Proton arc therapy: an assessment of dosimetric potential and investigation of novel strategies towards deliverable treatments

Abstract

The development of proton therapy is fundamentally about exploiting the property of protons to deliver improved dose distributions to patients compared to conventional radiotherapy. Following the widespread implementation of scanned beam proton therapy, Proton Arc Therapy (PAT) has the capability to further improve proton therapy by overcoming some of the limitations currently associated with proton therapy, which is typically delivered with a small number of beams (less than five). PAT has the potential to become the next-generation proton therapy technology due to its ability of improving the target dose conformality and organs-at-risk (OAR) sparing compared to the current technology, intensity-modulated proton therapy (IMPT). At the time of starting this work in 2017, there were only six papers reporting PAT. Most of the studies were retrospective plan comparisons which reported the improved dosimetric characteristics of PAT compared to IMPT, while only two studies reported the potential ways of delivering PAT, i.e., spot-scanning proton arc therapy or SPArc, and proton mono-energetic arc therapy or PMAT. However, at the time of starting this work, the research on PAT was still early stage (not applied clinically) and needed further evidence of PAT's advantages. The aim of this thesis was to gain fundamental understandings of PAT's dosimetric potential and to investigate ways to achieve practical deliveries in order to be closer to realising clinical PAT delivery. The work described in this thesis is split into three chapters following an introductory Chapter 1. Chapter 2 compared PAT and IMPT plans for one representative brain and two head and neck cases in order to gain understandings of the dosimetric differences between PAT and IMPT and the intrinsic robustness characteristics of PAT. Chapter 3 described a novel energy selection strategy, ELEANOR-PAT (energy layer selection based on coverage for PAT), which aimed to shorten the theoretical delivery time by strategically selecting energy layers compared to the full-energy PAT (FPAT). The ELEANOR-PAT plans were validated for eight ependymoma cases and assessed against clinical dose criteria. Other energy reduction strategies explored prior to the formation of ELEANOR-PAT were described as a separate section in section 3.2 before the main work on ELEANOR-PAT starting in section 3.3. Chapter 4 explored ways to make the ELEANOR-PAT plans from chapter 3 deliverable, by interpolating the selected energies to one per finer beam angle. An emulator was made to predict the delivered beam angle for each spot under the assumption of continuous gantry rotation. The interpolated ELEANOR-PAT plans were then compared to their emulated dose distributions to quantify any dose differences between the fixed-angle plan and continuous delivery. FPAT plans were found to improve the physical dose distributions under nominal scenarios, but when the dose distributions were evaluated under uncertainty scenarios the robustness was found to be dependent on individual cases. ELEANOR-PAT was shown to be a reliable energy reduction strategy for the eight ependymoma cases. ELEANOR-PAT not only preserved or improved the dose qualities from the IMPT plans but also reduced the total number of energy layers by 25 - 84% (compared to FPAT) which translated into theoretical delivery times of under three minutes for all eight cases. The interpolated ELEANOR-PAT plans under the continuous gantry rotation assumption agreed well in terms of calculated dose to the fixed-angle plans for all eight cases, as assessed using gamma analyses. Results from Chapter 3 and 4 show that ELEANOR-PAT is a reliable strategy for planning and delivering PAT. In this thesis, the potential dosimetric and delivery time benefits of PAT have been demonstrated for brain (ependymoma) cases. A novel strategy for practical PAT delivery has also demonstrated dosimetric advantages and generalisability to different ependymoma geometries. Theref

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Appleby (Supervisor), Ranald Mackay (Supervisor), Adam Aitkenhead (Supervisor) & Mike Merchant (Supervisor)