3D dosimetry in patients with early breast cancer undergoing Intraoperative Avidination for Radionuclide Therapy (IART) combined with external beam radiation therapy.

PURPOSE: Intraoperative Avidination for Radionuclide Therapy (IART) is a novel targeted radionuclide therapy recently used in patients with early breast cancer. It is a radionuclide approach with (90)Y-biotin combined with external beam radiotherapy (EBRT) to release a boost of radiation in the tumour bed. Two previous clinical trials using dosimetry based on the calculation of mean absorbed dose values with the hypothesis of uniform activity distribution (MIRD 16 method) assessed the feasibility and safety of IART. In the present retrospective study, a voxel dosimetry analysis was performed to investigate heterogeneity in distribution of the absorbed dose. The aim of this work was to compare dosimetric and radiobiological evaluations derived from average absorbed dose vs. voxel absorbed dose approaches.

METHODS: We evaluated 14 patients who were injected with avidin into the tumour bed after conservative surgery and 1 day later received an intravenous injection of 3.7 GBq of (90)Y-biotin (together with 185 MBq (111)In-biotin for imaging). Sequential images were used to estimate the absorbed dose in the target region according to the standard dosimetry method (SDM) and the voxel dosimetry method (VDM). The biologically effective dose (BED) distribution was also evaluated. Dose/volume and BED volume histograms were generated to derive equivalent uniform BED (EUBED) and equivalent uniform dose (EUD) values.

RESULTS: No "cold spots" were highlighted by voxel dosimetry. The median absorbed-dose in the target region was 20 Gy (range 15-27 Gy) by SDM, and the median EUD was 20.4 Gy (range 16.5-29.4 Gy) by the VDM; SDM and VDM estimates differed by about 6 %. The EUD/mean voxel absorbed dose ratio was >0.9 in all patients, indicative of acceptable uniformity in the target. The median BED and EUBED values were 21.8 Gy (range 15.9-29.3 Gy) and 22.8 Gy (range 17.3-31.8 Gy), respectively.

CONCLUSION: VDM highlighted the absence of significant heterogeneity in absorbed dose in the target. The EUD/mean absorbed dose ratio indicated a biological efficacy comparable to that of uniform distribution of absorbed dose. The VDM is recommended for improving accuracy, taking into account actual activity distribution in the target region. The radiobiological model applied allowed us to compare the effects of IART® with those of EBRT and to match the two irradiation modalities.