Interinstitutional Plan Quality Assessment of 2 Linac-Based, Single-Isocenter, Multiple Metastasis Radiosurgery Techniques

Dosimetric evaluation of LINAC-based single-isocenter multi-target multi-fraction stereotactic radiosurgery with more than 20 targets: comparing MME, HyperArc, and RapidArc

BACKGROUND

To compare the dosimetric quality of three widely used techniques for LINAC-based single-isocenter multi-target multi-fraction stereotactic radiosurgery (fSRS) with more than 20 targets: dynamic conformal arc (DCA) in BrainLAB Multiple Metastases Elements (MME) module and volumetric modulated arc therapy (VMAT) using RapidArc (RA) and HyperArc (HA) in Varian Eclipse.

METHODS

Ten patients who received single-isocenter fSRS with 20-37 targets were retrospectively replanned using MME, RA, and HA. Various dosimetric parameters, such as conformity index (CI), Paddick CI, gradient index (GI), normal brain dose exposures, maximum organ-at-risk (OAR) doses, and beam-on times were extracted and compared among the three techniques. Wilcoxon signed-rank test was used for statistical analysis.

RESULTS

All plans achieved the prescribed dose coverage goal of at least 95% of the planning target volume (PTV). HA plans showed superior conformity compared to RA and MME plans. MME plans showed superior GI compared to RA and HA plans. RA plans resulted in significantly higher low and intermediate dose exposure to normal brain compared to HA and MME plans, especially for lower doses of ≥ 8Gy and ≥ 5Gy. No significant differences were observed in the maximum dose to OARs among the three techniques. The beam-on time of MME plans was about two times longer than RA and HA plans.

CONCLUSIONS

HA plans achieved the best conformity, while MME plans achieved the best dose fall-off for LINAC-based single-isocenter multi-target multi-fraction SRS with more than 20 targets. The choice of the optimal technique should consider the trade-offs between dosimetric quality, beam-on time, and planning effort.

Stereotactic Radiosurgery of Multiple Brain Metastases: A Review of Treatment Techniques

The advancement of systemic targeted treatments has led to improvements in the management of metastatic disease, particularly in terms of survival outcomes. However, brain metastases remain less responsive to systemic therapies, underscoring the significance of local interventions for comprehensive disease control. Over the past years, the threshold for treating brain metastases through stereotactic radiosurgery has risen. Yet, as the number of treated metastases increases, treatment complexity and duration also escalate. This trend has made multi-isocenter radiosurgery treatments, such as those with the Gamma Knife, challenging to plan and lengthy for patients. In contrast, single-isocenter approaches employing linear accelerators offer an efficient and expeditious treatment option. This review delves into the literature, comparing different linear-accelerator-based techniques with each other and in relation to dedicated systems, focusing on dosimetric considerations and feasibility.

Two novel stereotactic radiotherapy methods for locally advanced, previously irradiated head and neck cancers patients

To determine the feasibility and utility of conebeam CT-guided stereotactic radiotherapy for locally recurrent, previously irradiated head and neck cancer (HNC) patients on the Halcyon, a ring delivery system (RDS). This research aims to quantify plan quality, treatment delivery accuracy, and overall efficacy by comparing against novel clinical TrueBeam HyperArc method. Ten recurrent HNC patients who were treated at our institution on TrueBeam (6MV-FFF) for 30 to 40 Gy in 3 to 5 fractions with noncoplanar HyperArc plans were re-planned on Halcyon (6MV-FFF). These plans were re-planned with the same Acuros-based dose engine. Additionally, we used site-specific full/partial coplanar VMAT arcs. PTV coverage, mean dose to GTV, maximum dose to organs-at-risk (OAR), beam-on time (BOT), and quality assurance (QA) results were investigated and compared. Halcyon provided highly conformal HNC SRT plans with slightly superior mean PTVD99 coverage (96.7% vs 95.5%, p = 0.071), and slightly lower mean GTV dose (37.8 Gy vs 38.2 Gy, p = 0.241) when compared to the HyperArc plans. Differences in plan conformality and maximum dose to OARs were statistically insignificant. Due to Halcyon's coplanar geometry, D2cm was significantly higher (p = 0.001) but Halcyon did result in a reduced normal brain dose by 1 Gy on average and up to 5.2 Gy in some cases. Halcyon provided similar patient-specific QA pass rates with a 2%/2mm gamma criteria (98.2% vs 98.5%) and independent in-house Monte Carlo second check results (97.7% vs 98.2%), suggesting identical treatment delivery accuracy. Halcyon plans resulted in slightly longer beam-on time (3.16 vs 2.30 minutes, p = 0.010), however door-to-door patient time is expected to be <10 minutes. Compared to clinical TrueBeam HyperArc, Halcyon SRT plans provided similar plan quality and treatment delivery accuracy with a potentially faster overall treatment using fully automated patient setup and verification. Rapid delivery of recurrent HNC SRT may reduce intrafraction motion errors while also improving patient compliance and comfort. To provide high-quality of HNC SRT similar to HyperArc, we recommend Halcyon users consider commissioning this novel method. This method will be useful for remote and underserved patient cohorts including Halcyon-only clinics as well.

A Comprehensive Evaluation of Single Isocenter Multiple Target SRS Plans and the Analytical Relationship between Plan Quality Indices with the Number and Volume of Targets

OBJECTIVE

To study the relationship of plan quality indices with the number and volume of target for 5 to 10 brain metastases treated with LINAC-based Single Isocenter Multiple Target (SIMT) Stereotactic radiosurgery (SRS) planning and to determine the maximum volume of spherical targets treated without exceeding the normal tissue tolerances.

METHODS

Spherical targets of 5 to 10 numbers per plan, with individual target volumes ranging from 0.025 cc to 11.5 cc, were simulated with randomly drawn planning target volumes (PTVs) within the brain. SIMT SRS plans were generated for the 21 Gy prescription dose with a 6 MV Flattening Filter Free photon beam. Target coverage, organ at risk sparing, plan quality indices, R50%, and gradient measure were studied. Mean brain dose, V12 for Brain minus PTV (BmP), V10, V12, V15, V18, V20, and V24 for brain volume were evaluated. Equations relating the gradient index, mean brain dose, and V12 (BmP) to the given number and volume of the targets were constructed.

RESULTS

PTV coverage D98 was 98.77 ± 1.37 %. The mean CIRTOG, QRTOG, HIRTOG, CIP, GI, and R50% of the individual targets were 1.02 ± 0.08, 0.94 ± 0.02, 1.49 ± 0.11, 0.91 ± 0.06, 4.74 ± 2.3, and 4.95 ± 2.67, respectively. The gradient measure achieved was in the range of 0.49 to 1.35 cm. The mean brain dose was in the range of 1.62 to 6.69 Gy. The mean V12 (BmP) per target obtained was 3.85 ± 2.83 cc.

CONCLUSION

Equations relating the number and volume of targets to the gradient measure, mean brain dose, V12 (BmP), and V10-24 can serve as a baseline for multiple brain metastases SIMT planning. The target volume for 5, 6, 7, 8, 9, and 10 targets that can be treated without exceeding V12 (BmP) is 6, 5, 4.7, 4, 3.7, and 3.4 cc, respectively, for the 21 Gy prescription dose.

Single-isocenter multiple-target stereotactic radiosurgery for multiple brain metastases: dosimetric evaluation of two automated treatment planning systems

Purpose

Automated treatment planning systems are available for linear accelerator (linac)-based single-isocenter multi-target (SIMT) stereotactic radiosurgery (SRS) of brain metastases. In this study, we compared plan quality between Brainlab Elements Multiple Brain Metastases (Elements MBM) software which utilizes dynamic conformal arc therapy (DCAT) and Varian HyperArc (HA) software using a volumetric modulated arc therapy (VMAT) technique.

Patients and methods

Between July 2018 and April 2021, 36 consecutive patients ≥ 18 years old with 367 metastases who received SIMT SRS at UPMC Hillman Cancer San Pietro Hospital, Rome, were retrospectively evaluated. SRS plans were created using the commercial software Elements MBM SRS (Version 1.5 and 2.0). Median cumulative gross tumor volume (GTV) and planning tumor volume (PTV) were 1.33 cm³ and 3.42 cm³, respectively. All patients were replanned using HA automated software. Extracted dosimetric parameters included mean dose (D_mean) to the healthy brain, volumes of the healthy brain receiving more than 5, 8,10, and 12 Gy (V_5Gy, V_8Gy, V_10Gy and V_12Gy), and doses to hippocampi.

Results

Both techniques resulted in high-quality treatment plans, although Element MBM DCAT plans performed significantly better than HA VMAT plans, especially in cases of more than 10 lesions). Median V_12Gy was 13.6 (range, 1.87–45.9) cm³ for DCAT plans and 18.5 (2.2–62,3) cm³ for VMAT plans (p < 0.0001), respectively. Similarly, V_10Gy, V_8Gy, V_5Gy (p < 0.0001) and median dose to the normal brain (p = 0.0001) were favorable for DCAT plans.

Conclusions

Both Elements MBM and HA systems were able to generate high-quality plans in patients with up to 25 brain metastases. DCAT plans performed better in terms of normal brain sparing, especially in patients with more than ten lesions and limited total tumor volume.

CBCT Verification of SRT for Patients With Brain Metastases

Background

The aim of our work is to demonstrate the role of image guidance and volumetric imaging in stereotactic radiotherapy (SRT) of brain metastases.

Methods

Between 2018 and 2020, 106 patients underwent intracranial stereotactic radiotherapy. 10 patients with metastatic brain tumors treated with SRT were randomly selected and included in our study model. Patients were scanned pre- and post-treatment with cone beam CT. Total of 100 verifications of 50 stereotaxic treatments were performed and analyzed.

Results

Population mean X, Y, Z values were -0.13 cm, -0.04 cm, -0.03 cm, respectively, rotation values 0.81°, 0.51°, 0.46°, respectively. Systematic error components for translational displacements pre corrections were as follows: 0.14 cm for X, 0.13 cm for Y and 0.1 cm for Z. Systematic error components of the post-treatment HR 3D CBCTs were as follows: 0.01 cm for X, 0.06 cm for Y and 0.04 cm for Z.

Conclusions

Population mean values close to 0 confirmed that there is no systematic variation in our system and the accuracy of our equipment and tools is reliable. HR 3D CBCT scans performed pre SRTs further refine patient and target volume setting, support medical decision making and eliminate the possibility of gross error.

Evaluation of two automated treatment planning techniques for multiple brain metastases using a single isocenter

Two automated treatment planning techniques were evaluated for multiple brain metastases using a single isocenter. One technique is knowledge-based planning (KBP) using a stereotactic radiosurgery (SRS) model in Eclipse treatment planning system (TPS); and the other is the Multiple Brain Mets (MBM) SRS technique in Brainlab Elements TPS. Eighteen plans each with 3-10 lesions were used for the study. Plan evaluation metrics included the planning target volume (PTV) coverage, conformity index (CI), total monitor units (MUs), plan optimization time, brain V_12 Gy, V_8 Gy, and V_5 Gy. Both the KBP and MBM planning techniques produced comparable plans to the manually generated clinical plans in terms of PTV coverage and CI. For irregularly shaped lesions, the KBP plans provided more conformal dose distribution to the PTV than the MBM plans. The KBP plans took significantly longer time to plan but have fewer MUs than the MBM plans. The MBM plans spared normal brain tissues better than the KBP plans in terms of V_5 Gy.

Simultaneous radiosurgery for multiple brain metastases: technical overview of the UCLA experience

PURPOSE/OBJECTIVE(S)

To communicate our institutional experience with single isocenter radiosurgery treatments for multiple brain metastases, including challenges with determining planning target volume (PTV) margins and resulting consequences, image-guidance translational and rotational tolerances, intra-fraction patient motion, and prescription considerations with larger PTV margins.

MATERIALS/METHODS

Eight patient treatments with 51 targets were planned with various margins using Elements Multiple Brain Mets SRS treatment planning software (Brainlab, Munich, Germany). Forty-eight plans with 0 mm, 1 mm and 2 mm margins were created, including plans with variable margins, where targets more than 6 cm away from the isocenter were planned with larger margins. The dosimetric impact of the margins were analyzed with V5Gy, V8Gy, V10Gy, V12Gy values. Additionally, 12 patient motion data were analyzed to determine both the impact of the repositioning threshold and the distributions of the patient translational and rotational movements.

RESULTS

The V5Gy, V8Gy, V10Gy, V12Gy volumes approximately doubled when margins change from 0 to 1 mm and tripled when change from 0 to 2 mm. With variable margins, the aggregated results are similar to results from plans using the lower of two margins, since only 12.2% of the targets were more than 6 cm away from the isocenter. With 0.5 mm re-positioning threshold, 57.4% of the time the patients are repositioned. Reducing the threshold to 0.25 mm results in 91.7% repositioning rate, due to limitations of the fusion algorithm and actual patient motion. The 90th percentile of translational movements in all directions is 0.7 mm, while the 90th percentile of rotational movements in all directions is 0.6 degrees. Median translations and rotations are 0.2 mm and 0.2 degrees, respectively.

CONCLUSIONS

Based on the data presented, we have switched our modus operandi from 2 to 1 mm PTV margins, with an eventual goal of using 0.5 and 1.0 mm variable margins when an automated margin assignment method becomes available. The 0.5 mm and 0.5 degrees repositioning thresholds are clinically appropriate with small residual patient movements.

Evaluation of a two-dimensional diode array for patient-specific quality assurance of HyperArc

PURPOSE

To evaluate a two-dimensional diode array for patient-specific quality assurance of VMAT stereotactic radiosurgery (SRS) plans.

METHODS

The diode array, an SRS MapCHECK (SRSMC), was composed of a 77 mm ×77 mm face-centered array having a spacing of 2.47 mm. Sixty SRS plans were selected from our clinical database, 30 for treatment of a single target and 30 for multiple targets. The target sizes ranged from 2.4 mm to 44.7 mm equivalent diameter (median 8.7 mm). The plans were delivered to the diode array. For multiple target plans, two measurements were obtained at two locations, one corresponding to the largest target and the other to the smallest target. Gamma using a 3%/1 mm criteria and the dose to the center diode were compared with radiochromic film (RCF). Dose to selected regions of the detector electronics was calculated.

RESULTS

The mean difference between the center diode and RCF was -1.2%. For a threshold of at least 95% of detectors/pixels having gamma < 1 for a 3%/1 mm criteria, SRSMC and RCF gave consistent results for 79 of the 90 measurements. For plans with an arc having a patient support angle of 90° or 270°, the median dose to the electronics was 0.65% of the prescription dose.

CONCLUSIONS

SRSMC is an efficient tool for accurate patient-specific quality assurance of VMAT single and multiple target radiosurgery, yielding similar clinical decisions as radiochromic film.

Practical Considerations for Single Isocenter LINAC Radiosurgery of Multiple Brain Metastases

The purpose of this paper is to summarize treatment guidelines for the performance of single isocenter LINAC radiosurgery of multiple brain metastases developed and used by 3 experienced centers. This article is not meant to provide consensus guidelines. Rather, this is a practical, "how we do it" reference without substantial discussion. To serve as a treatment reference, the great majority of the information is presented in topic-specific tables.

Predicting the effect of indirect cell kill in the treatment of multiple brain metastases via single-isocenter/multitarget volumetric modulated arc therapy stereotactic radiosurgery

Purpose

Due to spatial uncertainty, patient setup errors are of major concern for radiosurgery of multiple brain metastases (m‐bm) when using single‐isocenter/multitarget (SIMT) volumetric modulated arc therapy (VMAT) techniques. However, recent clinical outcome studies show high rates of tumor local control for SIMT‐VMAT. In addition to direct cell kill (DCK), another possible explanation includes the effects of indirect cell kill (ICK) via devascularization for a single dose of 15 Gy or more and by inducing a radiation immune intratumor response. This study quantifies the role of indirect cell death in dosimetric errors as a function of spatial patient setup uncertainty for stereotactic treatments of multiple lesions.

Material and Methods

Nine complex patients with 61 total tumors (2‐16 tumors/patient) were planned using SIMT‐VMAT with geometry similar to HyperArc with a 10MV‐FFF beam (2400 MU/min). Isocenter was placed at the geometric center of all tumors. Average gross tumor volume (GTV) and planning target volume (PTV) were 1.1 cc (0.02–11.5) and 1.9 cc (0.11–18.8) with an average distance to isocenter of 5.4 cm (2.2–8.9). The prescription was 20 Gy to each PTV. Plans were recalculated with induced clinically observable patient setup errors [±2 mm, ±2^o] in all six directions. Boolean structures were generated to calculate the effect of DCK via 20 Gy isodose volume (IDV) and ICK via 15 Gy IDV minus the 20 Gy IDV. Contributions of each IDV to the PTV coverage were analyzed along with normal brain toxicity due to the patient setup uncertainty. Induced uncertainty and minimum dose covering the entire PTV were analyzed to determine the maximum tolerable patient setup errors to utilize the ICK effect for radiosurgery of m‐bm via SIMT‐VMAT.

Results

Patient setup errors of 1.3 mm /1.3° in all six directions must be maintained to achieve PTV coverage of the 15 Gy IDV for ICK. Setup errors of ±2 mm/2° showed clinically unacceptable loss of PTV coverage of 29.4 ± 14.6% even accounting the ICK effect. However, no clinically significant effect on normal brain dosimetry was observed.

Conclusions

Radiosurgery of m‐bm using SIMT‐VMAT treatments have shown positive clinical outcomes even with small residual patient setup errors. These clinical outcomes, while largely due to DCK, may also potentially be due to the ICK. Potential mechanisms, such as devascularization and/or radiation‐induced intratumor immune enhancement, should be explored to provide a better understanding of the radiobiological response of stereotactic radiosurgery of m‐bm using a SIMT‐VMAT plan.

Treatment of multiple intracranial metastases in radiation oncology: a contemporary review of available technologies

The use of stereotactic radiosurgery to treat multiple intracranial metastases, frequently concurrently, has become increasingly common. The ability to accurately and safely deliver stereotactic radiosurgery treatment to multiple intracranial metastases (MIM) relies heavily on the technology available for targeting, planning, and delivering the dose. A number of platforms are currently marketed for such applications, each with intrinsic capabilities and limitations. These can be broadly categorised as cobalt-based, linac-based, and robotic. This review describes the most common representative technologies for each type along with their advantages and current limitations as they pertain to the treatment of multiple intracranial metastases. Each technology was used to plan five clinical cases selected to represent the clinical breadth of multiple metastases cases. The reviewers discuss the different strengths and limitations attributed to each technology in the case of MIM as well as the impact of disease-specific characteristics (such as total number of intracranial metastases, their size and relative proximity) on plan and treatment quality.

Single isocenter treatment planning techniques for stereotactic radiosurgery of multiple cranial metastases

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DCA in most cases is superior to VMAT for multi metastases single isocenter SRS.

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Normal brain V_12Gy was significantly reduced with DCA, predicting for lower S-NEC.

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Maximum doses to critical organs-at-risk were significantly lower with DCA.

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Conformity was comparable between VMAT and DCA.

Background and purpose

Whole brain radiation therapy use has decreased in favor of stereotactic radiosurgery (SRS) for the treatment of multiple brain metastases due to reduced neurotoxicity. Here we compare two single isocenter radiosurgery planning techniques, volumetric modulated arc therapy (VMAT) and dynamic conformal arcs (DCA) in terms of their dosimetric and delivery performance.

Materials and methods

Sixteen patients with 2– 18 brain metastases (total 103; median 4) previously treated with single fraction SRS were replanned for multiple lesion single isocenter treatments using VMAT and DCA using different treatment planning systems for each and three different plan geometries for DCA. Plans were evaluated using the Paddick conformity index, normal tissue V_12Gy, the probability for symptomatic brain necrosis (S-NEC), maximum organ-at-risk (OAR) point doses, and total number of monitor units (MU).

Results

Conformity was not significantly different between VMAT and DCA plans. VMAT plans showed a trend towards higher MU with a median difference between 18% and 24% (p ≤ 0.09). Median V_12Gy differences were 7.0 cm³–8.6 cm³ favoring DCA plans (p < 0.01). VMAT plans had median excess absolute and relative S-NEC risks compared to DCA plans of 8%–10% and 25%–31%, respectively (p < 0.01). Moreover for VMAT compared to DCA, maximum OAR doses were significantly higher for the brainstem (1.9 Gy; p < 0.01), chiasm (0.5 Gy; p ≤ 0.02), and optic nerves (0.5 Gy; p ≤ 0.04).

Conclusions

In most cases DCA plans were found to be dosimetrically superior to VMAT plans with reduced V_12Gy and associated risk for S-NEC. Maximum doses to important OARs showed significant improvement, increasing the ability for subsequent salvage treatments involving radiation.

Transition From Manual to Automated Planning and Delivery of Volumetric Modulated Arc Therapy Stereotactic Radiosurgery: Clinical, Dosimetric, and Quality Assurance Results

PURPOSE

Properly planned single isocenter volumetric modulated arc therapy (VMAT) radiosurgery plans exhibit high quality and efficiency. We report here the largest clinical experience to date, to our knowledge, comparing manual planning with a new automated platform designed to standardize and simplify radiosurgery planning and delivery processes.

METHODS

We treated 693 patients with single isocenter VMAT radiosurgical plans generated by either our conventional manual (mVMAT) or a recently implemented automated (HyperArc^TM) technique. All plans targeted the gross tumor volume without margin. Radiochromic film was used for patient-specific quality assurance (PSQA). We evaluated local control and toxicity data for a subgroup of 107 patients having 377 metastatic tumors that were treated with HyperArc.

RESULTS

The median Radiation Therapy Oncology Group (RTOG) conformity index was 1.14 and was not different between the 2 techniques. The median Paddick gradient index was 5.42 for HyperArc versus 7.09 for mVMAT (P < .001). The median mean brain doses were 4.6% and 5.1% for HyperArc and mVMAT, respectively (P = .04). The PSQA for both techniques met clinical criteria, but 97% of the HyperArc plans satisfied the gamma tolerance limit recommended by the American Association of Physicists in Medicine Task Group No. 218, compared with 94% of the mVMAT plans (P = .02). The median treatment-planning times were not significantly different. The median treatment times were 10.5 and 11.4 minutes for HyperArc and mVMAT, respectively (P < .001). The Kaplan-Meier estimate of local control was 90.1% at 1 year.

CONCLUSIONS

HyperArc produces high-quality radiosurgical plans that are at least as good as mVMAT plans created by an expert manual planner with easier planning and more efficient delivery workflow. A less experienced planner can produce very high-quality radiosurgical plans even for patients with more than 10 targets. The use of a single-isocenter technique for multiple targets with no PTV margin did not compromise clinical outcomes, and 1-year local control for treated targets remained congruent with historical series.

Dynamic conformal arcs-based single-isocenter VMAT planning technique for radiosurgery of multiple brain metastases

Multiple small beamlets in the delivery of highly modulated single-isocenter HyperArc VMAT plan can lead to dose delivery errors associated with small-field dosimetry, which can be a major concern for stereotactic radiosurgery for multiple brain lesions. Herein, we describe and compare a clinically valuable dynamic conformal arc (DCA)-based VMAT (DCA-VMAT) approach for stereotactic radiosurgery of multiple brain lesions using flattening filter free beams to minimize this effect. Original single-isocenter HyperArc style VMAT and DCA-VMAT plans were created on 7 patients with 2 to 8 brain lesions (total 35 lesions) for 10 MV- flattening filter free beam. 20 Gy was prescribed to each lesion. For identical planning criteria, DCA-VMAT utilizes user-controlled field aperture shaper before VMAT optimization. Plans were evaluated for conformity and target coverage, low- and intermediate dose spillages to brain volume that received more than 30% (V30%) and 50% (V50%) of prescription dose. Additionally, mean brain dose, V8, V12 and maximal dose to adjacent organs-at-risk (OAR) including hippocampi were reported. Total monitor units, beam modulation factor, treatment delivery efficiency, and accuracy were recorded. Comparing with original VMAT, DCA-VMAT plans provided similar tumor dose, target coverage and conformity, yet tighter radio-surgical dose distribution with lower dose to normal brain V30% (p = 0.009), V50% (p = 0.05) and other OAR including lower dose to hippocampi. Lower total number of monitor units and smaller beam modulation factor reduced beam on time by 2.82 min (p < 0.001), on average (maximum up to 3.8 min). Beam delivery accuracy was improved by 8%, on average (p < 0.001) and maximum up to 13% in some cases for DCA-VMAT plans. This novel DCA-VMAT approach provided excellent plan quality, reduced dose to normal brain, and other OAR while significantly reducing beam-on time for radiosurgery of multiple brain lesions-improving patient compliance and clinic workflow. It also provided less MLC modulation through the targets-potentially minimizing small field dosimetry errors as demonstrated by quality assurance results. Incorporating DCA-based VMAT optimization in HyperArc module for radiosurgery of multiple brain lesions merits future investigation.

The development and testing of a novel spherical radiotherapy phantom system for the commissioning and patient-specific quality assurance of mono-isocentric multiple mets SRS plans

PURPOSE

To develop a single radiotherapy phantom system capable of performing both patient-specific quality assurance (QA) measurements and commissioning measurements for mono-isocentric LinAc-based stereotactic radiosurgery ("mLSRS") treatment plans.

METHODS

Design A 20-cm diameter spherical phantom was designed which contained within it a film cartridge. The surface of the sphere was machined to display a set of angular markings at both the equator and the meridian representing a spherical coordinate system. A stand was designed which allows for free rotation about any vector passing through the center of the sphere. A program was created using Python 3 to: (a) Compute the measurement setup necessary to intersect exactly one film plane with three user specified dicom points contained within the QA plan; (b) Extract the intersection dose plane from the three dimensional DICOM dose file and; (c) Generate a synthetic computed tomography (CT) in the exact measurement geometry which is subsequently used for phantom positioning during the QA measurement.

TESTING

To assess the functionality of the phantom system dynamic conformal arc mLSRS plans that were generated by a clinically commissioned multiple metastasis treatment planning system (BrainLab Elements version 2.0) using patient-specific data. A total of seven patient plans were created that contained a total of 31 targets {<Volume> = (0.382 ± 0.534) cc: Range [0.051, 2.05] cc, <Off-Axis Distance> = (30 ± 16) mm: Range[0, 55] mm} 27 of which were directly measured with film and analyzed. Each planned isocenter was mapped to the phantom's center and the dose was recomputed. From the phantom dose distribution dicom points of interest were selected in sets of three and input into the provided software. The software computed the plane that intersects with the entered three points and instructed the user on the setup geometry to place the film in the intersecting plane. The software then generated a synthetic CT scan with embedded fiducial markers rotated into the setup orientation. This CT was then used as the setup reference image in ExacTrac image guidance system (tolerance 0.7 mm & 0.5deg). All plans were delivered on a Varian Truebeam linear accelerator with HDMLC, Exactrac and a 6 degree of freedom couch. After delivery of each test plan a 10 × 10 reference field was delivered to a known dose approximately equal to the maximum dose contained within the plan for film calibration. The test film was scanned simultaneously with the 10 × 10 reference film and a film that received zero dose using an Epson 10000XL flatbed scanner after waiting 24 hours. The test film was scaled according to the reference film and analyzed via the gamma analysis (3%, 1 mm, 10%) implemented in Ashland Film QA Pro software.

RESULTS

The spherical phantom system was able to perform validation measurements on a variety of patient-specific plan geometries. The average gamma pass-rate γ(3%, 1 mm,10%) for all measurements was 96.7% (σ = 3.6%).

CONCLUSIONS

A novel spherical radiotherapy phantom system has been designed and tested on clinically relevant test plans.