Optimisation of Hypo Fractionation in Radiation Therapy

Stereotactic Ablative Radiotherapy (SABR) represents a significant breakthrough in the field of radiotherapy, offering a highly effective treatment option for a variety of primary and oligometastatic disease sites. As SABR becomes ever more widespread, it is important to evaluate dose distribution, exploit dose heterogeneity and improve plan quality where feasible, to ensure the highest achievable dose to the target with the steepest dose gradients outside the target possible. The main goal of the research presented in this thesis was to address the limitations of widely accepted volume based metrics used to assess dose gradients outside the Planning Treatment Volume (PTV) and prescription dose coverage in SABR plans. A robust and comprehensive dose evaluation methodology based on a spatial analysis was developed that removes the associated subjectivity and weaknesses associated with the current standard of practice. The methodology developed can be used to enhance the quality assessment of SABR plans in the clinic and automate checks.
Automation is made possible by setting acceptability criteria, developed in this thesis through the use of clinical SABR cases. Dose gradient differences for lung and other body sites and 6MV flattened and flattening filter free (FFF) beams is also examined. The methodology developed for plan review helped formulate approaches to examine the basis for the current practice of omitting a Clinical Target Volume (CTV) in SABR plans. While a CTV is not used in Lung SABR, there is no published evidence on the rationale for this. This work assessed incidental CTV coverage in VMAT Lung SABR plans as a function of prescription, motion and volume. In the axial direction, the 50 Gy EQD2 and 20.8 Gy EQD2 coverage was sufficient for typical lung SABR dose prescription ranges from 151Gy BED10 to 105Gy BED10. However, any decrease of margins in the cranial caudal direction may prove detrimental to outcome, as coverage of the 50 Gy EQD2 would be compromised. Coverage of the 20.8 Gy EQD2 would be further compromised. Importantly this work also showed that, CTV coverage in the coronal plane was improved when motion was > 5 mm. A final aim of the research presented in this thesis was to exploit dose inhomogeneity within radiation fields. Hypo-fractionated lung SABR has often been avoided when tumours are close to the chest wall. Our strategic objective was the reduction of fraction number, while maintaining target biological effective dose coverage without increasing chest wall toxicity predictors. The work presented here concluded there is merit to further refine known chest wall risk adapted strategies by using dose heterogeneity. Tumours with a PTV distance of between 0.5 cm to - 0.5 cm from chest wall, can typically be treated with 54 Gy in 3 fractions optimised to reduce chest wall toxicity.