Inclusion of geometrical uncertainties in radiotherapy treatment planning by means of coverage probability

Purpose: Following the ICRU-50 recommendations, geometrical uncertainties i n tumor position during radiotherapy treatments are generally included in t he treatment planning by adding a margin to the clinical target volume (CTV ) to yield the planning target volume (PTV), We have developed a method for automatic calculation of this margin. Methods and Materials: Geometrical uncertainties of a specific patient grou p can normally be characterized by the standard deviation of the distributi on of systematic deviations in the patient group (Sigma) and by the average standard deviation of the distribution of random deviations (sigma), The C TV of a patient to be planned can be represented in a 3D matrix in the trea tment room coordinate system with voxel values one inside and zero outside the CTV, Convolution of this matrix with the appropriate probability distri butions for translations and rotations yields a matrix with coverage probab ilities (CPs) which is defined as the probability for each point to be cove red by the CTV, The PTV can then be chosen as a volume corresponding to a c ertain iso-probability level. Separate calculations are performed for syste matic and random deviations, Iso-probability volumes are selected in such a way that a high percentage of the CTV volume (on average > 99%) receives a high dose (> 95%), The consequences of systematic deviations on the dose d istribution in the CTV can be estimated by calculation of dose histograms o f the CP matrix for systematic deviations, resulting in a so-called dose pr obability histogram (DPH), A DPH represents the average dose volume histogr am (DVH) for all systematic deviations in the patient group. The consequenc es of random deviations can be calculated by convolution of the dose distri bution with the probability distributions for random deviations. Using the convolved dose matrix in the DPH calculation yields full information about the influence of geometrical uncertainties on the dose in the CTV, Results: The model is demonstrated to be fast and accurate for a prostate, cervix, and lung cancer case, A CTV-to-PTV margin size which ensures at lea st 95% dose to (on average) 99% of the CTV, appears to be equal to about 2 Sigma + 0.7 sigma for three all cases. Because rotational deviations are in cluded, the resulting margins can be anisotropic, as shown for the prostate cancer case, Conclusion: A method has been developed for calculation of CTV-to-PTV margi ns based on the assumption that the CTV should be adequately irradiated wit h a high probability. (C) 1999 Elsevier Science Inc.