Are the CTV-to-PTV margins currently used in online adaptive radiotherapy for prostate cancer too large? The impact of the distribution of microscopic disease on treatment margin requirements.

Abstract

Planning target volume (PTV) margin recipes assume all parts of the target are equally important. For the prostate clinical target volume (CTV) this is invalid. We evaluated the impact of the spatial probability distribution of microscopic disease in the prostate on CTV-to-PTV margins. A prostate with a volume of 44 cm³ was defined as CTV_prostate. Homogenous dose distributions were created with margins ranging 0-5 mm. The gross tumor volume (GTV) was assumed covered with a separate margin. Microscopic satellites were sampled within the CTV_prostate from a histopathology-based probability distribution for a range of numbers (1-10) and sizes (0.02-0.2 cm³) to define CTV_satellites. Geometric errors were sampled from a 3D Gaussian distribution, simulating online adaptive treatment of 5 fractions. Each CTV was shifted with respect to the dose according to each total error. The PTV margin ensuring 95 % of the prescribed dose to the CTV_satellites in 90 % of simulations was determined and compared with CTV_prostate. For systematic errors with width (Σ) 0.5 mm and random errors with width (σ_r) 1.5 mm, the margin for the CTV_prostate was 3 mm, whereas for each definition of CTV_satellites this margin was 0-1 mm. For σ_r = 2.7 mm, a margin of 5 mm was adequate for the CTV_prostate and 2-3 mm for all except the most favourable and unfavourable CTV_satellites definition. The CTV-to-PTV margins used in online adaptive radiotherapy for prostate cancer can be reduced by ∼2 mm, if the GTV is covered with an adequate margin.