Technical note: Optimization functions for re‐irradiation treatment planning

Abstract

AbstractBackgroundAlthough re‐irradiation is increasingly used in clinical practice, almost no dedicated planning software exists.PurposeStandard dose‐based optimization functions were adjusted for re‐irradiation planning using accumulated equivalent dose in 2‐Gy fractions (EQD2) with rigid or deformable dose mapping, tissue‐specific α/β, treatment‐specific recovery coefficients, and voxelwise adjusted EQD2 penalization levels based on the estimated previously delivered EQD2 (EQD2deliv).MethodsTo demonstrate proof‐of‐concept, 35 Gy in 5 fractions was planned to a fictitious spherical relapse planning target volume (PTV) in three separate locations following previous prostate treatment on a virtual human phantom. The PTV locations represented one repeated irradiation scenario and two re‐irradiation scenarios. For each scenario, three re‐planning strategies with identical PTV dose‐functions but various organ at risk (OAR) EQD2‐functions was used: reRTregular: Regular functions with fixed EQD2 penalization levels larger than EQD2deliv for all OAR voxels. reRTreduce: As reRTregular, but with lower fixed EQD2 penalization levels aiming to reduce OAR EQD2. reRTvoxelwise: As reRTregular and reRTreduce, but with voxelwise adjusted EQD2 penalization levels based on EQD2deliv. PTV near‐minimum and near‐maximum dose (D98%/D2%), homogeneity index (HI), conformity index (CI) and accumulated OAR EQD2 (α/β = 3 Gy) were evaluated.ResultsFor the repeated irradiation scenario, all strategies resulted in similar dose distributions. For the re‐irradiation scenarios, reRTreduce and reRTvoxelwise reduced accumulated average and near‐maximum EQD2 by ˜1–10 Gy for all relevant OARs compared to reRTregular. The reduced OAR doses for reRTreduce came at the cost of distorted dose distributions with D98% = 92.3%, HI = 12.0%, CI = 73.7% and normal tissue hot spots ≥150% for the most complex scenario, while reRTregular (D98% = 98.1%, HI = 3.2%, CI = 94.2%) and reRTvoxelwise (D98% = 96.9%, HI = 6.1%, CI = 93.7%) fulfilled PTV coverage without hot spots.ConclusionsThe proposed re‐irradiation‐specific EQD2‐based optimization functions introduce novel planning possibilities with flexible options to guide the trade‐off between target coverage and OAR sparing with voxelwise adapted penalization levels based on EQD2deliv.