Radiobiological comparison between Cobalt-60 and Iridium-192 high-dose-rate brachytherapy sources: Part I-cervical cancer

Purpose This study aimed to compare the biological effective doses (BEDs) to clinical target volume (CTV) and organs at risk (OARs) for cervical cancer patients treated with high-dose-rate (HDR) Iridium-192 (Ir-192) or Cobalt-60 (Co-60) brachytherapy (BT) boost and to determine if the radiobiological differences between the two isotopes are clinically relevant. Methods Considering all radiosensitivity parameters and their reported variations, the BEDs to CTV and OARs during HDR Co-60/Ir-192 BT boost were evaluated at the voxel level. The anatomical differences between individuals were also taken into account by retrospectively considering 25 cervical cancer patients. The intrafraction repair, proliferation, hypoxia-induced radiosensitivity heterogeneity, relative biological effectiveness (RBE), and source aging dose-rate variation were also taken into account. The comparisons in CTV were performed based on equivalent uniform BED (EUBED). Results Considering nominal parameters with no RBE correction, the CTV EUBEDs were almost similar with a median ratio of similar to 1.00 (p < 0.00001), whereas RBE correction resulted in 3.9%-5.5% (p = 0.005, median = 4.8%) decrease for Co-60 with respect to Ir-192. For OARs, the median values of D-2cc (in EQD2(3)) for Co-60 were lower than that of Ir-192 up to 9.2% and 11.3% (p < 0.00001) for nominal parameters and fast repair conditions, respectively. In addition, for a nominal value (reported range) of radiosensitive parameters, the CTV EUBED differences of up to 6% (5%-10%) were assessed for HDR-BT component. Conclusion The RBE values are the most important cause of discrepancies between the two sources. By comparing BED/EUBEDs to CTV and OARs between Co-60 and Ir-192 sources, this numerical study suggests that a dose escalation to similar to 4% is feasible and safe while sparing well the surrounding normal tissues. This 4% dose escalation should be benchmarked with clinical evidences (such as the results of clinical trials) before it can be used in clinical practice.