Technical note: Optimization functions for re-irradiation treatment planning

Although re-irradiation is increasingly used in clinical practice, almost no dedicated planning software exists.

Standard 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 (EQD2_deliv ).

To 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: 1) reRT_regular : Regular functions with fixed EQD2 penalization levels larger than EQD2_deliv for all OAR voxels. 2) reRT_reduce : As reRT_regular , but with lower fixed EQD2 penalization levels aiming to reduce OAR EQD2. 3) reRT_voxelwise : As reRT_regular and reRT_reduce , but with voxelwise adjusted EQD2 penalization levels based on EQD2_deliv . PTV near-minimum and near-maximum dose (D_98% /D_2% ), homogeneity index (HI), conformity index (CI) and accumulated OAR EQD2 (α/β = 3 Gy) were evaluated.

For the repeated irradiation scenario, all strategies resulted in similar dose distributions. For the re-irradiation scenarios, reRT_reduce and reRT_voxelwise reduced accumulated average and near-maximum EQD2 by ˜1-10 Gy for all relevant OARs compared to reRT_regular . The reduced OAR doses for reRT_reduce came at the cost of distorted dose distributions with D_98% = 92.3%, HI = 12.0%, CI = 73.7% and normal tissue hot spots ≥150% for the most complex scenario, while reRT_regular (D_98% = 98.1%, HI = 3.2%, CI = 94.2%) and reRT_voxelwise (D_98%  = 96.9%, HI = 6.1%, CI = 93.7%) fulfilled PTV coverage without hot spots.

The 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 EQD2_deliv .