Dosimetric study of uniform scanning proton therapy planning for prostate cancer patients with a metal hip prosthesis, and comparison with volumetric-modulated arc therapy.

Suresh Rana,Omar Zeidan,Wen Hsi,Niek Schreuder,Chihyao Cheng,Yuanshui Zheng,Gary Larson,Carlos Vargas

doi:10.1120/jacmp.v15i3.4611

Abstract

The main purposes of this study were to 1) investigate the dosimetric quality of uniform scanning proton therapy planning (USPT) for prostate cancer patients with a metal hip prosthesis, and 2) compare the dosimetric results of USPT with that of volumetric‐modulated arc therapy (VMAT). Proton plans for prostate cancer (four cases) were generated in XiO treatment planning system (TPS). The beam arrangement in each proton plan consisted of three fields (two oblique fields and one lateral or slightly angled field), and the proton beams passing through a metal hip prosthesis was avoided. Dose calculations in proton plans were performed using the pencil beam algorithm. From each proton plan, planning target volume (PTV) coverage value (i.e., relative volume of the PTV receiving the prescription dose of 79.2 CGE) was recorded. The VMAT prostate planning was done using two arcs in the Eclipse TPS utilizing 6 MV X‐rays, and beam entrance through metallic hip prosthesis was avoided. Dose computation in the VMAT plans was done using anisotropic analytical algorithm, and calculated VMAT plans were then normalized such that the PTV coverage in the VMAT plan was the same as in the proton plan of the corresponding case. The dose‐volume histograms of calculated treatment plans were used to evaluate the dosimetric quality of USPT and VMAT. In comparison to the proton plans, on average, the maximum and mean doses to the PTV were higher in the VMAT plans by 1.4% and 0.5%, respectively, whereas the minimum PTV dose was lower in the VMAT plans by 3.4%. The proton plans had lower (or better) average homogeneity index (HI) of 0.03 compared to the one for VMAT (HI = 0.04). The relative rectal volume exposed to radiation was lower in the proton plan, with an average absolute difference ranging from 0.1% to 32.6%. In contrast, using proton planning, the relative bladder volume exposed to radiation was higher at high‐dose region with an average absolute difference ranging from 0.4% to 0.8%, and lower at low‐ and medium‐dose regions with an average absolute difference ranging from 2.7% to 10.1%. The average mean dose to the rectum and bladder was lower in the proton plans by 45.1% and 22.0%, respectively, whereas the mean dose to femoral head was lower in VMAT plans by an average difference of 79.6%. In comparison to the VMAT, the proton planning produced lower equivalent uniform dose (EUD) for the rectum (43.7 CGE vs. 51.4 Gy) and higher EUD for the femoral head (16.7 CGE vs. 9.5 Gy), whereas both the VMAT and proton planning produced comparable EUDs for the prostate tumor (76.2 CGE vs. 76.8 Gy) and bladder (50.3 CGE vs. 51.1 Gy). The results presented in this study show that the combination of lateral and oblique fields in USPT planning could potentially provide dosimetric advantage over the VMAT for prostate cancer involving a metallic hip prosthesis.PACS number: 87.55.D‐, 87.55.ne, 87.55.dk

Highlights

There has been growing interest in treating prostate cancer using protons, which have finite range in tissue, sharp lateral penumbra, and near-zero exit dose when compared to conventional MV X-rays.[1]. Since the majority of the proton dose can be deposited in a region called the spread-out Bragg peak (SOBP) with sharp dose falloff after SOBP, proton therapy can spare the critical structures that are adjacent to the target volume
If a prostate cancer patient has a metallic hip prosthesis, the proton treatment plan can no longer have the second lateral field because proton beam passing through metallic hip prosthesis is not recommended.[5] a proton prostate plan involving a metallic hip prosthesis requires the beam setup to include an angled beam too, but such beam arrangement may lead to an increase in dose to the critical structures depending on the orientation of the angled beam
Since the current literature on proton therapy lacks the dosimetric data for prostate cases involving prosthesis, we aim to investigate the dosimetric quality of uniform scanning proton therapy (USPT) planning using three fields for prostate cancer patients with a metallic hip prosthesis

Summary

Introduction

There has been growing interest in treating prostate cancer using protons, which have finite range in tissue, sharp lateral penumbra, and near-zero exit dose when compared to conventional MV X-rays (photons).(1) Since the majority of the proton dose can be deposited in a region called the spread-out Bragg peak (SOBP) with sharp dose falloff after SOBP, proton therapy can spare the critical structures that are adjacent to the target volume. Several researchers have investigated proton therapy planning of prostate cancer using mostly parallel opposed lateral fields or slightly angled fields.[6,7,8,9,10,11,12,13,14,15,16] For prostate cases that do not involve a metallic hip prosthesis, we generally use two parallel opposed lateral fields for proton planning at ProCure Proton Therapy Center, Oklahoma City. If a prostate cancer patient has a metallic hip prosthesis, the proton treatment plan can no longer have the second lateral field because proton beam passing through metallic hip prosthesis is not recommended.[5] a proton prostate plan involving a metallic hip prosthesis requires the beam setup to include an angled beam too, but such beam arrangement may lead to an increase in dose to the critical structures depending on the orientation of the angled beam. Tang et al[17] demonstrated that anterior–oblique beams can significantly reduce dose to the anterior rectal wall, at high-dose levels, when compared to beam arrangement having two parallel opposed lateral proton fields

Objectives

Methods

Results

Conclusion