3D-dosimetry and radiobiological evaluations in early breast cancer patients undergoing intraoperative avidination for radionuclide therapy (IART)

Objectives IART is a novel targeted radionuclide therapy (TRT) based on the avidin-biotin-90Y system, recently applied in early breast cancer patients. It represents an anticipated boost irradiating the residual mammary gland immediately after surgery and shortening the conventional external-beam-radiotherapy. IART is able to deliver a Biologically Effective Dose (BED) of 21 Gy by the injection of 90Y-biotin. Previous standard-dosimetry assessed IART feasibility and safety. Retrospectively, a voxel-dosimetry analysis was performed to investigate dose distribution heterogeneity. Dosimetric and radiobiological data derived from standard vs. voxel methods were compared.

Methods Fourteen patients were evaluated. Avidin(100mg) was injected in tumour bed during surgery, after tumour removal, and targeted by systemic injection of 90Y-biotin+111In-biotin (3.7GBq+185MBq, respectively) one day later. Sequential images (planar+SPECT) were used to estimate the average dose in the target region (standard method). For voxel dose calculation, a software was developed and S-voxel factors for 90Y calculated by MonteCarlo simulation. Dose-volume and BED-volume histograms were generated to derive Equivalent-Uniform-Biological-Effective-Dose (EUBED) and Equivalent-Uniform-Dose (EUD) (α/β=10,μrepair=0.5 h-1,α=0.3 Gy-1).

Results No cold spots were highlighted by voxel-dosimetry. Median(range) dose in target region was 20 (15-27) Gy(standard-method); median(range) EUD was 20.4 (16.5-29.4) Gy (voxel-method). EUD/mean dose among voxels ratio was >0.9 in all patients, indicative of acceptable uniformity in the target. Median (range) BED and EUBED were 21.8 (15.9-29.3)Gy and 22.8 (17.3-31.8) Gy.

Conclusions Voxel method highlighted the absence of cold spot in the target. EUD/mean dose ratio indicated biological efficacy comparable to that of uniform dose distribution