Development of a technique using VMAT and robust optimisation to replace the use of surface bolus during radiotherapy for patients post-mastectomy.

Abstract

Radiotherapy following surgery is routine practice for patients that have had a mastectomy and are at high risk of the cancer recurring. To ensure that the chest-wall receives an adequate radiation dose a tissue equivalent material, or bolus, 1cm thick is placed on the patient surface. At the Queen Elizabeth Hospitals Birmingham (QEHB) this is applied for 7 of the 15 fractions of treatment. This technique however requires the creation of two treatment plans, increasing planning time in the patient pathway. In addition, the lack of flexibility of the bolus can cause air gaps between the skin and material affecting the surface dose and since the bolus is only required for a number of the fractions, treatment errors can occur if it is omitted by mistake. The aim of the research was to investigate a single, no bolus planning solution for these patients to reduce these issues. For a sample of 8 patients it was shown that bolus increased the target volume receiving 95% of the prescription dose by 7.7% compared to using no bolus at all. The use of VMAT could replicate these dose distributions, including superficial doses, without the need for bolus. Although the VMAT plans did produce a low dose bath which in some cases increased the doses to organs at risk, the plans still met all the required dose constraints. However, fluence loading in the surface region (to overcome the build-up effect) means that VMAT plans show unacceptable changes in dose distribution with changes in patient position or contour. Combining VMAT with robust optimisation significantly reduced dose differences caused by perturbation. These plans however still resulted in distributions that did not meet accepted dose constraints within the target structure, potentially causing undesired side effects for the patients. The use of robust optimisation also compromised the non-perturbed plans, reducing the dose enhancement effect to the patient surface. A single plan solution using the VMAT technique and combined with robust optimisation produced plans that mimic the clinical plans, without the use of bolus. Although the robust optimisation significantly reduced the variation in dose due to perturbation, these plans are still susceptible to patient movement and can result in tolerance doses being exceeded. The combination of VMAT and robust optimisation shows promise in producing a single, no bolus plan solution for chest-wall irradiation, but further work is required to quantify patient motion if the technique is to be applied clinically.

Date of Award	5 May 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	RMS UnKnown (Main Supervisor)