherapy for brain metastases: potential pitfalls in image guidance based on bony anatomy alignment

INTRODUCTION

This study sought to evaluate the potential geometrical change and/or displacement of the target relative to the cranium during fractionated stereotactic radiotherapy (FSRT) for treating newly developed brain metastases.

METHODS

For 16 patients with 21 lesions treated with image-guided frameless FSRT in 5 or 10 fractions using a 6-degree-of-freedom image guidance system-integrated platform, the unenhanced computed tomography or T2-weighted magnetic resonance images acquired until the completion of FSRT were fused to the planning image datasets for comparison. Significant change was defined as ≥3-mm change in the tumour diameter or displacement of the tumour centroid.

RESULTS

FSRT was started 1 day after planning image acquisition. Tumour shrinkage, deviation and both were observed in 2, 1 and 1 of the 21 lesions, respectively, over a period of 7-13 days. Tumour shrinkage or deviation resulted in an increase or decrease in the marginal dose to the tumour, respectively, and a substantial increase in the irradiated volume for the surrounding tissue irrespective of the pattern of alteration. No obvious differences in the clinical and treatment characteristics were noted among the populations with or without significant changes in tumour volume or position.

CONCLUSION

Target deformity and/or deviation can unexpectedly occur even during relatively short-course FSRT, inevitably leading to a gradual discrepancy between the planned and actually delivered doses to the tumour and surrounding tissue. To appropriately weigh the treatment outcome against the planned dose distribution, target deformity and/or deviation should also be considered in addition to the immobilisation accuracy, as image guidance with bony anatomy alignment does not necessarily guarantee accurate target localisation until completion of FSRT.

Cerebral cyst formation following stereotactic ablative irradiation for non-nasopharyngeal head and neck malignancies: imaging findings and relevant dosimetric parameters

OBJECTIVE

To describe the clinical characteristics, imaging findings and relevant dosimetric parameters of cases presenting with cerebral cyst formation (CCF) after single or oligo-fractionated stereotactic radiotherapy (SRT) for non-nasopharyngeal head and neck malignancies (HNMs).

METHODS

We identified four cases with the follow-up duration of 5.7-9.1 years from SRT. The irradiated sites included the middle ear in one case and the ethmoid sinus in three cases, two of the latter possessed brain invasion. The chronological changes in MR images and the dose-volume histogram of the adjacent brain tissue were evaluated.

RESULTS

CCF with or without multiple septi presented with a latency of 29-86 months (median, 45.5 months), which was preceded by either non-specific parenchymal enhancement or typical radiation necrosis. In three cases, CCF adjacent to the frontal base resultantly caused mass effect, and two of these three cases required surgical intervention at 38 and 54 months, respectively, after SRT for alleviation of symptoms. The relation of the irradiated brain volumes to the biological equivalent dose based on the linear-quadratic (LQ) and LQ-cubic models was represented as a threshold.

CONCLUSION

When contemplating SRT for HNM cases, caution should be exercised to the dose-volume relation-ship of the adjacent brain tissue, especially the frontal base, as well as other critical structures, and long-term vigilant follow-up is also mandatory.

ADVANCES IN KNOWLEDGE

CCF can occur as an unusual consequence of late brain injury with variable but mostly long latency following SRT for non-nasopharyngeal HNMs adjacent to the brain, even superficial parts that were previously irradiated via conventional radiotherapy.

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.

Prescription to 50-75% isodose line may be optimum for linear accelerator based radiosurgery of cranial lesions

PURPOSE

The prescribed percentage-isodose-line (PIDL) in linac-based SRS varies among institutions. For plans with similar tumor coverage and conformity index, the one with sharper dose falloff outside the tumor volume would be preferred because the probability of brain necrosis is related to the irradiated volume (for example V_12Gy) outside the tumor. The aim of this study is to investigate the optimal isodose line that yields the steepest dose falloff for linac-based SRS using dynamic conformal arc technique (DCA).

METHODS

30 patients with brain tumors were retrospectively studied. The MLC-based DCA was used for planning. For each patient, 5-7 plans with different PIDLs but similar conformity indices were generated. All plans were normalized such that 95% of target volume receives the prescription dose (PD). Gradient index was calculated. The plan with minimum GI was considered optimal.

RESULTS

Optimal GI decreases (3.9 to 2.2) as target volumes increases (0.15 to 50.1cm³), and the optimal PIDL shifts to higher percentile. Median optimal PIDL is 40.0±7.2% (range 33.2-53.1%) for targets <1cm³ and 62.3±8.3% (range 44.6-72.9%) for those >1cm³. The average planned PIDL used for treatment was 83.6±3.3%. The lower optimal PIDL results in smaller V_0.5PD and higher mean dose to the tumor.

CONCLUSION

The optimal PIDL appears to be between 50% and 75% which is lower than the commonly used PIDLs in published studies. Larger targets tend to have higher optimal PIDLs. By choosing an optimal PIDL, we could reduce the volume of irradiated normal brain while delivering higher mean dose to the tumor.

Clinical outcomes of single or oligo-fractionated stereotactic radiotherapy for head and neck tumors using micromultileaf collimator-based dynamic conformal arcs

PURPOSE

To assess the clinical outcomes of single or oligo-fractionated stereotactic radiotherapy (SRT) using dynamic conformal arcs (DCA) for head and neck tumors (HNTs).

METHODS

Thirty-four consecutive patients with 35 lesions treated between 2005 and 2009 were retrospectively evaluated, of whom 85.7 % had recurrent or metastatic disease, and 45.7 and 34.3 % had previous radiotherapy and surgery, respectively. The median SRT dose was 22.3 Gy (11.2-32.8) in 2-4 fractions with a median interval of 7 days and 10.4 Gy (9.2-12.4) in one fraction. SRT was combined with upfront conventionally fractionated RT in 48.6 % of patients.

RESULTS

The median follow-up periods were 18.4 months (2-84.1) for the entire cohort and 49.6 months for the survivors. The 1- and 2-year local control (LC) rates were 84.3 and 70.5 %, with the 1- and 2-year overall survival (OS) rates of 78.6 and 51.6 %. LC was significantly better for tumor volumes <25.6 cm(3) (p = 0.001). OS was significantly longer in patients without any disease outside the SRT site (p < 0.001), whereas LC after the SRT did not affect the OS. Late adverse events occurred in 9 patients, including cranial nerve (CN) injury (grade 3/4) in 2, brain radionecrosis in 5 (grade 1), and fatal bleeding in 2 patients harboring uncontrolled lesions abutting the carotid artery.

CONCLUSIONS

DCA-based SRT can confer relatively long-term LC with acceptable toxicity in selected patients with HNTs. The patients with CN involvement or tumor volume ≥25.6 cm(3) were deemed unsuitable for this treatment regimen.

Consideration of optimal isodose surface selection for target coverage in micro-multileaf collimator-based stereotactic radiotherapy for large cystic brain metastases: comparison of 90%, 80% and 70% isodose surface-based planning

OBJECTIVE

This study aims to compare dynamic conformal arc (DCA) plans based on different-percentage isodose surfaces (IDSs), normalised to 100% at the isocentre, for target coverage (TC; dose prescription) in stereotactic radiotherapy for large cystic brain metastases.

METHODS

The DCA plans were generated for 15 targets (5 spherical models and 10 metastatic brain lesions) based on 90%, 80% and 70% IDSs for dose prescription to attain ≥99% TC values using the Novalis Tx platform. These plans were optimised mainly by leaf margin and/or collimator angle adjustment, while similar arc arrangements were used.

RESULTS

TC values were equivalent among the three plans. Conformity index values were similar between the 80% and 70% plans, while they were worse in the 90% plans. Mean doses (D(mean)) of the interior 3 mm rind structure were highest in the 70% plans, followed by the 80% plans and lowest in the 90% plans. D(mean) of the exterior 3 mm rind structure and the ratio of 50%/100% isodose volumes (Paddick's gradient index values) were highest in the 90% plans, followed by 80% and lowest in the 70% plans.

CONCLUSIONS

These results suggest that the 70% IDS plans might be beneficial for both tumour control and reducing toxicity to surrounding normal tissue if appropriate dose conformity and precise treatment set-up are ensured. The 90% IDS plans are unfavourable in view of inferior dose gradient outside the target and should be limited to cases in which the target dose homogeneity is given the highest priority.

Significance of target location relative to the depth from the brain surface and high-dose irradiated volume in the development of brain radionecrosis after micromultileaf collimator-based stereotactic radiosurgery for brain metastases

The objective of this study was to investigate the factors that potentially lead to brain radionecrosis (RN) after micromultileaf collimator-based stereotactic radiosurgery (SRS) for brain metastases. We retrospectively evaluated 131 lesions with a minimum follow-up of 6 months, 43.5% of which received prior whole-brain radiotherapy (WBRT). The three-tiered location grade (LG) was defined, as follows, for each target by considering mainly the depth from the brain surface: grade 1 (superficial), involving the region at a depth of ≤5 mm from the brain surface; grade 2 (deep), located at a depth of >5 mm from the brain surface; and grade 3 (central), located in the brainstem, cerebellar peduncle, diencephalon, or basal ganglion. The predictive factors for RN, including high-dose irradiated isodose volumes (IIDVs) and LG, were evaluated by univariate and multivariate analysis. Symptomatic RN (S-RN) and asymptomatic RN (A-RN) were observed in 8.4% and 6.9% of cases, respectively. Multivariate analysis indicated that the significant factors for both types of RN were LG, V12 Gy, and V22 Gy in all cases; V22 Gy and LG for the non-WBRT cases; and V15 Gy and LG for the WBRT cases. For the non-WBRT cases, the cutoff values of V22 Gy were 2.62 and 2.14 cm(3) for S-RN and both RN, respectively. For the WBRT cases, the cutoff values of V15 Gy were 5.61 and 5.20 cm(3) for S-RN and both RN, respectively. In addition to the IIDV data, LG helps predict the risk of RN. High-dose IIDV, V22 Gy, was also significantly correlated with RN, particularly for patients treated with SRS alone.

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.