The benefit of MR-only radiotherapy treatment planning for anal and rectal cancers: A planning study.

David Bird,Ann M Henry,Alexandra Gilbert,Richard Speight,Rachel Cooper,Nathalie Casanova,Bashar Al‐Qaisieh,David L Buckley,Michael G Nix,Mark Teo,David Sebag‐Montefiore,Hazel Mccallum

doi:10.1002/acm2.13423

Abstract

IntroductionLimited evidence exists showing the benefit of magnetic resonance (MR)‐only radiotherapy treatment planning for anal and rectal cancers. This study aims to assess the impact of MR‐only planning on target volumes (TVs) and treatment plan doses to organs at risks (OARs) for anal and rectal cancers versus a computed tomography (CT)‐only pathway.Materials and methodsForty‐six patients (29 rectum and 17 anus) undergoing preoperative or radical external beam radiotherapy received CT and T2 MR simulation. TV and OARs were delineated on CT and MR, and volumetric arc therapy treatment plans were optimized independently (53.2 Gy/28 fractions for anus, 45 Gy/25 fractions for rectum). Further treatment plans assessed gross tumor volume (GTV) dose escalation. Differences in TV volumes and OAR doses, in terms of Vx Gy (organ volume (%) receiving x dose (Gy)), were assessed.ResultsMR GTV and primary planning TV (PTV) volumes systematically reduced by 13 cc and 98 cc (anus) and 44 cc and 109 cc (rectum) respectively compared to CT volumes. Statistically significant OAR dose reductions versus CT were found for bladder and uterus (rectum) and bladder, penile bulb, and genitalia (anus). With GTV boosting, statistically significant dose reductions were found for sigmoid, small bowel, vagina, and penile bulb (rectum) and vagina (anus).ConclusionOur findings provide evidence that the introduction of MR (whether through MR‐only or CT‐MR pathways) to radiotherapy treatment planning for anal and rectal cancers has the potential to improve treatments. MR‐related OAR dose reductions may translate into less treatment‐related toxicity for patients or greater ability to dose escalate.

Highlights

Limited evidence exists showing the benefit of magnetic resonance (MR)-only radiotherapy treatment planning for anal and rectal cancers
Statistically significant dosimetric reductions were found on MR plans for TA B L E 2 The magnetic resonance (MR) target volume (TV) differences in volume compared to computed tomography (CT) and the mean sensitivity and specificity overlap for each target volume between MR and CT over the whole patient cohort, where effect size is the systematic difference between MR and CT volumes
We provide evidence that utilizing an MR-only radiotherapy pathway for anal and rectal cancers makes statistically significant changes to TV volumes and treatment plan organs at risks (OARs) doses, in terms of reductions in volume (∼100 cc for planning TV (PTV)/PTVA) and dose-volume (5%–20%) compared to a CT-only pathway. These TV and treatment plan changes can be considered evidence of benefit as smaller TVs result in less irradiated tissue, and lower normal tissue doses can be expected to lead to reduced organ toxicities.[18]

Summary

Introduction

Limited evidence exists showing the benefit of magnetic resonance (MR)-only radiotherapy treatment planning for anal and rectal cancers. Magnetic resonance imaging (MRI)-only radiotherapy treatment planning is the use of an MRI scan alone to plan radiotherapy treatments These techniques require the generation of a “synthetic-computed tomography (CT)” (computer generated) dataset as MRI does not directly provide the patient density information required to allow dose calculation that is usually obtained from CT.[1,2,3] MR-only planning techniques have developed considerably in recent years, with commercial syntheticCT (sCT) solutions available and specialist centers treating prostate cancers.[1,2,3] a remaining challenge to wide-spread adoption is the lack of evidence within the literature demonstrating the impact of MR-only radiotherapy treatment planning to patients, in terms of improving treatments compared to standard pathways.[1]. For anal and rectal cancers, there is no evidence in the literature quantifying the impact of MR-only radiotherapy treatment planning to patient treatments

Methods

Results

Conclusion