Influence of inhomogeneous radiosensitivity distributions and intrafractional organ movement on the tumour control probability of focused IMRT in prostate cancer

Purpose To evaluate the influence of radioresistance and intrafractional movement on the tumour control probability (TCP) in IMRT prostate treatments using simultaneous integrated boosts to PSMA-PET/CT-delineated GTVs. Materials and methods 13 patients had PSMA-PET/CT prior to prostatectomy and histopathological examination. Two GTVs were available: GTV-PET and GTV-histo, which is the true cancer volume. Focused IMRT plans delivering 77 Gy in 35 fractions to the prostate and 95 Gy to PTV-PET were produced. For random portions of the true cancer volume, α and α/β were uniformly changed to represent different radiosensitivity reductions. TCP was calculated (linear quadratic model) for the true cancer volume with and without simulated intrafractional movement. Results Intrafractional movement increased the TCP by up to 10.2% in individual cases and 1.2% averaged over all cases for medium radiosensitivity levels. At lower levels of radiosensitivity, movement decreased the TCP. Radiosensitivity reductions of 10–20% led to TCP reductions of 1–24% and 10–68% for 1% and 5% affected cancer volume, respectively. There is no linear correlation but a sudden breakdown of TCPs within a small range of radiosensitivity levels. Conclusion TCP drops significantly within a narrow range of radiosensitivity levels. Intrafractional movement can increase TCP when the boost volume is surrounded by a sufficiently high dose plateau.