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Purpose: In this modelling study we pursued two main goals. The first was toestablish a new CTV-to-PTV expansion which considers the closest and mostcritical organ at risk (OAR). The second goal was to investigate the impact ofthe planning target volume (PTV) margin size on the tumor control probability(TCP) dependent on the geometrical setup uncertainties. The aim was to achievea smaller margin expansion close to the OAR while allowing a moderately largerexpansion in less critical areas further away from the OAR and most importantlywhile maintaining the TCP. Methods and Materials: Imaging data of radiationtherapy (RT) plans from pet dogs which had undergone RT for brain tumor wereused to estimate the clinic specific rotational setup uncertainties. AMonte-Carlo methodology used to quantify the implications of thoseuncertainties on the TCP. A computational CTV-to-PTV expansion method based onprobability density was established. All required software modules weredeveloped and integrated into a software package which directly interacts withthe Varian Eclipse treatment planning system. Results: The rotational setupuncertainties were obtained from 44 dog patients. Their frequency distributionswere analysed, and the mean values and standard deviations were determined. Toinvestigate the impact on the TCP, several uniform and non-isotropic PTVtargets were created. Standardized RT plans with equal optimization constraintswere defined and automatically applied and calculated on these targets. Theresulting TCPs were then evaluated and compared. Conclusion: The non-isotropicmargins were found to result in larger TCPs with smaller margin excess volume.Further we have presented an additional application of the newly establishedCTV-to-PTV expansion method for radiation therapy of the spinal axis of humanpatients.

The ROSMA-PTV concept and the impact of rotational setup uncertainties on the tumor control probability in canine brain tumors