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

Abstract

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 (192 Ir) or Cobalt-60 (60 Co) brachytherapy (BT) boost and to determine if the radiobiological differences between the two isotopes are clinically relevant. Considering all radiosensitivity parameters and their reported variations, the BEDs to CTV and OARs during HDR 60 Co/192 Ir 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). Considering nominal parameters with no RBE correction, the CTV EUBEDs were almost similar with a median ratio of ∼1.00 (p<0.00001), whereas RBE correction resulted in 3.9%-5.5% (p=0.005, median=4.8%) decrease for 60 Co with respect to 192 Ir. For OARs, the median values of D2cc (in EQD23 ) for 60 Co were lower than that of 192 Ir 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. The RBE values are the most important cause of discrepancies between the two sources. By comparing BED/EUBEDs to CTV and OARs between 60 Co and 192 Ir sources, this numerical study suggests that a dose escalation 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.