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

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