Geometric and dosimetric uncertainties in intracranial stereotatctic treatments for multiple nonisocentric lesions

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.

Gantry triggered x-ray verification during single-isocenter stereotactic radiosurgery: Increased certainty for a no-margin strategy

BACKGROUND

Single-isocenter linac-based stereotactic radiosurgery (SRS) has emerged as a dedicated treatment option for multiple brain metastases. Consequently, image-guidance for patient positioning and motion management has become very important. The purpose of this study was to analyze intra-fraction errors measured with stereoscopic x-rays and their impact on the dose distribution.

MATERIALS AND METHODS

Treatments were planned with non- coplanar dynamic conformal arcs for 33 patients corresponding to 127 brain lesions and 356 arcs. Intra-arc positioning errors were measuredusing stereoscopic x-rays (ExacTrac Dynamic, Brainlab), triggered during arc delivery. Couch corrections above 0.7 mm and 0.5° were always applied. Intra-arc positioning data was analyzed. The dose impact was evaluated by applying the measured errors to the dose given in each arc.

RESULTS

Median residual errors were 0.10 mm, 0.13 mm and 0.08 mm for the lateral, longitudinal and vertical directions and 0.10°, 0.08° and 0.13° for the pitch, roll and yaw angles respectively. 90% of the treatment arcs showed shifts of less than 0.4 mm and 0.4°in all directions. Dosimetric impact of motion showed the largest losses in coverage on small targets. All targets achieved at least 95% of the prescription dose to 95% of their volume, even when planned without margins.

CONCLUSIONS

Intra-fractional errors measured during beam delivery were found to be notably low with a dose impact that showed acceptable target coverage when applying these intra-arc errors to the dose distributions of the individual treatment arcs. Using an adequate immobilization and intra-fraction imaging prior to and during irradiation, no margins need to be added to compensate for intra-fraction motion.

How much rotational error is clinically acceptable for single-isocenter/two-lesion lung SBRT treatment on halcyon ring delivery system (RDS)?

PURPOSE

SBRT treatment of two separate lung lesions via single-isocenter/multi-target (SIMT) plan on Halcyon RDS could improve patient comfort, compliance, patient throughput, and clinic efficiency. However, aligning two separate lung lesions synchronously via a single pre-treatment CBCT scan on Halcyon can be difficult due to rotational patient setup errors. Thus, to quantify the dosimetric impact, we simulated loss of target(s) coverage due to small, yet clinically observable rotational patient setup errors on Halcyon for SIMT treatments.

METHODS

Seventeen previously treated 4D-CT based SIMT lung SBRT patients with two separate lesions (total 34 lesions, 50 Gy in five fractions to each lesion) on TrueBeam (6MV-FFF) were re-planned on Halcyon (6MV-FFF) using a similar arc geometry (except couch rotation), dose engine (AcurosXB algorithm), and treatment planning objectives. Rotational patient setup errors of [± 0.5⁰ to ± 3.0⁰] on Halcyon were simulated via Velocity registration software in all three rotation axes and recalculated dose distributions in Eclipse treatment planning system. Dosimetric impact of rotational errors was evaluated for target coverage and organs at risk (OAR).

RESULTS

Average PTV volume and distance to isocenter were 23.7 cc and 6.1 cm. Average change in Paddick's conformity indexes were less than -5%, -10%, and -15% for 1°, 2°, and 3°, respectively for yaw, roll, and pitch rotation directions. Maximum drop off of PTV(D100%) coverage for 2° rotation was -2.0% (yaw), -2.2% (roll), and -2.5% (pitch). With ±1° rotational error, no PTV(D100%) loss was found. Due to anatomical complexity: irregular and highly variable tumor sizes and locations, highly heterogenous dose distribution, and steep dose gradient, no trend for loss of target(s) coverage as a function of distance to isocenter and PTV size was found. Change in maximum dose to OAR were acceptable per NRG-BR001 within ±1.0° rotation, but were up to 5 Gy higher to heart with 2° in the pitch rotation axis.

CONCLUSION

Our clinically realistic simulation results show that rotational patient setup errors up to 1.0° in any rotation axis could be acceptable for selected two separate lung lesions SBRT patients on Halcyon. Multivariable data analysis in large cohort is ongoing to fully characterize Halcyon RDS for synchronous SIMT lung SBRT.

The spatial accuracy of ring-mounted halcyon linac versus C-arm TrueBeam linac for single-isocenter/multi-target SBRT treatment

Stereotactic body radiotherapy (SBRT) treatment of oligometastatic lesions via single-isocenter/multi-target (SIMT) plan is more efficient than using multi-isocenter/multitarget SBRT. This study quantifies the spatial positioning accuracy of 2 commercially available LINAC systems for SIMT treatment pertaining to the potential amplification of error as a function of the target's distance-to-isocenter. We compare the Ring-Gantry Halcyon LINAC equipped with the fast iterative conebeam-CT (iCBCT) for image-guided SIMT treatment, and the SBRT-dedicated C-Arm TrueBeam with standard pretreatment CBCT imaging. For both systems, Sun Nuclear's MultiMet Winston-Lutz Cube phantom with 6 metallic BBs distributed at different planes up to 7 cm away from the isocenter was used. The phantom was aligned and imaged via CBCT, and then couch corrections were applied. To treat all 6 BBs, an Eclipse 10-field 3D-conformal Field-in-Field (2×2 cm² MLC field to each BB) plan for varying gantry, collimator, and couch (TrueBeam only) positions was developed for both machines with 6MV-FFF beam. The plan was delivered through ARIA once a week. The EPID images were analyzed via Sun Nuclear's software for spatial positioning accuracy. On TrueBeam, the treatment plan was delivered twice: once with 3DoF translational corrections and once with PerfectPitch 6DoF couch corrections. The average 3D spatial positioning accuracy was 0.55 ± 0.30 mm, 0.54 ± 0.24 mm, and 0.56 ± 0.28 mm at isocenter, and 0.59 ± 0.30 mm, 0.69 ± 0.30 mm, and 0.70 ± 0.35 mm at 7 cm distance-to-isocenter for Halcyon, TrueBeam 3DoF, and TrueBeam 6DoF, respectively. This suggests there are no clinically significant deviations of spatial uncertainty between the platforms with the distance-to-isocenter. On both platforms, our weekly independent measurements demonstrated the reproducibility for less than 1.0 mm positional accuracy of off-axis targets up to 7 cm from the isocenter. Due to this, no additional PTV-margin is suggested for lesions within 7 cm of isocenter. This study confirms that Halcyon can deliver similar positional accuracy to SBRT-dedicated TrueBeam to off-axis targets up to 7 cm from isocenter. These results further benchmark the spatial uncertainty of our extensively used SBRT-dedicated TrueBeam LINAC for SIMT SBRT treatments.

Optimization of treatment isocenter location in single-isocenter LINAC-based stereotactic radiosurgery for management of multiple brain metastases

PURPOSE

Single-isocenter linear accelerator (LINAC)-based stereotactic radiosurgery (SRS) has become a promising treatment technique for the management of multiple brain metastases. Because of the high prescription dose and steep dose gradient, SRS plans are sensitive to geometric errors, resulting in loss of target coverage and suboptimal local tumor control. Current planning techniques rely on adding a uniform and isotropic setup margin to all gross tumor volumes (GTVs) to account for rotational uncertainties. However, this setup margin may be insufficient, since the magnitude of rotational uncertainties varies and is dependent upon the distance between a GTV and the isocenter. In this study, we designed a framework to determine the optimal isocenter of a single-isocenter SRS plan for multiple brain metastases using stochastic optimization to mitigate potential errors resulting from rotational uncertainties.

METHODS

Planning target volumes (PTVs), defined as GTVs plus a 1-mm margin following common SRS planning convention, were assumed to be originally treated with a prescription dose and therefore covered by the prescription isodose cloud. The dose distribution, including the prescription isodose, was considered invariant assuming small rotations throughout the study. A stochastic optimization scheme was developed to determine the location of the optimal isocenter, so that the prescription dose coverage of rotated GTVs, equivalent to the intersecting volumes between the rotated GTVs and original PTVs, was maximized for any random small rotations about the isocenter. To evaluate the coverage of GTVs, the expected V 100 % undergoing random rotations was approximated as the sample average V 100 % undergoing a predetermined number of rotations. The expected V 100 % of each individual GTV and total GTVs was then compared between the plans using the optimal isocenter and the center-of-mass (CoM), respectively.

RESULTS

Twenty-two patients previously treated for multiple brain metastases in a single institute were included in this retrospective study. Each patient was initially treated for more than three brain metastases (mean: 7.6; range: 3-15) with the average GTV volume of 0.89 cc (range: 0.03-11.78 cc). The optimal isocenter found for each patient was significantly different from the CoM, with the average Euclidean distance between the optimal isocenter and the CoM being 4.36 ± 2.59 cm. The dose coverage to GTVs was also significantly improved (paired t-test; p < 0.001) when the optimal isocenter was used, with the average V 100 % of total GTVs increasing from 87.1% (standard deviation as std: 11.7%; range: 39.9-98.2%) to 94.2% (std: 5.4%; range: 77.7-99.4%). The volume of a GTV was positively correlated with the expected V 100 % regardless of the isocenter used (Spearman coefficient: ρ = 0.66 ; p < 0.001). The distance between a GTV and the isocenter was negatively correlated with the expected V 100 % when the CoM was used ( ρ = - 0.21 ; p = 0.004), however no significant correlation was found when the optimal isocenter was used ( ρ = - 0.11 ; p = 0.137).

CONCLUSION

The proposed framework provides an effective approach to determine the optimal isocenter of single-isocenter LINAC-based SRS plans for multiple brain metastases. The implementation of the optimal isocenter results in SRS plans with consistently higher target coverage despite potential rotational uncertainties, and therefore significantly improves SRS plan robustness against random rotational uncertainties.

Single-isocenter stereotactic non-coplanar arc treatment of 200 patients with brain metastases: multileaf collimator size and setup uncertainties

Purpose

The purpose of this study was to evaluate our 2 years’ experience with single-isocenter, non-coplanar, volumetric modulated arc therapy (VMAT) for brain metastasis (BM) stereotactic radiosurgery (SRS).

Methods

A total of 202 patients treated with the VMAT SRS solution were analyzed retrospectively. Plan quality was assessed for 5 mm (120) and 2.5 mm (high-definition, HD) central leaf width multileaf collimators (MLCs). For BMs at varying distances from the plan isocenter, the geometric offset from the ideal position for two image-guided radiotherapy workflows was calculated. In the workflow with ExacTrac (BrainLAB, München, Germany; W‑ET), patient positioning errors were corrected at each couch rotation. In the workflow without ExacTrac (W-noET), only the initial patient setup correction was considered. The dose variation due to rotational errors was simulated for multiple-BM plans with the HD-MLC.

Results

Plan conformity and quality assurance were equivalent for plans delivered with the two MLCs while the HD-MLC plans provided better healthy brain tissue (BmP) sparing. 95% of the BMs had residual intrafractional setup errors ≤ 2 mm for W‑ET and 68% for W‑noET. For small BM (≤1 cc) situated >3 cm from the plan isocenter, the dose received by 95% of the BM decreased in median (interquartile range) by 6.3% (2.8–8.8%) for a 1-degree rotational error.

Conclusion

This study indicates that the HD-MLC is advantageous compared to the 120-MLC for sparing healthy brain tissue. When a 2-mm margin is applied, W‑noET is sufficient to ensure coverage of BM situated ≤ 3 cm of the plan isocenter, while for BM further away, W‑ET is recommended.

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

PURPOSE

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

METHOD

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

RESULTS

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

CONCLUSION

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

Dosimetric study between a single isocenter dynamic conformal arc therapy technique and Gamma Knife radiosurgery for multiple brain metastases treatment: impact of target volume geometrical characteristics

Purpose

To compare linac-based mono-isocentric radiosurgery with Brainlab Elements Multiple Brain Mets (MBM) SRS and the Gamma Knife using a specific statistical method and to analyze the dosimetric impact of the target volume geometric characteristics. A dose fall-off analysis allowed to evaluate the Gradient Index relevancy for the dose spillage characterization.

Material and methods

Treatments were planned on twenty patients with three to nine brain metastases with MBM 2.0 and GammaPlan 11.0. Ninety-five metastases ranging from 0.02 to 9.61 cc were included. Paddick Index (PI), Gradient Index (GI), dose fall-off, volume of healthy brain receiving more than 12 Gy (V_12Gy) and DVH were used for the plan comparison according to target volume, major axis diameter and Sphericity Index (SI). The multivariate regression approach allowed to analyze the impact of each geometric characteristic keeping all the others unchanged. A parallel study was led to evaluate the impact of the isodose line (IDL) prescription on the MBM plan quality.

Results

For mono-isocentric linac-based radiosurgery, the IDL around 70–75% was the best compromise found. For both techniques, the GI and the dose fall-off decreased with the target volume. In comparison, PI was slightly improved with MBM for targets < 1 cc or SI > 0.78. GI was improved with GP for targets < 2.5 cc. The V_12Gy was higher with MBM for lesions > 0.4 cc or SI < 0.84 and exceeded 10 cc for targets > 5 cc against 6.5 cc with GP. The presence of OAR close to the PTV had no impact on the dose fall off values. The dose fall-off was higher for volumes < 3.8 cc with GP which had the sharpest dose fall-off in the infero-superior direction up to 30%/mm. The mean beam-on time was 94 min with GP against 13 min with MBM.

Conclusions

The dose fall-off and the V_12Gy were more relevant indicators than the GI for the low dose spillage assessment. Both evaluated techniques have comparable plan qualities with a slightly improved selectivity with MBM for smaller lesions but with a healthy tissues sparing slightly favorable to GP at the expense of a considerably longer irradiation time. However, a higher healthy tissue exposure must be considered for large volumes in MBM plans.

Is there a volume threshold of brain metastases for Linac-based stereotactic radiotherapy?

PURPOSE

To investigate whether there is a volume threshold in target volume of brain metastases below which a small cone size and sharp penumbra in Gamma Knife (GK) may provide improved plan quality when compared to Volumetric Modulated Arc Therapy (VMAT)-based stereotactic radiosurgery (SRS).

METHODS

For patients treated on GK SRS for brain metastases in 2018-2019 in our institution, 121 patients with two and three targets were identified. Twenty-six patients with two or three brain metastases (total of 76 lesions) were selected for this study. Two VMAT plans, SmartArc (Pinnacle) and HyperArc (Eclipse), were generated retrospectively for each patient. Plan quality was evaluated based on RTOG conformity index (CI), Paddick gradient index (GI), normal tissue (NT) V12Gy and V4.5Gy. By using the receiver operating characteristic (ROC) curve for both VMAT plans (SmartArc and HyperArc) and metrics of RTOG CI and NT V12Gy, we compared GK plans to SmartArc and HyperArc plans separately to determine the threshold volume.

RESULTS

For SmartArc plans, both ROC curve analyses showed a threshold volume of 0.4 cc for both CI and NT V12Gy. For HyperArc plans, the threshold volumes were 0.2 cc for the CI and 0.5 cc for NT V12Gy. GK plans produced improved dose distribution compared to VMAT for targets ≤0.4 cc, but HyperArc was found to have competing results with GK in terms of CI and NT V12Gy. For targets > 0.4 cc, both SmartArc and HyperArc showed better plan quality when compared to the GK plans.

CONCLUSIONS

Target volumes ≤0.4 cc may require a small cone size and sharp penumbra in GK while for target volumes >0.4 cc, VMAT-based SRS can provide improved overall plan quality and faster treatment delivery.

Dosimetric impact of rotational errors on the quality of VMAT-SRS for multiple brain metastases: Comparison between single- and two-isocenter treatment planning techniques

Purpose

In the absence of a 6D couch and/or assuming considerable intrafractional patient motion, rotational errors could affect target coverage and OAR‐sparing especially in multiple metastases VMAT‐SRS cranial cases, which often involve the concurrent irradiation of off‐axis targets. This work aims to study the dosimetric impact of rotational errors in such applications, under a comparative perspective between the single‐ and two‐isocenter treatment techniques.

Methods

Ten patients (36 metastases) were included in this study. Challenging cases were only considered, with several targets lying in close proximity to OARs. Two multiarc VMAT plans per patient were prepared, involving one and two isocenters, serving as the reference plans. Different degrees of angular offsets at various orientations were introduced, simulating rotational errors. Resulting dose distributions were evaluated and compared using commonly employed dose‐volume and plan quality indices.

Results

For single‐isocenter plans and 1^⁰ rotations, plan quality indices, such as coverage, conformity index and D_95%, deteriorated significantly (>5%) for distant targets from the isocenter (at> 4–6 cm). Contrarily, for two‐isocenter plans, target distances to nearest isocenter were always shorter (≤4 cm), and, consequently, 1^⁰ errors were well‐tolerated. In the most extreme case considered (2^⁰ around all axes) conformity index deteriorated by on‐average 7.2%/cm of distance to isocenter, if one isocenter is used, and 2.6%/cm, for plans involving two isocenters. The effect is, however, strongly associated with target volume. Regarding OARs, for single‐isocenter plans, significant increase (up to 63%) in D_max and D_0.02cc values was observed for any angle of rotation. Plans that could be considered clinically unacceptable were obtained even for the smallest angle considered, although rarer for the two‐isocenter planning approach.

Conclusion

Limiting the lesion‐to‐isocenter distance to ≤4 cm by introducing additional isocenter(s) appears to partly mitigate severe target underdosage, especially for smaller target sizes. If OAR‐sparing is also a concern, more stringent rotational error tolerances apply.

Intracranial Stereotactic Radiation Therapy With a Jawless Ring Gantry Linear Accelerator Equipped With New Dual Layer Multileaf Collimator

Purpose

To test the feasibility of a simplified, robust, workflow for intracranial stereotactic radiation therapy (SRT) using a ring gantry linear accelerator (RGLA) equipped with a dual-layer stacked, staggered, and interdigitating multileaf collimator.

Materials and Methods

Twenty recent clinical SRT cases treated using a radiosurgery c-arm linear accelerator were anonymized. From these data sets, a new planning workflow was developed and used to replan these cases, which then were compared to their clinical counterparts. Population-based dose-volume histograms were analyzed for target coverage and sparing of healthy brain. All plans underwent plan review and quality assurance and were delivered on an end-to-end verification phantom using image guidance to simulate treatment.

Results

The RGLA plans were able to meet departmental standards for target coverage and organ-at-risk sparing and showed plan quality similar to the clinical plans. RGLA plans showed increases in the 50% isodose in the axial plane but decreases in the sagittal and coronal planes. There were no statistically significant differences in the homogeneity index or number of monitor units between the 2 systems. There were statistically significant increases in conformity and gradient indices, with median values of 1.09 versus 1.11 and 2.82 versus 3.13, respectively, for the c-arm versus RGLA plans. These differences were not believed to be clinically significant because they met clinical goals. The population-based dose-volume histograms showed target coverage and organ-at-risk sparing similar to that of the clinical plans. All plans were able to meet the departmental quality assurance requirements and were delivered under image guidance on an end-to-end phantom with measurements agreeing within 3% of the expected value. RGLA plans showed a median reduction in delivery time of ≈50%.

Conclusions

This work describes a simplified and efficient workflow that could reduce treatment times and expand access to SRT to centers using an RGLA.

Optimization of MLC parameters for TPS calculation and dosimetric verification: application to single isocenter radiosurgery of multiple brain lesions using VMAT

Linac and MLC-based stereotactic radiosurgery (SRS) using single-isocenter-multiple-target (SIMT) VMAT has become increasingly popular in the management of multi-focal cranial metastases. However, significant geometrical and dosimetric challenges exist due to the typically small target volumes and in most cases, non-isocentric locations. To the best of our knowledge, there hasn't been a study in the optimization of MLC parameters, in the context of SIMT SRS, to ensure TPS calculation accuracy. In this work, we set out to optimize the dosimetric leaf gap (DLG) for the HD MLC installed on dedicated stereotactic Varian STx systems using a diverse group of 21 clinical SRS and SBRT plans. These plans featured a broad range of target sizes and target-to-isocenter distances that are typical of the stereotactic cases treated on these systems. Dose discrepancies between TPS calculations and verification measurements using a previously validated diode array Delta⁴ (ScandiDos) were minimized in a balanced manner to accommodate the variety of stereotactic plans. A DLG of 0.6 mm was found to be 'optimal' for the HD MLC and for the 'typical' plans treated on our STx systems. The finding was independently verified using commercially available 3D polymer gel dosimeter CrystalBall^TM (MGS Research Inc.). 3D verification for 6 SIMT SRS plans, consisted of 5 to 15 targets, achieved an average gamma score of 97.3% (σ = 2.0%) on 3%/2 mm criteria with a cutoff isodose level of 20%. We further examined the practice of routine dosimetric verifications including the selection of appropriate detectors and optimal gamma parameters. We found that the commonly used standard 3%/3 mm criteria would have resulted in all but 4 (out of 2840) clinical plans achieving a gamma score of 95% or better, and therefore, losing sensitivity to detect potential dosimetric discrepancies. Based on the characteristics of stereotactic plans, a more stringent distance-to-agreement parameter is needed.

Development of a rotational set-up correction device for stereotactic head radiation therapy: A performance evaluation

We developed a new head supporting device to provide accurate correction of rotational setup during image-guided radiation therapy (IGRT), evaluating its correction performance and the efficacy of dose distribution in stereotactic radiotherapy (SRT) using a helical tomotherapy (HT) system. The accuracy of rotational motion was measured using an electronic inclinometer; we compared device angles and measurement values from 0.0° to 3.0°. The correction accuracy was investigated based on the distance between rotational centers in the device and on megavoltage computed tomography (MVCT); the correction values were compared using distances in the range of 0.0-9.0 cm using a head phantom with a rotational error of 1.5°. For an SRT with a simultaneous integrated boost plan and a rotational error of 3.0° in yaw angle using a head phantom, and for a single-isocenter SRT for multiple brain metastases in the data of three patients, dosimetric efficacy of the HT unit was evaluated for calculated dose distributions with MVCT after rotational correction. This device can correct pitch and yaw angles within 0.3° and can be corrected to within 0.5° for each rotational angle according to the result of MVCT correction regardless of the rotational center position. In the head phantom study, the device had a beneficial impact on rotational correction; D99% for the target improved by approximately 10% with rotational correction. Using patient data with the device, the mean difference based on the treatment planning data was 0.3% for D99% and -0.1% for coverage index to the target. Our rotational setup correction device has high efficacy, and can be used for IGRT.

Characterization of a novel 3D printed patient specific phantom for quality assurance in cranial stereotactic radiosurgery applications

In single-isocenter stereotactic radiosurgery/radiotherapy (SRS/SRT) intracranial applications, multiple targets are being treated concurrently, often involving non-coplanar arcs, small photon beams and steep dose gradients. In search for more rigorous quality assurance protocols, this work presents and evaluates a novel methodology for patient-specific pre-treatment plan verification, utilizing 3D printing technology. In a patient's planning CT scan, the external contour and bone structures were segmented and 3D-printed using high-density bone-mimicking material. The resulting head phantom was filled with water while a film dosimetry insert was incorporated. Patient and phantom CT image series were fused and inspected for anatomical coherence. HUs and corresponding densities were compared in several anatomical regions within the head. Furthermore, the level of patient-to-phantom dosimetric equivalence was evaluated both computationally and experimentally. A single-isocenter multi-focal SRS treatment plan was prepared, while dose distributions were calculated on both CT image series, using identical calculation parameters. Phantom- and patient-derived dose distributions were compared in terms of isolines, DVHs, dose-volume metrics and 3D gamma index (GI) analysis. The phantom was treated as if the real patient and film measurements were compared against the patient-derived calculated dose distribution. Visual inspection of the fused CT images suggests excellent geometric similarity between phantom and patient, also confirmed using similarity indices. HUs and densities agreed within one standard deviation except for the skin (modeled as 'bone') and sinuses (water-filled). GI comparison between the calculated distributions resulted in passing rates better than 97% (1%/1 mm). DVHs and dose-volume metrics were also in satisfying agreement. In addition to serving as a feasibility proof-of-concept, experimental absolute film dosimetry verified the computational study results. GI passing rates were above 90%. Results of this work suggest that employing the presented methodology, patient-equivalent phantoms (except for the skin and sinuses areas) can be produced, enabling literally patient-specific pre-treatment plan verification in intracranial applications.

Dosimetric Effect of Rotational Setup Errors in Single-Isocenter Volumetric-Modulated Arc Therapy of Multiple Brain Metastases

AIM

The aim of this study is to investigate the dosimetric effects of rotational uncertainties in patient positioning on target coverage in stereotactic radiosurgery (SRS) of multiple brain metastases using single isocenter volumetric modulated arc therapy (VMAT).

MATERIALS AND METHODS

Ten SRS cases with multiple brain metastases (2-8) planned with single isocenter non-coplanar VMAT technique were investigated in this study. Pitch, yaw and roll displacements of 1, 3 and 5° were simulated about isocenter along each axes and nine plans were obtained for each case. Gross tumor volume (GTV) coverage obtained on simulated plans were compared with the original plan on four metrics of mean dose (Dmean), minimum dose to GTV (Dmin), dose to 95% of GTV (D95) and the volume covered by the 95% of the prescribed dose (V95).

RESULTS

At 1° rotation about 4% of the PTV had V95 and D95 values <95%. The minimum dose obtained across all PTV was 85.1% and there was not much change in the mean dose values obtained. The PTV volume which had V95 and D95 values <95% were in the range of 0.05 - 0.07cc and at a radial distance of 6.2 cm - 7.2 cm. At 3° rotation almost 50% of the PTV had V95 and D95 values <95%. The minimum dose obtained across all PTV was 48.3% and the mean dose reduced to as low as 78.8%. At 5° rotation almost 74% of the PTV had V95 and D95 values <95%. The minimum dose obtained across all PTV was 21.2% and the mean dose reduced to as low as 49.2%.

CONCLUSION

Our results indicate that correcting rotational uncertainties is critical in single-isocenter, multi-target SRS. For rotational deviations, radial distance of the target from isocenter along the respective axis has a strong influence on target coverage. For rotational setup deviation at a given radial distance larger targets tend to have lesser geometric miss compared to smaller targets. Mathematical model for spherical targets can be used to estimate V95 for given rotational errors.

Optimization based trajectory planning for real-time 6DoF robotic patient motion compensation systems

Purpose

Robotic stabilization of a therapeutic radiation beam with respect to a dynamically moving tumor target can be accomplished either by moving the radiation source, the patient, or both. As the treatment beam is on during this process, the primary goal is to minimize exposure of normal tissue to radiation as much as possible when moving the target back to the desired position. Due to the complex mechanical structure of 6 degree-of-freedom (6DoF) robots, it is not intuitive as to what 6 dimensional (6D) correction trajectory is optimal in achieving such a goal. With proportional-integrative-derivative (PID) and other controls, the potential exists that the controller may generate a trajectory that is highly curved, slow, or suboptimal in that it leads to unnecessary exposure of healthy tissue to radiation. This work investigates a novel feedback planning method that takes into account a robot’s mechanical joint structure, patient safety tolerances, and other system constraints, and performs real-time optimization to search the entire 6D trajectory space in each time cycle so it can respond with an optimal 6D correction trajectory.

Methods

Computer simulations were created for two 6DoF robotic patient support systems: a Stewart-Gough platform for moving a patient’s head in frameless maskless stereotactic radiosurgery, and a linear accelerator treatment table for moving a patient in prostate cancer radiation therapy. Motion planning was formulated as an optimization problem and solved at real-time speeds using the L-BFGS algorithm. Three planning methods were investigated, moving the platform as fast as possible (platform-D), moving the target along a straight-line (target-S), and moving the target based on the fastest descent of position error (target-D). Both synthetic motion and prior recorded human motion were used as input data and output results were analyzed.

Results

For randomly generated 6D step-like and sinusoidal synthetic input motion, target-D planning demonstrated the smallest net trajectory error in all cases. On average, optimal planning was found to have a 45% smaller target trajectory error than platform-D control, and a 44% smaller target trajectory error than target-S planning. For patient head motion compensation, only target-D planning was able to maintain a ≤0.5mm and ≤0.5deg clinical tolerance objective for 100% of the treatment time. For prostate motion, both target-S planning and target-D planning outperformed platform-D control.

Conclusions

A general 6D target trajectory optimization framework for robotic patient motion compensation systems was investigated. The method was found to be flexible as it allows control over various performance requirements such as mechanical limits, velocities, acceleration, or other system control objectives.

A study for the dosimetric evaluation of rotational setup error for lung stereotactic body radiation therapy

Purpose

To investigate the necessity of rotational shifts by considering dosimetric impact of rotational errors on stereotactic body radiation therapy (SBRT).

Materials and methods

20 lung patients with the lesion size <5 cm treated with SBRT have been selected for dosimetric analysis. Three-dimensional dose has been rotationally shifted (±1°, ±3°, ±5° for pitch, roll and yaw) and overlaid to the original computed tomography images. The dose–volume histograms of 18-rotational plans of each patient were compared to those of the original plan.

Results

No significant dosimetric differences were observed in target coverage. For all of the cases up to 5° in any couch angle dose differences of D99 and D95 were <3%. Variations of conformity index were observed to be less than 0·05. None of the organ at risk doses exceeded the dose limit. The V20 differences of the ipsilateral and the total lungs were less than 0·4%.

Conclusion

It has been found to be unnecessary to perform rotational shifts up to 5° for lung SBRT treatments; the translational shift is sufficient for the cases used in this study. This method may be applied and tested after planning and before treatment initiation to rule out exceptionally extreme cases.

Towards frameless maskless SRS through real-time 6DoF robotic motion compensation

Stereotactic radiosurgery (SRS) uses precise dose placement to treat conditions of the CNS. Frame-based SRS uses a metal head ring fixed to the patient's skull to provide high treatment accuracy, but patient comfort and clinical workflow may suffer. Frameless SRS, while potentially more convenient, may increase uncertainty of treatment accuracy and be physiologically confining to some patients. By incorporating highly precise robotics and advanced software algorithms into frameless treatments, we present a novel frameless and maskless SRS system where a robot provides real-time 6DoF head motion stabilization allowing positional accuracies to match or exceed those of traditional frame-based SRS. A 6DoF parallel kinematics robot was developed and integrated with a real-time infrared camera in a closed loop configuration. A novel compensation algorithm was developed based on an iterative closest-path correction approach. The robotic SRS system was tested on six volunteers, whose motion was monitored and compensated for in real-time over 15 min simulated treatments. The system's effectiveness in maintaining the target's 6DoF position within preset thresholds was determined by comparing volunteer head motion with and without compensation. Comparing corrected and uncorrected motion, the 6DoF robotic system showed an overall improvement factor of 21 in terms of maintaining target position within 0.5 mm and 0.5 degree thresholds. Although the system's effectiveness varied among the volunteers examined, for all volunteers tested the target position remained within the preset tolerances 99.0% of the time when robotic stabilization was used, compared to 4.7% without robotic stabilization. The pre-clinical robotic SRS compensation system was found to be effective at responding to sub-millimeter and sub-degree cranial motions for all volunteers examined. The system's success with volunteers has demonstrated its capability for implementation with frameless and maskless SRS treatments, potentially able to achieve the same or better treatment accuracies compared to traditional frame-based approaches.

The spatial accuracy of two frameless, linear accelerator-based systems for single-isocenter, multitarget cranial radiosurgery

Single‐isocenter, multitarget cranial stereotactic radiosurgery (SRS) is more efficient than using an isocenter for each target, but spatial positioning uncertainties can be magnified at locations away from the isocenter. This study reports on the spatial accuracy of two frameless, linac‐based SRS systems for multitarget, single‐isocenter SRS as a function of distance from the isocenter. One system uses the ExacTrac platform for image guidance and the other localizes with cone beam computed tomography (CBCT). For each platform, a phantom with 12 target BBs distributed up to 13.8 cm from the isocenter was aligned starting from five different initial offsets and then imaged with the treatment beam at seven different gantry and couch angles. The distribution of the resulting positioning errors demonstrated the value of adding a 1‐mm PTV margin for targets up to about 7–8 cm from the isocenter. For distances 10 cm or more, the CBCT‐based alignment remained within 1.1 mm while the ExacTrac‐based alignment differed by up to 2.2 mm.

Single-Isocenter Multiple-Target Stereotactic Radiosurgery: Risk of Compromised Coverage

PURPOSE

To determine the dosimetric effects of rotational errors on target coverage using volumetric modulated arc therapy (VMAT) for multitarget stereotactic radiosurgery (SRS).

METHODS AND MATERIALS

This retrospective study included 50 SRS cases, each with 2 intracranial planning target volumes (PTVs). Both PTVs were planned for simultaneous treatment to 21 Gy using a single-isocenter, noncoplanar VMAT SRS technique. Rotational errors of 0.5°, 1.0°, and 2.0° were simulated about all axes. The dose to 95% of the PTV (D95) and the volume covered by 95% of the prescribed dose (V95) were evaluated using multivariate analysis to determine how PTV coverage was related to PTV volume, PTV separation, and rotational error.

RESULTS

At 0.5° rotational error, D95 values and V95 coverage rates were ≥95% in all cases. For rotational errors of 1.0°, 7% of targets had D95 and V95 values <95%. Coverage worsened substantially when the rotational error increased to 2.0°: D95 and V95 values were >95% for only 63% of the targets. Multivariate analysis showed that PTV volume and distance to isocenter were strong predictors of target coverage.

CONCLUSIONS

The effects of rotational errors on target coverage were studied across a broad range of SRS cases. In general, the risk of compromised coverage increased with decreasing target volume, increasing rotational error and increasing distance between targets. Multivariate regression models from this study may be used to quantify the dosimetric effects of rotational errors on target coverage given patient-specific input parameters of PTV volume and distance to isocenter.