High Dose Rate Brachytherapy (HDR BT) has become a standard treatment as a boost or monotherapy for different tumors such as gynecological malignancies, prostate cancers, sarcoma, and breast cancers [1]. In most of modern clinical studies in BT, one of the commercial Ir-192 sources was employed. Recently though, Co-60 HDR BT sources have been used worldwide. Co-60 with a lower dose rate (about one-half), can deliver the prescribed dose two times longer than Ir-192 [2]. Moreover, because of their different photon spectra, these two sources show some discrepancies in dose distributions. Before expanding the Iridium-based results to Cobalt-based clinical treatments, It may be questioned whether these differences can meaningfully influence the biologically effective dose (BED) distributions inside the tumor?

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