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.

[PURPOSE] 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.

[MATERIALS AND METHODS] A prostate with a volume of 44 cm was defined as CTV. 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 from a histopathology-based probability distribution for a range of numbers (1-10) and sizes (0.02-0.2 cm) to define CTV. 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 in 90 % of simulations was determined and compared with CTV.

[RESULTS] For systematic errors with width (Σ) 0.5 mm and random errors with width (σ) 1.5 mm, the margin for the CTV was 3 mm, whereas for each definition of CTV this margin was 0-1 mm. For σ = 2.7 mm, a margin of 5 mm was adequate for the CTV and 2-3 mm for all except the most favourable and unfavourable CTV definition.

[CONCLUSION] 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.