Individualising anti-cancer treatment: Optimising tracking accuracy and dose delivery in Stereotactic Body Radiotherapy (SBRT)

MD (res)Introduction: Stereotactic Body Radiotherapy (SBRT) is a form of highly focal radiation therapy. Treatment delivery is largely guided (“tracked”) by gold-marker fiducials for non-spinal body sites. CyberKnife (CK) is purpose-built todeliver SBRT. Methods: An experimental system, using a radiotherapy “phantom”, was designed to assess the accuracy of the CK system in imaging static and migrating fiducials (Chapter 2). An assumption in fiducial-tracked treatments is that the exact arrangement of fiducials at planning CT is maintained at treatment. It is also crucial that relative Organ At Risk (OAR) and tumour position, is consistent between planning CT scan and treatment. The validity of this assumption was assessed by comparing fiducial locations on Planning and Treatment CTs (Chapter 3). The feasibility of achieving consistent bladder filling, and the impact of consistent filling on the ability to track translations/rotations in prostate cancer therapy was explored (Chapter 4). Uncertainties in treatment planning/delivery for CK prostate patients was explored, and ideal planning margins were calculated (Chapter 5). Optimum SBRT dose for localised pancreatic cancer, lymph node oligometastasis and oligometastatic breast cancer was explored (Chapter 6). Results: Imaging of fiducial position was accurate and reproducible across a clinically appropriate tracking range. However, findings highlighted the need for vigilance at treatment delivery. The reliability and “trackability” of implanted fiducials, as well as consistency of OAR position, varied according to tumour and implantation site. Guidelines were generated accordingly. Bladder filling/Margins guidelines have been generated. Radiobiological analysis has indicated that there is scope for cautious dose escalation in the SBRT treatment of pancreatic cancer. Analysis of SBRT-treated lymph node oligometastases has demonstrated that Local Control is 100% when SBRT is prescribed to a threshold 72Gy10. Conclusion: Refinements in patient preparation, fiducial placement, and dose/fractionation selection can optimise tracking accuracy and dose delivery in SBRT.THS