Optimising treatment outcomes using Stereotactic Body Radiotherapy (SBRT) for prostate cancer

Aims: to optimise linear accelerator-based prostate stereotactic ablative radiotherapy (SABR) through planning studies, tumour control probability (TCP) and normal tissue complication probability (NTCP) calculations and radiation-induced second primary cancer (RISPC) risk assessment.
Methods: A planning study was performed to develop a class solution for prostate SABR. A second planning study delivered boosts to dominant intra-prostatic lesions (DILs) and TCP and NTCP were calculated. A third planning study compared prostate SABR planning using flattened and flattening filter free (FFF) beams. A systematic review examined RISPC risk following prostate radiotherapy. A final study estimated RISPC risks following prostate SABR in comparison to other contemporary radiation techniques.
Results: Prostate SABR was optimal using a single anterior arc which resulted in highly conformal plans, lower rectal doses and improved delivery times and monitor unit requirements for most patients. Boosting DILs resulted in small TCP increases, but the benefit was offset by increases in NTCP. SABR to the whole prostate without DIL boosting resulted in high TCP and low NTCP. Plans using flattened and FFF beams were dosimetrically similar but FFF resulted in reduced delivery times. Clinical evidence, largely based on older radiation techniques, suggests that prostate radiotherapy increases RISPC risk. Clinical evidence concerning risk following modern techniques is too immature to draw firm conclusions. The final study demonstrated that SABR techniques resulted in lower estimated RISPC risks in all organs compared to conventionally fractionated techniques, while FFF techniques reduced RISPC risks in out-of-field organs.
Conclusions: Linear accelerator-based prostate SABR delivered with a single partial arc is optimal and high levels of TCP and low levels of NTCP are predicted from whole prostate SABR. FFF allows faster treatment delivery. Second malignancy risk is lower using SABR, particularly with FFF, compared to conventionally fractionated techniques. Phase III trials are required to investigate prostate SABR in practice.