The biological effectiveness of Intra-Operative RadioTherapy (IORT) beams

Intra Operative Radiation Therapy (IORT) is a combined treatment modality in which a large single dose of radiation is delivered at the time of surgery for tumor resection. The recent development of small dedicated electron accelerators has led to widespread use of IORT in clinical routine. The dose per pulse and dose rates delivered by IORT are about 100- and 20-fold greater than in conventional radiation therapy. The possible biological effects due to the delivery modality of IORT are presently not taken into account in treatment planning. We studied the biological effectiveness of the very peculiar IORT beams (electrons delivered at very high dose rate, high dose per pulse and a total dose delivered in a single fraction) by measuring the clonogenic survival following the electron beams, as a function of dose rate and dose per pulse, compared to that elicited by photons for the breast cancer carcinoma MCF-7 cell line. Early breast carcinoma represents one of the tumors of choice for undergoing IORT. Thus far, our results do not seem to support an effect depending on either the dose per pulse or the dose rate in the IORT clinically used range. However, our data point to a higher RBE at the very high dose rates. Although these rates are not used in the clinic, the latter results are being further investigated.