Modeling normal tissue complication probability from repetitive computed tomography scans during fractionated high-dose-rate brachytherapy and external beam radiotherapy of the uterine cervix

Purpose: To calculate the normal tissue complication probability (NTCP) of late radiation effects on the rectum and bladder from repetitive CT scans d uring fractionated high-dose-rate brachytherapy (HDRB) and external beam ra diotherapy (EBRT) of the uterine cervix and compare the NTCP with the clini cal frequency of late effects. Methods and Materials: Fourteen patients with cancer of the uterine cervix (Stage IIb-IVa) underwent 3-6 (mean, 4.9) CT scans in treatment position du ring their course of HDRB using a ring applicator with an Iridium stepping source. The rectal and bladder walls were delineated on the treatment-plann ing system, such that a constant wall volume independent of organ filling w as achieved. Dose-volume histograms (DVH) of the rectal and bladder walls w ere acquired, A method of summing multiple DVHs accounting for variable dos e per fraction were applied to the DVHs of HDRB and EBRT together with the Lyman-Kutcher NTCP model fitted to clinical dose-volume tolerance data from recent studies. Results: The D-mean of the DVH from EBRT was close to the D-max for both th e rectum and bladder, confirming that the DVH from EBRT corresponded with h omogeneous whole-organ irradiation. The NTCP of the rectum was 19.7% (13.5% , 25.9%) (mean and 95% confidence interval), whereas the clinical frequency of late rectal sequelae (Grade 3-4, RTOG/EORTC) was 13% based on material from 200 patients. For the bladder the NTCP was 61.9% (46.8%, 76.9%) as com pared to the clinical frequency of Grade 3-4 late effects of 14%. If only 1 CT scan from HDRB was assumed available, the relative uncertainty (standar d deviation or SD) of the NTCP value for an arbitrary patient was 20-30%, w hereas 4 CT scans provided an uncertainty of 12-13%. Conclusion: The NTCP for the rectum was almost consistent with the clinical frequency of late effects, whereas the NTCP for bladder was too high. To o btain reliable (SD of 12-13%) NTCP values, 3-4 CT scans are needed during 5 -7 fractions of HDRB treatments. (C) 2000 Elsevier Science Inc.