Monte Carlo Simulations for Dosimetry in Prostate Radiotherapy with Different Intravesical Volumes and Planning Target Volume Margins.

Wei Lv,Dong Yu,Hengda He,Qian Liu

doi:10.1371/journal.pone.0159497

Abstract

In prostate radiotherapy, the influence of bladder volume variation on the dose absorbed by the target volume and organs at risk is significant and difficult to predict. In addition, the resolution of a typical medical image is insufficient for visualizing the bladder wall, which makes it more difficult to precisely evaluate the dose to the bladder wall. This simulation study aimed to quantitatively investigate the relationship between the dose received by organs at risk and the intravesical volume in prostate radiotherapy. The high-resolution Visible Chinese Human phantom and the finite element method were used to construct 10 pelvic models with specific intravesical volumes ranging from 100 ml to 700 ml to represent bladders of patients with different bladder filling capacities during radiotherapy. This series of models was utilized in six-field coplanar 3D conformal radiotherapy simulations with different planning target volume (PTV) margins. Each organ’s absorbed dose was calculated using the Monte Carlo method. The obtained bladder wall displacements during bladder filling were consistent with reported clinical measurements. The radiotherapy simulation revealed a linear relationship between the dose to non-targeted organs and the intravesical volume and indicated that a 10-mm PTV margin for a large bladder and a 5-mm PTV margin for a small bladder reduce the effective dose to the bladder wall to similar degrees. However, larger bladders were associated with evident protection of the intestines. Detailed dosimetry results can be used by radiation oncologists to create more accurate, individual water preload protocols according to the patient’s anatomy and bladder capacity.

Highlights

Stability of the target volume and radiotoxicity to organs at risk (OAR) are two principal factors that should be considered in radiotherapy planning
The mass of the bladder wall remained constant during the Finite element (FE) analysis; it changed in the course of reducing the resolution of the Visible Chinese Human (VCH) and reverse modeling to the FE model
The results of the FE analysis showed that the bladder wall mainly extended along the anterior–posterior and superior–inferior directions during bladder filling

Summary

Introduction

Stability of the target volume and radiotoxicity to organs at risk (OAR) are two principal factors that should be considered in radiotherapy planning. The bladder is located above the region to be irradiated, and its shape and volume change. If the bladder volume decreases, the radiation dose absorbed by the bladder wall, small intestine, and rectum increases, which may lead to serious organ toxicity [1,5,6]. In order to maintain stable interfraction and intrafraction volumes of the bladder, patients are prepared by preloading a certain volume of water to either fill or void the bladder before prostate radiation treatment [7]. Image-guided radiotherapy (IGRT) restricts OAR toxicity by tracking the position of tumors and further reducing planning target volume (PTV) margin [9]. Because of the radiation toxicity and difficulty in maintaining and measuring the intravesical volume, few clinical studies have performed quantitative dosimetry with different bladder volumes and PTV margins. Anthropomorphous computational models have been developed to simulate the dosimetry in radiation treatment

Methods

Results

Discussion

Conclusion