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OBJECTIVES: The aim of this study was to determine the number of trade-off explored (TO) library plans required for building a RapidPlan (RP) library that would generate the optimal clinical treatment plan. METHOD: We developed two RP models, one each for the two clinical sites, head neck (HN) and cervix. The models were created using 100 plans and were validated using 70 plans (VP) for each site respectively. Each of the two libraries comprising 100 TO plans were divided into five different subsets of library plans comprising of 20, 40, 60, 80, and 100 plans, leading to five different RP model for each site. For every validation patient, a TO plan (TO_VP) was created. For every patient, five RP-plans were automatically generated using RP models. The dosimetric parameters of the six plans (TO_VP + five RP-plans) were compared using Pearson correlation and Greenhouse-Geisser analysis. RESULTS: PTV D95% in six competing plans varied between 97.6±0.7% and 98.1±0.6% in HN cases and 98.8±0.3% and 99.0±0.4% in cervix cases. Overall, for both sites the mean variations in OAR doses or volumes were within 50cGy, 0.5% and 0.2cc between library plans, and if TO_VP was included the variations deteriorated to 180 cGy, 0.4% and 15cc. All OARs in both sites, except D0.1 ccspine, showed a statistically insignificant variation between all plans. CONCLUSION: Dosimetric variation among various output plans generated from five RapidPlan libraries is minimal and clinically insignificant. The optimal output plan can be derived from the least weighted library consisting of 20 plans. ADVANCES IN KNOWLEDGE: This article shows that, when the constituent plans are subjected to trade-off exploration, the number of constituent plans for a knowledge-based planning module is not relevant in terms of its dosimetric output.
RESUMO
PURPOSE: The purpose of this study is to evaluate the dosimetric and treatment delivery characteristics of volumetric modulated arc therapy technique (VMAT)-based craniospinal axis irradiation (CSI) between ring gantry Halcyon (HAL) and C-arm based Novalis Tx (NTx) linear accelerator. Set-up margin and treatment delivery time for both machines were also taken into account. MATERIALS AND METHODS: Fifteen patients, 4 females and 11 males treated between March 2019 and February 2022 within the age group 4-56 years simulated in the supine position and were planned for multiple isocentre VMAT technique in ring gantry Halcyon and C-Arm Novalis linear accelerator for 6FFF and 6 MV flatten beam energy. The number of isocenters was the same in both the machines, usually three for adult adolescent age group patients and two for pediatric patients. Total on-couch time and the patient positional shift were captured for each isocenter during each session of treatment. Margins were calculated using Herk's formula of margin = 2.5Σ +0.7σ. Dosimetry, on-couch time, and set-up margin were compared between two competing arms. RESULTS: Ninety-five percent of PTV coverage (P = 0.333), volume receiving 107% (P = 0.676), total MU (P = 0.818) in both the arms were comparable and statically insignificant. Low-dose spillage such as D20% (P = 0.212) and D50% (P = 0.008) was lesser in HAL comparable to NTx. CI and HI were statically insignificant. Out of 26 organs at risk (OAR), only 3 organs showed a statically significant dose difference. The mean and maximum setup margin in any linear direction was 0.45 and 0.53 cm for HAL and 0.37 and 0.56 cm for NTx and, variation was statistically insignificant (0.23 < P < 0.47). On-couch time was 4.0 ± 5.5 min lesser for HAL and the difference in on-couch time between the two arms was statistically different. CONCLUSION: Even though the majority of the delivery parameters such as gantry speed, dose rate, beam characteristic (flatten or unflatten), MLC width, and speed between the ring gantry HAL and C-arm NTx linear accelerators were distinctly different, they offered no or minimal difference in the dose distribution and in the setup margin. HAL gives a faster treatment time delivery, which could be crucial for some selective cases such as patients receiving treatment under general anesthesia.
