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) or Cobalt-60 ( 60 Co) 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 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 doserate 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 ∼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 D 2cc (in EQD2 3 ) 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. Conclusion: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.

Radiobiological and dosimetric comparison of 60Co versus 192Ir high-dose-rate intracavitary-interstitial brachytherapy for cervical cancer

“…Moreover, the bladder and rectum had higher doses than the intestines due to their location being closer to the source of radiation. It has previously been demonstrated that 60Co produces a larger hot spot than 192Ir in the CTV of tumors and that the dose rate falls more rapidly as the distance from the source increases [32][33][34]. Compared to the published results, the internal, external, and obturator lymph nodes etc., (locations far away from the center of the source) receive a lower dose when using 60Co for ICBT [35].…”

“…The repair of sublethal damage to normal tissues may not be completed in time, so the control of the tumor cannot be separated from the time of the entire radiotherapy [ 45 ]. In the study conducted by Dayyani, M. et al, considering the influence of RBE correction and a patient's full treatment regimen, the BED produced by 60Co was lower, so it was recommended that the prescribed dose be increased by 4% when using a 60Co source; the EQD2 of OARs after dose escalation was close to or even lower than that of 192Ir [ 33 ]. However, our study was mainly conducted to compare the physical dose and radiobiological differences in a single IC-ISBT irradiation dose with regard to the choice of radioactive source, so the other treatments, including the EBRT that was performed and the remaining four ISBTs, were treated uniformly and were simplified.…”