ESTRO 2024 - Abstract Book

S5954

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

Purpose/Objective:

MCO in radiotherapy allows for optimal treatment planning through achieving a balance between treatment targets and organs-at-risks (OARs). Previous studies demonstrated its benefit for improving planning-target-volume (PTV) coverage and OARs sparing [1, 2]. However, discrepancy between navigated and deliverable plans limits its application [3]. The purpose of this study was to determine the discrepancy between the navigated and deliverable plans of the Eclipse and RayStation MCO with dosimetric comparison of their deliverable plans.

Material/Methods:

Twenty-five prostate cancer cases were selected for volumetric modulated arc therapy (VMAT) planning. All treatment plans were created with MCO and same planning parameters, including the number of field and collimator angle on Eclipse version 15.6.04 (Varian Medical Systems) and RayStation version 12A (RaySearch Laboratories) planning systems. Planning objectives followed the in-house protocol with minor adjustments for specific patient anatomy. The navigated and deliverable plans were generated using the MCO module. Two major conflicting planning objectives were chosen, namely PTV76 D98% and rectum D50%, resulting in two-objective trade-off during MCO navigation. Five sets of data in the navigated and deliverable plan were collected for each patient with a total number of 125 sets of data for each planning system. The discrepancy was evaluated using cumulative dose volume histogram (DVH) and pareto front. Plan in middle of the pareto curve was selected for dosimetric comparison. Mean corresponding doses for PTV and OARs, conformity index (CI), and heterogeneity index (HI) of the two systems were compared through two-tailed t-test with P value <0.05 to indicate statistical significance. Our results show the two systems had statistically significant percentage differences (P<0.001) of mean PTV76 D98% (Eclipse: 0.887% versus RayStation: 3.56%) and rectum D50% (Eclipse: -2.49% versus RayStation: -1.96%). Although RayStation shows a greater discrepancy in PTV D98%, it had better dose coverag in final deliverable plan compared to navigated plans. Similarly, Eclipse MCO had greater discrepancies in rectal dose sparing but with a more clinically favorable outcome, i.e., better rectal dose sparing in the final deliverable plan compared to the navigated plans. Pareto front shows the discrepancies between the navigated and deliverable plans of the two systems. For Eclipse, the differences between the navigated and deliverable plans were consistent. However, for RaysStation, smallest difference was noted at the beginning of the curves with increasing differences afterwards. The CI of target volume in the final deliverable plans indicates better dose coverage in Eclipse (1.050) when compared to RayStation (0.872). The absolute PTV76 D98% of Eclipse and RayStation were 76.073 and 70.737 Gy respectively. This shows that RayStation MCO resulted in insufficient dose coverage to the clinical target. In terms of OARs sparing, RayStation allowed a greater dose sparing. For example, the maximum of bladder was 79.144 Gy while that of Eclipse was 82.321 Gy. Besides, RayStation demonstrates a greater rectal dose sparing (Eclipse: 81.248Gy versus RayStation: 78.437 Gy). Results:

Conclusion: