ESTRO 2021 Abstract Book

S1100

ESTRO 2021

S. Sundar 1 , S. Chilukuri 1 , K. Patro 2 , M. Sawant 2 , R. S 3 , M. Arjunan 2 , D. Sharma 2 , R. Jalali 4 1 Apollo Proton Cancer Centre, Radiation Oncology, Chennai, India; 2 Apollo Proton Cancer Centre, Medical Physics, Chennai, India; 3 Apollo Proton Cancer Centre, Medical Physics , Chennai, India; 4 Apollo Proton Cancer Centre, Radiation Oncology , Chennai, India Purpose or Objective To compare the dosimetry between pencil beam scanning proton bream therapy (PBT) and helical Tomotherapy (HT) in patients of high-risk prostate cancers requiring elective pelvic nodal irradiation using moderate hypo-fractionated regimen and to estimate late toxicities based on previously published normal tissue complication probability (NTCP) models Materials and Methods Twelve consecutive patients diagnosed and treated with PBT for high-risk prostate cancers and requiring elective pelvic nodal irradiation were included in this study. PBT and HT plans were developed to deliver a dose of 50CGE in 25 fractions to elective pelvic lymph-nodes with simultaneous boost of 68CGE to prostate and bilateral seminal vesicles. Two field multi-field optimization was used to generate proton therapy plans. The dose volume parameters for targets and all the relevant organs at risk were compared between these plans. The doses received to pelvic musculature (levator ani, external sphincter and iliococcygeus), bladder wall and trigone were recorded for each of the plans. History of use of anti-coagulants and transurethral resection of prostate was also recorded. These were used to estimate late toxicities based on published NTCP models (Schaake, et al, Radiotherapy Oncology 2016 for late rectal toxicities and Schaake, et al, Plos One 2018 for late urinary toxicities) after conversion to 2Gy equivalent doses. One-Sample Kolmogorov-Smirnov test was used to analyze distribution of data and based on that either a Paired T-test or a Wilcoxon matched-pair signed rank test was used. Results IMPT and HT plans achieved adequate target coverage without any statistically significant difference between them. Better sparing of dose to bladder, rectum and bowel bag were achieved in PBT plans; whereas doses to penile bulb and femoral heads were higher in PBT plans (Table-1). PBT plans showed significantly large differences in volumes of bladder and rectum receiving 15-50Gy compared to HT plans. The average risk for Grade II rectal bleeding, fecal incontinence and stool frequency for HT and PBT plans were 12.4% vs. 4.5% (p=0.003); 2.6% vs. 0.2% (p=0.016); and 2.25% vs. 0.25% (p=0.007) respectively. Similarly, the average risk for grade II dysuria, urinary incontinence and grade 1 hematuria were 12% vs. 10.3% (p=0.023); 7.7% vs. 4.5% (p=0.015); and 15.1% vs. 9.3% (p=0.011) respectively.

Conclusion PBT plans demonstrated better sparing of rectal and bladder doses especially to the volumes receiving 15- 55Gy. The estimated late bladder and rectal toxicities based on the published NTCP models significantly favored PBT plans.