Towards real-time PGS range monitoring in proton therapy of prostate cancer

Paulo Magalhaes Martins,Benjamin Ackermann,Thomas Tessonnier,Joao Seco,Stephan Brons,Hugo Freitas

doi:10.1038/s41598-021-93612-y

Abstract

Proton therapy of prostate cancer (PCPT) was linked with increased levels of gastrointestinal toxicity in its early use compared to intensity-modulated radiation therapy (IMRT). The higher radiation dose to the rectum by proton beams is mainly due to anatomical variations. Here, we demonstrate an approach to monitor rectal radiation exposure in PCPT based on prompt gamma spectroscopy (PGS). Endorectal balloons (ERBs) are used to stabilize prostate movement during radiotherapy. These ERBs are usually filled with water. However, other water solutions containing elements with higher atomic numbers, such as silicon, may enable the use of PGS to monitor the radiation exposure of the rectum. Protons hitting silicon atoms emit prompt gamma rays with a specific energy of 1.78 MeV, which can be used to monitor whether the ERB is being hit. In a binary approach, we search the silicon energy peaks for every irradiated prostate region. We demonstrate this technique for both single-spot irradiation and real treatment plans. Real-time feedback based on the ERB being hit column-wise is feasible and would allow clinicians to decide whether to adapt or continue treatment. This technique may be extended to other cancer types and organs at risk, such as the oesophagus.

Highlights

Proton therapy of prostate cancer (PCPT) was linked with increased levels of gastrointestinal toxicity in its early use compared to intensity-modulated radiation therapy (IMRT)
The authors pointed out that the sample size for the proton cohort was quite small because the study included patients diagnosed from 1992 to 2005, a period when proton therapy was in its relative infancy and only passively scattered proton therapy (PSPT) was available
We describe the implementation of an Endorectal balloons (ERBs) inflated with a mixture of water and silicon dioxide (SiO2 ) to wirelessly monitor the proton range in PCPT via prompt gamma rays

Summary

Introduction

Proton therapy of prostate cancer (PCPT) was linked with increased levels of gastrointestinal toxicity in its early use compared to intensity-modulated radiation therapy (IMRT). More recent studies have demonstrated more favourable toxicity outcomes with proton t herapy[20,22,23] Another drawback of PCPT is the irradiation of the femoral heads, which lay in the path of the beam to the prostate. An in vivo diode-based range verification system was developed and commissioned at the Massachusetts General Hospital (MGH)[35,36] This system was designed for passive-scattering proton delivery and relies on a 3 × 4 matrix of 1 mm diodes mounted in a water balloon. The rectal wall was shown to receive doses of 1.6% for anterior fields and 0.4% for AO fields[36] Another technique to improve gastrointestinal toxicity and mitigate the uncertainties in proton relative biological effectiveness (RBE) is the use of rectal hydrogel spacers located between the prostate and the rectum[37,38,39]. Reduction in the dose to the rectal wall by means of an ERB has been observed by several authors[40,51,52]

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