Statistic and dosimetric criteria to assess the shift of the prescribed dose for lung radiotherapy plans when integrating point kernel models in medical physics: are we ready?

The dose calculation algorithms integrated in a radiotherapy Treatment Planning System (TPS) compute the medical prescribed dose (PD) into a representation of the delivered dose (DD), of the same expected value in gray, to the patient, itself translated in monitor units (MUs) actually delivered by the radiotherapy machine. This is a fundamental fiducial chain between the treatment desired by the medical oncologist and the physical dose, and clinical effect, truly obtained in the patient. As everyone knows the relation between the PD and the DD is not yet exact in all anatomical situations or with all calculation algorithms. Since the generalization of predictive and personalized dosimetry with 2 and 3D dose distributions and dose volume histograms (DVH) one could easily forget this caveat and imagine to see the truth on treatment plan. This is almost right for density rather homogeneous anatomic regions as brain, pelvis, abdomen, etc. but it is still a search for density very heterogeneous regions as chest because of the very low density of lungs. The progress toward always better calculation algorithms is not linear. Impressive progresses have been made considering heterogeneities but with low consideration regarding the real physical processes of dose deposition, and more recently increasing consideration is given to dose deposition mechanism’s going closer and closer to Monte Carlo simulation results, taken as reference. Most of the Radiation Oncology departments had, in the recent years, to go through successive changes of dose calculation algorithms according to the evolution of those software’s. In this paper, the evolution of the relation between the PD and the DD is examined through the most current situation of change from pencil beam convolution with modified batho (PBC-MB) to anisotropic analytical algorithm (AAA). The point of view is the quantification of the altered PD to consider, when one wishes to keep on with the same physical DD or clinical results, when implementing such changes (1-3). This concern should, of course be extended to organs at risk (OARs), or integrate dose escalation, but this is out of the scope of this report which focus on the methodological issues