The influence of source decay on BED in2 high dose rate brachytherapy for cervical cancer : considering different tumor cell lines and normal tissues

Abstract

OBJECTIVE: This study focuses on the influence of intra-fraction DNA dam-15 age repair and cellular re-population on biologically effective dose (BED) cased by the source decay at different time points in a source exchange cycle , in order to examine the effect of source decay for different tumor cell lines and normal tissues in Ir192 high dose rate (HDR) brachytherapy (BT) for cervical cancer. METHODS: 24 patients who accepted HDR BT were chosen in this study. Radiobiological parameters of T1/2, Tk, Tp and α/β from the recommendations of the AAPM TG137 and ICRU89 then were considered in a new full form BED formula, which was used to replaced the common simplified formula that ignore the intrafraction repair and interfraction repair as well as the repopulation. Fraction times were normalized to require 4.26.6 min for 78 Gy prescription dose of plans at 10 Ci of sources activities according to the different clinical tumour volumes in HDR BT (HRCTVs). Calculations of different BED values were carried out in HRCTVs about two commonly clinical classifications and nine cervical cell lines according the different values of α/β and BED formulas respectively. Organs at risk (OARs) were also statistically analyzed, as well as the duration of each treatment have been recorded. RESULTS: Compared to the simplified BED formula based values (BED), the full form formula based BED values (BEDg) showed significant difference, which were decreased more and more obviously with the source decay from 10 Ci to 2Ci in cervical cancer patients. With the α/β value of 10 for tumour, the maximum percent values (%) of changing BED(∆BED = BED-BEDg) by two formulas (Gy, x±s) for the source activity were 3.05 ± 0.47,and 2.415 ± 0.38 in D90% and D100% in HRCTV, respectively. Focusing on the clinical classifications and cell lines, the top first three percent values of ∆BED over 5Gy were 14.06±1.67 and 11.08±1.43; 9.92±1.19 and 7.51±1.01; 7.57±1.05 and 5.90±0.87 in D90% and D100% for HRCTV with the α/β values of 10.00 (stage I and II cervix carcinoma), 16.46 (HX156c), 11.40 (HX155c). The similar results also showed in OARs, that with source decay, the maximum percent values of ∆BED (α/β = 3) in bladder was 13.37±2.27 at D0.1cc; in the rectum was 11.92±2.10 at D0.1cc; in the sigmoid and small intestine bowel were 12.45±2.27 and 11.91±2.62 at D0.1cc, respectively. The duration of each treatment fraction also had a positive correlation with the source decay. With the source activity decay, the treatment time has changed obviously, which was from 5-8 minutes (10 Ci) to 27-33 minutes(2 Ci). Conclusion: This study confirmed that the source delay has a great effect on the BED of the cervical cancer tissue with different cell lines and the normal tissues. The full form BED considering repair and repopulation can more accurate assessment of absorbed dose in the HDR BT for cervical cancer, especially for the hyper fraction schemes with single fraction dose at the 2Ci source in HDR BT. The simplified BED calculation that dose not take into account the source activity may not be appropriate in the HDR BT of cervical cancer.