Browsing by Subject "SRS"

Abstract

Purpose: Updated recommendations for pre-treatment QA of patient-specific intensity modulated radiation therapy (IMRT) and Volumetric modulated arc therapy (VMAT) quality assurance (QA) were recently published by the AAPM task group TG-218. While the traditionally most common QA analysis is to use a Gamma index with dose & spatial analysis criteria of 3% & 3mm, respectively, TG-218 recommends a tighter spatial tolerance of 2mm for standard IMRT QA, and that even tighter tolerances should be considered for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Our purpose is to report our experience with applying the TG-218 recommendations to a large clinical SRS and SBRT program. In addition, a new SRS technique was recently developed at Duke, called Conformal Arc Informed Volumetric Modulated Arc Therapy (CAVMAT), which is designed to be less sensitive to configuration and delivery errors. We measured the agreement of CAVMAT for pre-treatment QA and compared it to the current standard (VMAT) to evaluate whether CAVMAT is more robust to delivery errors than VMAT.

Methods: We re-analyzed the pre-treatment QA with respect to the TG-218 recommendations. For Portal Dosimetry (Varian Medical Systems, Palo Alto, CA), this included IMRT brain (n=25) and SBRT / hypofractionated image guided radiotherapy (HIGRT) cases that utilize flattened photon beams (n=18). For Delta4 (ScandiDos, Madison, WI) this included single target SRS (n=24), multiple target SRS (n=25), SBRT cases using VMAT (n=74), and SBRT cases using IMRT with FFF photons (n=23). For ArcCHECK (Sun Nuclear, Melbourne, FL)), we take 25 single target VMAT SRS cases and 25 multiple target VMAT SRS cases. For SRS MapCHECK(Sun Nuclear, Melboume, FL), we analyze 10 multiple target VMAT SRS cases with 16 targets. A Gamma analysis was performed with 6 spatial/dose criteria combinations: 3%/3mm, 3%/2mm, 3%/1mm, 2%/1mm, 4%/1mm, 5%/1mm. We then calculated the TG-218 action limit and tolerance limit per plan type and compared to the “universal” TG-218 action limit of 90% having a Gamma <1.

To compare CAVMAT and VMAT, log file analysis and pre-treatment QA was performed for 10 patients with 20 plans (10 VMAT, 10 CAVMAT) with 46 targets in total. 10 VMAT plans were re-planned using CAVMAT, and the dosimetric effect due to treatment delivery errors was quantified for V6Gy, V12Gy, and V16Gy of healthy brain along with the maximum, average and minimum doses of each target. Gamma analysis of VMAT and CAVMAT plans was performed using Delta4 and SRS MapCHECK with 3% / 1mm, 2% / 1mm, 1% / 1mm criteria to assess the agreement during patient specific quality assurance.

Result: For Portal Dosimetry QA of IMRT brain and SBRT/HIGRT using a 3%/1mm criteria, the TG-218 action limit was 99.68, and 90.14, respectively; with 3.68% and 3.68% of cases failing the universal 90% criteria. For Delta4 QA of single target SRS, multiple target SRS, and SBRT IMRT with FFF using a 3%/1mm criteria, the TG-218 action limit was 93.64, 97.12, and 92.01, respectively; with 0%, 0%, and 0% of cases failing the universal 90% criteria. For Delta4 QA of SBRT VMAT using a 4%/1mm criteria, the TG-218 action limit was 94.47, with 100% passing. For ArcCHECK QA of single target and multiple target SRS VMAT using a 3%/2mm criteria, the TG-218 action limit was 98.06 and 96.59 respectively, with 100% passing. For SRS MapCHECK QA of multiple target SRS VMAT cases using 3%1mm criteria, the TG-218 action limit was 99.24 with 100% passing.

The average increase in V6Gy, V12Gy, V16Gy due to treatment delivery errors as quantified using the trajectory logfile was 0.94 ± 1.43, 0.90 ± 1.38%, and 1.23 ± 1.54% respectively for VMAT, and 0.035 ± 0.14%, 0.14 ± 0.18%, and 0.28 ± 0.24% for CAVMAT. The average change to target maximum, average, and minimum dose due to delivery errors was 0.53 ± 0.46%, 0.52 ± 0.46%, and 0.53 ± 0.56%, for VMAT, and 0.16  0.18%, 0.11  0.08%, and 0.03  0.24% for CAVMAT. There was no significant difference in magnitude of MLC discrepancies during delivery for VMAT and CAVMAT. For Gamma analysis with strict 1% / 1mm criteria, the average passing rate of VMAT gamma analysis is 94.53 ± 4.42%, while that of CAVMAT is 99.28 ± 1.74%.

Conclusion: For most QA devices, spatial tolerance of pre-treatment QA for SRS/SBRT can be tightened to 1mm while still maintaining an in-control QA process. The gamma criteria to 3%/1mm for all SRS cases and SBRT with IMRT and transitioning to a 4%1mm criteria for SBRT with VMAT have a spatial tolerance that is appropriate for the radiotherapy technique while not resulting in an excessive false positive failure rate. The CAVMAT treatment planning technique resulted in superior gamma analysis passing rate for each gamma analysis criteria.

Purpose:

A new development in linac-based intracranial stereotactic radiosurgery (SRS) and extracranial stereotactic body radiation therapy (SBRT) is treatment of multiple targets using single isocenter volumetric modulated arc therapy (VMAT) technique, dramatically reducing treatment time while maintaining high target conformality and steep dose gradients between targets and surrounding organs at risk (OAR). In VMAT, the gantry rotates around the patient while continuously delivering radiation. Throughout the VMAT arc, the beam is modulated based on an inverse optimization algorithm in order to spare organs at risk. Single isocenter multi-target VMAT has already been implemented for intracranial SRS and is increasingly used for extracranial SBRT treatments. Despite the increasing popularity of this technique, certain inherent clinically meaningful challenges warrant further investigation. Specifically, single isocenter, multifocal SRS and SBRT can result in small volumes targets with a large off-axis distance from the treatment isocenter. Consequently, angular errors in the collimator, patient support assembly (PSA), or gantry could have an increased impact on target coverage, warranting a re-evaluation of routine linear accelerator QA tolerance recommendations in TG-142. Also, questions have arisen regarding the ability of clinical dose calculation algorithms to calculate dose accurately for these cases at large off-axis distances. Specifically, it is questionable whether or not the MLC model used is sophisticated enough to accurately model the dose off axis. This is of concern because the MLC leaves have different dimensions outside of the HD region, but the dosimetric-leaf-gap model used is the same for both regions. Applying the single isocenter technique to extracranial SBRT of oligometastases introduces additional unique challenges that must be addressed. These include greater intra and inter-fractional setup uncertainties, and dosimetric interplay since immobilization is more difficult and internal motion is non-negligible. The purpose of this thesis is to explore these specific physics and treatment planning considerations for single isocenter multi-target intracranial radiosurgery and extracranial SBRT.

Materials and Methods:

Intracranial SRS

For single isocenter multifocal stereotactic radiosurgery, we evaluated potential dose deviations from mechanical errors in PSA, collimator, and gantry angle within the tolerance recommended by TG-142 for radiosurgery machines. Systematic errors in PSA, collimator, and gantry angle were introduced at the recommended tolerance levels into both multifocal SRS plans and traditional single target SRS using dynamic conformal arcs, and the resulting dosimetric effect were quantified within the treatment planning system. In addition, we quantified the accuracy of the treatment planning system dose calculation algorithm for targets located at large off-axis distances with 3D Slicer analysis software. The dose distribution from the treatment planning system was compared to the distribution measured using a high-resolution 3D dosimetry system (PRESAGE®/Optical-CT). Comparisons were made using DVH and gamma analysis (criteria: 2mm/3%, 3mm/3%, and 3mm/5%). DVH comparisons included a shell analysis in which we compared the dose from 2mm within the target to the target’s surface, the surface to 2mm outside the target, and 2mm to 4mm outside the target for both the plan and the dosimeter.

Extracranial SBRT

For applying the single isocenter multifocal technique to extracranial oligometastases, we propose a treatment method that addresses intra and inter-fractional motion as well as dosimetric interplay. The developed technique uses a Single Isocenter with Distinct Optimizations (SIDO) in which all Volumetric Modulated Arc Therapy (VMAT) fields share an isocenter but each field treats only one target. When necessary, setup uncertainties from rotations and deformations are mitigated by applying a couch translation between VMAT arcs, and interplay is minimized using dynamic conformal arcs (DCAs) as the starting point for inverse optimization. We evaluated this planning technique using relevant dose indices including conformity index, gradient index and modulation factor.

Results:

Intracranial SRS

Induced errors at TG-142 tolerance levels showed the greatest change in multifocal SRS target coverage for collimator and gantry rotations, while minimal change in coverage was noted for errors in PSA rotation. For single isocenter cases, the largest dose discrepancies were a result of 1° errors in the collimator and gantry angles, specifically with respect to the volume of the PTV receiving the prescription dose. These errors caused up to 33% and 18% deviations, respectively, to the volume of the PTV receiving the prescription dose with mean deviations of 5% and 2%, respectively. When the collimator and gantry errors in single isocenter plans were reduced to 0.5°, the discrepancies in the volume of the PTV receiving the prescription dose were reduced to a max value of <5% for the gantry and collimator with mean values of approximately 1%. For 1° errors in DCA plans, however, deviations to the volume of the PTV receiving the prescription dose did not exceed 5% for the collimator, couch, or gantry and similar results were seen in all other dosimetric indices investigated.

A preliminary analysis of the Eclipse dose calculation algorithm in comparison to actual dose delivered to targets shows agreement with 89.46%, 94.87%, and 96.39% of voxels having a passing gamma index with criteria of 2mm/3%, 3mm/3%, and 3mm/5% respectively (distance from isocenter ranged from 0-10cm). Targets within 8cm of the isocenter showed less than 2% discrepancy between the plan and measurement with respect to the percent of the target receiving the prescription dose. The target at 10cm from the isocenter, however, had a 15% discrepancy between the plan and measurement with respect to the percent of the target receiving the prescription dose and therefore warrants further investigation.

Extracranial SBRT

As the distance between targets increases, the probability for requiring a second translation between treatment arcs also increases. Assuming a margin of 5mm and considering six lung and five liver patient cases, a second translational shift would be required 0%, ~10%, and ~25% of the time for target separations of 5cm, 10cm, and 15cm respectively.

For greater than 3cm separation between targets in extracranial SBRT, SIDO and SIDO with DCA have an average conformity index of 0.862, and 0.864 respectively, which is comparable to the average conformity of traditional multifocal treatment techniques at these target separations of 0.901. When separation between PTVs is less than 3cm, however, traditional single isocenter VMAT has superior conformity with a mean value of 0.875, as opposed to 0.772 and 0.782 for SIDO and SIDO with DCA respectively; and decreasing conformity with decreasing target separation. SIDO with DCA had superior GI over all other planning techniques for almost all cases with a mean value of 7.31 across all target separations. SIDO with DCA even performed better than the DCA technique with a mean GI of 7.43 across all target separations, which was assumed to be the best method for obtaining a desirable GI. SIDO with DCA had a comparable MF to the DCA plans and was closer to 1 than all other planning techniques. The mean MF values across all target separations for SIDO with DCA and DCA were 1.17 and 0.83 respectively.

Conclusions:

Intracranial SRS

Institutions utilizing a single isocenter VMAT technique for multifocal disease should pay careful attention to the angular mechanical tolerances in designing a robust and complete QA program, especially with respect to the collimator and gantry recommended tolerances. We recommend reducing collimator and gantry tolerances from 1.0° to 0.5° to decrease the potential magnitude of deviations between the planning and delivered dose distributions. The PRESAGE®/Optical-CT 3D dosimetry system verified the accuracy of the Eclipse dose calculation algorithm to within 2% for targets located up to 8cm from the isocenter. Further investigation is required for more distal targets, as they did not have sufficient agreement.

Extracranial SBRT

A single isocenter approach for SBRT treatment of extracranial oligometastases may be feasible using the proposed SIDO and SIDO with DCA treatment planning techniques. SIDO for extracranial oligometastases allows flexibility to mitigate spatial uncertainties from rotation and deformation, and has comparable dosimetry to traditional VMAT with low modulation when inverse optimization begins with DCAs. These advantages make SIDO beneficial for target separations of greater than 3cm, however, for target separations less than 3cm a traditional single isocenter technique is more appropriate.

Purpose: Linac based radiosurgery to multiple metastases is commonly planned with VMAT as it effectively achieves high conformality to complex target arrangements. However, as the number of targets increases, VMAT may struggle to block between targets, which may lead to highly modulated and/or nonconformal MLC trajectories. This phenomenon is particularly apparent in multi-target geometries as targets often share the same MLC leaf pair, creating an MLC opening and yielding insufficient inter-target collimation. Given the complex geometries, multi-target SRS necessitates high degrees of dosimetric accuracy. Dosimetric accuracy may be impacted by beam commissioning as a single dosimetric leaf gap (DLG) must be selected and used for all targets, geometries, and MLCs. Conformal Arc Informed VMAT (CAVMAT) aims to reduce healthy tissue dose by utilizing simplified MLC trajectories and by producing more conformal dose distributions. It is hypothesized that the simplified leaf motion and reduced complexity of CAVMAT may reduce sensitivity to commissioning and treatment delivery uncertainty.

Materials & Methods: CAVMAT is a hybrid treatment planning technique which combines the conformal MLC trajectories of dynamic conformal arcs with the MLC modulation and versatility of inverse optimization. CAVMAT has three main steps. First, targets are assigned to subgroups to maximize MLC blocking between targets. Second, arc weights are optimized to achieve the desired target dose, while minimizing MU variation between arcs. Third, the optimized conformal arc plan serves as the starting point for limited inverse optimization to improve dose conformity to each target. Twenty multifocal VMAT cases were re-planned with CAVMAT with 20Gy applied to each target. The total volume receiving 2.5 Gy, 6 Gy, 12 Gy, and 16 Gy, conformity index, treatment delivery time, and the total MU were used to compare the VMAT and CAVMAT plans. Of the 20 VMAT plans, 10 were selected and replanned with CAVMAT, at DLG values of 0.4 mm, 0.8 mm, and 1.2 mm and the change in V6Gy [cc], V12Gy [cc], V16Gy [cc], and target dose was quantified. The 10 VMAT and CAVMAT plans were delivered to a Delta4 QA phantom and dose agreement was quantified using gamma index with 3%/1mm, 2%/1mm, and 1%/1mm criteria. Trajectory log files were collected and analyzed to quantify MLC positioning errors during delivery. 16 targets were selected, with at least one target from each plan, and were delivered to an SRS Mapcheck QA phantom to evaluate dose difference per DLG.

Results: After replanning the 20 VMAT cases, CAVMAT reduced the average V2.5Gy[cc] by 25.25±19.23%, V6Gy[cc] by 13.68±18.97%, V12Gy[cc] by 11.40±19.44%, and V16Gy[cc] by 6.38±19.11%. CAVMAT improved conformity by 3.81±7.57%, while maintaining comparable target dose. MU for the CAVMAT plans increased by 24.35±24.66%, leading

to an increased treatment time of ~2 minutes. For the DLG analysis, the 10 VMAT plans

demonstrated an average sensitivity to variation of V6Gy [cc], V12Gy [cc], V16Gy [cc] of 35.83 ± 9.48%/mm,

34.12 ± 6.61%/mm, and 39.22 ± 8.41%/mm, respectively, compared to 23.18 ± 4.53 %/mm, 22.45± 4.28 %/mm, and 24.88 ± 4.91 %/mm for CAVMAT. VMAT was found to be roughly twice as sensitive as CAVMAT to changes in target doses for a varying DLG. For the plans delivered to the Delta4 CAVMAT demonstrated an improved dose agreement, with the strictest criteria of 1%/1mm resulting in a passing rate of 94.53 ± 4.42% for VMAT compared to 99.28 ± 1.74% for CAVMAT. Log file analysis demonstrated CAVMAT’s improved resistance to treatment delivery uncertainties, though the difference compared to VMAT is not expected to be substantial.

Conclusions: The CAVMAT technique successfully eliminated insufficient MLC blocking between targets prior to the inverse optimization, leading to less complex treatment plans and improved tissue sparing. CAVMAT is more robust to dosimetric and treatment delivery uncertainties and better maintains target dose and healthy tissue sparing. The reduced complexity, inherent tissue sparing, improved conformity, and reduced sensitivity to uncertainties indicates CAVMAT to be a promising method to treat brain metastases.

Objective

Methods

Results

Conclusions

Abstract

Purpose

Single-isocenter multi-target (SIMT) Volumetric Modulated Arc Therapy (VMAT) technique can produce highly conformal dose distributions and short treatment delivery times for the treatment of multiple brain metastases. SIMT radiosurgery using VMAT is primarily limited to linear accelerators utilizing 2.5mm leaf width MLCs. We explore feasibility of applying this technique to linear accelerators utilizing MLCs with leaf width of 5mm to broaden the applicability of SIMT radiosurgery using VMAT to include the greater number of linear accelerators with standard 5mm MLCs.

Methods

Twenty patients with 3-10 intracranial brain metastases originally treated with 2.5 mm leaf width MLCs were re-planned using standard 5mm leaf width MLCs and the same treatment geometry (3-5 VMAT arcs). Conformity index, low (V30%), and moderate (V50%) isodose spill were selected for analysis. V12Gy was also analyzed for single fraction cases. We tested the effects of several strategies to mitigate degradations of dose quality values when 5 mm leaf width MLCs were used; these included duplicating each VMAT arc with altered collimator angles by 10°, 15°, and 90°, and adding 1-2 VMAT arcs, with all arcs equally spaced.

Results

Wider MLCs caused small changes in total MUs (5827±2334 vs 5572±2220, p=.006), and Conformity Index (CI) (2.22%±0.05%, p=.045), but produced more substantial increases in brain V30%[%] and V50%[%] (27.75%±0.16% and 20.04%±0.13% respectively, p < .001 for both), and V12Gy[cc] (16.91%±0.12%, p < .001).

Conclusion

SIMT radiosurgery delivered via VMAT using 5mm leaf width MLCs can achieve similar CI compared to that using 2.5mm leaf width MLCs but with moderately increased isodose spill, which can be only partially mitigated by increasing the number of VMAT arcs.