ESTRO 2021 Abstract Book

S1596

ESTRO 2021

PO-1874 Impact of Arc length on unilateral dose volume parameters and plan delivery: A Dosimetry study P. Agarwal 1 , S.D. Pattanaik 1 , D. Epili 1 , A. Mukherji 1,1 , S. Pradhan 1 1 Homi Bhabha Cancer Hospital, Radiation Oncology, Varanasi, India Purpose or Objective To analyze the impact of arc length on dose volume parameters for the carcinoma of buccal mucosa (unilateral) and on plan delivery using volumetric modulated arc therapy (VMAT) technique. Materials and Methods For this retrospective study, ten previously treated buccal mucosa (unilateral) patients were selected randomly having a volume range from 284cc to 843cc. For each patient, 4 plans were generated, resulted in a total of 40 plans. The plans were generated using two arcs of four different arc lengths 210 degree (Plan 1), 190 degree (Plan 2), 150 degree (Plan 3) and 360 degree (Plan 4) with 15 and 345 collimator angles. Dose prescription was 60 Gy in 30 fractions. The plans were generated using 6XFF photon beam (Eclipse Varian Medical System, Palo Alto, USA, version 15.5.11) and dose calculation was performed by Acuros algorithm. Planning optimization was carried out from single optimization without tweaking any parameters. Plan 1 was kept as the base plan for comparing the rest plans. For each plan, dose volume parameters for PTV and organ at risks (OARs) were analyzed statistically using paired t-test and Box-Whisker plots. For each plan, fluence verification was performed using EPID (True Beam Varian Medical System). Gamma analysis parameters for dose difference and distance to agreement of 3% &3 mm and 2% &2 mm were analyzed. Results The dose volume parameters results are tabulated in Table 1. On comparing plan 1 and 2, a statistically significant difference was found in their Coverage index(CI), Conformity index(COIN) and integral dose(ID). The difference in the parameters, excluding monitor units (MUs), between plan 3 and plan 1 were found to be statistically highly significant. In case of plan 4, MUs and ID were of higher statistically significant values in comparison with plan 1. From Box and whisker plots (Fig 1), the Gamma was above 97% for all the plans for 3% & 3 mm. However, for 2% & 2 mm, the Gamma was found to be the least for plan 3 among all the plans.

Conclusion As per the results mentioned above, shorter arc length plans (plan 1 and plan 2) result in comparable plans as compared to full arc length plans (plan 4), indicating that shorter length plans may be used, to improve the throughput without compromising the plan quality. However, very short arc plans (plan 3) is not recommended dosimetrically.