Abstract
MATERIALS AND METHODS
A prostate with a volume of 44 cm3 was defined as CTVprostate. 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 CTVprostate from a histopathology-based probability distribution for a range of numbers (1-10) and sizes (0.02-0.2 cm3) to define CTVsatellites. 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 CTVsatellites in 90 % of simulations was determined and compared with CTVprostate.
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.
RESULTS
For systematic errors with width (Σ) 0.5 mm and random errors with width (σr) 1.5 mm, the margin for the CTVprostate was 3 mm, whereas for each definition of CTVsatellites this margin was 0-1 mm. For σr = 2.7 mm, a margin of 5 mm was adequate for the CTVprostate and 2-3 mm for all except the most favourable and unfavourable CTVsatellites definition.
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.