Comparative evaluation of dose accumulation strategies in the clinical reirradiation setting

Background and purpose: Dose accumulation in reirradiation can contribute to both treatment outcomes and safe cumulative doses. We demonstrated consensus strategies on dose accumulation for three clinical reirradiation scenarios. Materials and methods: For a lung patient, prescription, point and volumetric-based dose accumulations were generated, while isodose transfer also highlighted a visual approach. In a head and neck case, three-dimensional conformal radiotherapy (3DRT) fields were reconstructed from treatment records. For a brachytherapy gynaecological case, isodose transfer and full volumetric dose accumulation were generated. For pertinent organs at risk (OARs) equieffective doses (EQD2Gy) enabled comparisons between the different methods. Results: Large variations in cumulative EQD2Gy values in the lung case were calculated. Assuming prescription EQD2Gy, values were 181.9, 161.4 and 154.6 Gy to the spinal canal, brachial plexus and pericardium. Median cumulative point EQD2Gy values for these organs were 15.9, 81.9, 66.6 Gy and similarly, volumetric dose values were 13.9, 68.9 and 55.2 Gy respectively. The head and neck re-simulation showed significant process limitations, especially for brainstem (11.8 – 34.4 Gy EQD2Gy). For the brachytherapy case, cumulative EQD2Gy values were similar across direct point and volumetric dose summation (88.5 vs 88.0 Gy for bladder; 83.6 vs 80.0 Gy for rectum). Conclusions: Different reirradiation dose accumulation strategies can lead to significant variations in reported OAR cumulative dose. Prescription and point-based dose methods tend to overestimate cumulative dose, which could justify their application in time critical situations, but may limit treatment options. More advanced volumetric-based methods demonstrated improved consistency in cumulative dose reporting.