Multicenter Study of Dose Accumulation for Reirradiation: Intercenter Variation in Radiobiological Dose Scaling Introduces Clinically Relevant Differences

Aims We investigated intercenter variation in reirradiation dose accumulation due to (i) varying use of image registration and (ii) varying handling of radiobiological dose scaling in dose accumulation for eight patient cases covering eight different anatomical regions. Materials and Methods Eight centers mapped the dose from the previous CT to the reirradiation CT using either rigid or deformable image registration and accumulated doses in (i) physical dose and (ii) equieffective dose in 2 Gy fractions (EQD2) with their institutional α/β values. If used in their clinic, centers could apply dose scaling factors (DSFs) to account for recovery between treatments. We analyzed the accumulated dose to one selected dose-limiting organ at risk (OAR) per patient case. Results Intercenter variation in accumulated near-maximum dose (D 1% ) due to image registration alone (physical dose) ranged from 0.7 Gy for the aorta (lung case) to 9.5 Gy for the mandible (head and neck case). EQD2 scaling increased intercenter variation in accumulated D 1% for all OARs, except for the heart (breast case). Application of DSFs changed the D 50% intercenter range compared to EQD2-scaled doses from 1.6 Gy to 5.8 Gy for the bladder (anal case), from 0.6 Gy to 1.2 Gy for the chiasm (brain case), and from 7.6 Gy to 10.9 Gy for the bowel bag (rectum case). Conclusion Lack of consensus in radiobiological-dose scaling considerably increases intercenter variation in reirradiation dose accumulation across a diverse range of tumor sites.