Intensity-modulated radiotherapy for whole pelvis irradiation in prostate cancer: A dosimetric and plan robustness study between photons and protons

Ashley L K Ong; K W Ang; Zubin Master; Sharon M M Wong; Jeffrey K L Tuan

doi:10.1016/j.tipsro.2018.02.001

Intensity-modulated radiotherapy for whole pelvis irradiation in prostate cancer: A dosimetric and plan robustness study between photons and protons

Tech Innov Patient Support Radiat Oncol. 2018 Apr 3:6:11-19. doi: 10.1016/j.tipsro.2018.02.001. eCollection 2018 Jun.

Authors

Ashley L K Ong¹, K W Ang¹, Zubin Master¹, Sharon M M Wong^{1

2

3}, Jeffrey K L Tuan^{1

4}

Affiliations

¹ Division of Radiation Oncology, National Cancer Centre Singapore, Singapore.
² Health and Social Sciences Cluster, Singapore Institute of Technology, Singapore.
³ College of Allied Health, SingHealth Academy, Singapore.
⁴ Duke-National University of Singapore Graduate Medical School, Singapore.

Abstract

Purpose: To evaluate the dosimetric impact and plan robustness of using Pencil Beam Scanning (PBS) in patients that requires prophylactic pelvic lymph nodes (PLNs) irradiation for prostate cancer.

Material and methods: Five intermediate to high-risk prostate patients previously treated using volumetric modulated arc therapy (VMAT), were selected for this study. Comparative proton radiotherapy plans were generated, where a three-field intensity modulated proton therapy (IMPT) plan was for the phase 1 planning target volume (PTV1) with PLNs. A technique with two posterior oblique fields using single field uniform dose (SFUD) was used for phase 2 (PTV2) volume, that comprises of the prostate and proximal seminal vesicles (Pro + proxSVs). Plan evaluation was performed on PTV coverage and dose to the organs at risk (OARs) using VMAT plans as a baseline (BL). Robust analysis on clinical target volume (CTV) coverage for the PBS plans was simulated with a 3 and 5 mm setup errors and a 3.5% range uncertainty.

Results: For target coverage, PTV1 and PTV2 showed negligible differences with a comparable homogeneity index (HI) values for both modalities. Proton plans produced a statistically significant lower mean dose to the bladder (32.5 Gy(RBE) vs. 46.5 Gy) and rectum (33.6 Gy(RBE) vs. 42.7 Gy). Dose to the bladder and rectum was equivalent at the high dose region. For the bowel cavity, the mean dose for proton plans were 45% lower compared to VMAT plans. Similarly, proton plans were able to achieve an overall reduction in integral dose for both treatment phase. CTV coverage remained high with all the simulated setup and range errors.

Conclusions: Proposed beam geometries for PTV1 and PTV2 proton plans presented good treatment accuracy with similar target coverage as the VMAT plans. Better sparing of OARs was achieved at the low-medium dose region for the proton plans.

Keywords: IMPT; PBS; Pelvic lymph nodes; Prostate cancer; Robust analysis; VMAT.