Application of piecewise VMAT technique to whole-brain radiotherapy with simultaneous integrated boost for multiple metastases

Optimization method for determining vertices in lattice radiotherapy

Purpose

This study presents an optimization method for arranging lattice radiotherapy (LRT) targets to enhance the contrast between peak and valley doses, aiming to improve the treatment effectiveness and precision.

Materials and methods

The LRT target comprises multiple sphere-like vertices generated using the optimization method, which involves four steps: 1) generating a volume for vertex arrangement, 2) determining initial positions and size of packing units, 3) determining initial positions and size of all the vertices and 4) optimizing the final vertex positions by using adaptive simulated annealing (ASA). Volumetric modulated arc therapy plans were retrospectively regenerated using the initial vertices produced by closest packing (Plan_Clo) and vertices obtained after ASA optimization (Plan_Opt). The peak-to-valley index (PVI) that characterizes the difference between peak and valley doses was introduced to evaluate the performance.

Results

A statistically significant difference was observed in the average PVI between Plan_Clo and Plan_Opt (p = 0.000). The average PVI ratio for Plan_Opt compared to Plan_Clo was 5.95 ± 4.87 (range: 1.24-16.80).

Conclusion

The proposed optimization method for determining LRT target vertices has been validated, demonstrating a significant improvement in the PVI. ASA optimization, combined with closest packing, effectively enhanced the peak-to-valley dose difference in LRT, showcasing its potential for advancing treatment planning.

A dosimetric comparison of non-coplanar volumetric modulated arc therapy and non-coplanar fixed field intensity modulated radiation therapy in hippocampus-avoidance whole-brain radiation therapy with a simultaneous integrated boost for brain metastases

Objective

The aim of this study was to investigate the dosimetric differences between non-coplanar volumetric modulated arc therapy (VMAT) and non-coplanar fixed-field intensity-modulated radiotherapy (IMRT) in hippocampus-avoidance whole-brain radiation therapy with a simultaneous integrated boost (HA-WBRT+SIB) for brain metastases using the Monaco treatment planning system (TPS).

Method

A total of 22 patients with brain metastases were retrospectively enrolled. Two radiotherapy treatment plans were designed for each patient: non-coplanar VMAT and non-coplanar fixed field IMRT. The dose distribution of targets and organs at risk (OAR), the number of monitor units (MUs), and pre-treatment plan verification were compared between the two plans while meeting the prescribed dose requirements of the target volume.

Results

There were no significant differences in V50, V55, Dmax, heterogeneity index (HI) and conformity index (CI) of target PGTV between the two plans (p>0.05). For PTV-brain-SIB, there was no significant difference in D98% between IMRT and VMAT (p=0.103). VMAT significantly improved the V30 of PTV-brain-SIB (p<0.001), decreased HI (p=0.003), and increased CI (p<0.001). There were no significant differences in the Dmax to the brain stem, left and right lens, optic chiasm, pituitary gland, and left and right hippocampus between the two plans (p>0.05). Compared with IMRT, VMAT significantly reduced the Dmax to the left and right eyes (p<0.001) and significantly increased the Dmax to the right inner ear (p=0.010). There was no significant difference in the Dmax to the left inner ear between VMAT and IMRT (p=0.458). Compared with IMRT, VMAT significantly reduced the Dmax to the left optic nerve (p=0.006), but significantly increased the Dmax to the right optic nerve (p=0.001). There was no significant difference in the Dmax to the left and right hippocampus between VMAT and IMRT (p>0.05), but VMAT significantly increased the D100% (p<0.05) compared with IMRT. Compared with VMAT, IMRT significantly reduced the MU (p<0.001) but VMAT has a higher treatment efficiency than IMRT, with an average reduction of 41 seconds (294.1 ± 16.4 s for VMAT, 335.8 ± 34.9 s for IMRT, p<0.001). Under the conditions of 3%/2 mm, and 2%/2 mm, the gamma passing rate of the IMRT QA was improved compared to VMAT, with an average increase of 0.6%, p=0.013, and 1.7%, p<0.001, respectively.

Conclusion

Both non-coplanar VMAT and non-coplanar fixed field IMRT based on the Monaco TPS produce clinically acceptable results for HA-WBRT+SIB in patients with brain metastases. Compared with IMRT, VMAT has better dose distribution in the target volume and treatment efficiency, but IMRT can better protect the hippocampus and reduce the number of MUs.

Impact of delivery time factor on treatment time and plan quality in tomotherapy

Delivery time factor (DTF) is a new parameter introduced by the RayStation treatment planning system for tomotherapy treatment planning. This study investigated the effects of this factor on various tomotherapy plans. Twenty-five patients with cancer (head and neck, 6; lung, 9; prostate, 10) were enrolled in this study. Helical tomotherapy plans with a field width of 2.5 cm, pitch of 0.287, and DTF of 2.0 were created. All the initial plans were recalculated by changing the DTF parameter from 1.0 to 3.0 in increments of 0.1. Then, DTF's impact on delivery efficiency and plan quality was evaluated. Treatment time and modulation factor increased monotonically with increasing DTF. Increasing the DTF by 0.1 increased the treatment time and modulation factor by almost 10%. This relationship was similar for all treatment sites. Conformity index (CI), homogeneity index, and organ at risk doses were improved compared to plans with a DTF of 1.0, except for the CI in the lung cancer case. However, the improvement in most indices ceased at a certain DTF; nevertheless, treatment time continued to increase following an increase in DTF. DTF is a critical parameter for improving the quality of tomotherapy plans.

Treatment plan prescreening for patient-specific quality assurance measurements using independent Monte Carlo dose calculations

PURPOSE

This study proposes a method to identify plans that failed patient-specific quality assurance (QA) and attempts to establish a criterion to prescreen treatment plans for patient-specific QA measurements with independent Monte Carlo dose calculations.

MATERIALS AND METHODS

Patient-specific QA results measured with an ArcCHECK diode array of 207 patients (head and neck: 25; thorax: 61; abdomen: 121) were retrospectively analyzed. All patients were treated with the volumetric modulated arc therapy (VMAT) technique and plans were optimized with a Pinnacle v16.2 treatment planning system using an analytical algorithm-based dose engine. Afterwards, phantom verification plans were designed and recalculated by an independent GPU-accelerated Monte Carlo (MC) dose engine, ArcherQA. Moreover, sensitivity and specificity analyzes of gamma passing rates between measurements and MC calculations were carried out to show the ability of MC to monitor failing plans (ArcCHECK 3%/3 mm,<90%), and attempt to determine the appropriate threshold and gamma passing rate criterion utilized by ArcherQA to prescreen treatment plans for ArcCHECK measurements. The receiver operator characteristic (ROC) curve was also utilized to characterize the performance of different gamma passing rate criterion used by ArcherQA.

RESULTS

The thresholds for 100% sensitivity to detect plans that failed patient-specific QA by independent calculation were 97.0%, 95.4%, and 91.0% for criterion 3%/3 mm, 3%/2 mm, and 2%/2 mm, respectively, which corresponded to specificities of 0.720, 0.528, and 0.585, respectively. It was shown that the 3%/3 mm criterion with 97% threshold for ArcherQA demonstrated perfect sensitivity and the highest specificity compared with other criteria, which may be suitable for prescreening treatment plans treated with the investigated machine to implement measurement-based patient-specific QA of patient plans. In addition, the area under the curve (AUC) calculated from ROC analysis for criterion 3%/3 mm, 3%/2 mm, and 2%/2 mm used by ArcherQA were 0.948, 0.924, and 0.929, respectively.

CONCLUSIONS

Independent dose calculation with the MC-based program ArcherQA has potential as a prescreen treatment for measurement-based patient-specific QA. AUC values (>0.9) showed excellent classification accuracy for monitoring failing plans with independent MC calculations.