Dose rate correction in medium dose rate brachytherapy for carcinoma cervix

Purpose: To establish the magnitude of brachytherapy dose reduction require d for stage IIB and III carcinoma cervix patients treated by external radia tion and medium dose rate (MDR) brachytherapy at a dose rate of 220 +/- 10 cGy/h at point A. Materials and methods: In study-I, at the time of MDR brachytherapy applica tion at a dose rate of 220 +/- 10 cGy/h at point A, patients received eithe r 3060 cGy, a 12.5% dose reduction (MDR-12.5), or 2450 cGy, a 30% dose redu ction (MDR-30), to point A and they were compared to a group of previously treated LDR patients who received 3500 cGy to point A at a dose rate of 55- 65 cGy/h. Study-II was a prospective randomized trial and patients received either 2450 cGy, a 30% dose reduction (MDR-II (30)) or 2800 cGy, a 20% dos e reduction (MDR-II (20), at point A. Patients were evaluated for local con trol of disease and morbidity. Results: In study-I the 5-year actuarial local control rate in the MDR-30 a nd MDR-12.5 groups was 71.7 +/- 10% and 70.5 +/- 10%, respectively, compare d to 63.4 +/- 10% in the LDR group. However, the actuarial morbidity tall g rades) in the MDR-12.5 group was 58.5 +/- 14% as against 34.9 +/- 9% in the LDR group (P < 0.05). Similarly, the grade III and IV morbidity also in th e MDR-12.5 group was 12.5 +/- 9% as against 5.3 +/- 5% in the LDR group (P < 0.05). No statistically significant difference in morbidity was seen betw een the MDR-30 and LDR groups. In study-II the 3-year actuarial local contr ol rate in the MDR-II (30) and MDR-II (20) groups was 66.6 +/- 10% and 74.8 +/- 9%, respectively. There was a significant correlation between the rect al BED received and the percentage of patients developing rectal morbidity. Only 10% of patients receiving a rectal BED of (100 < 120) Gy(3) developed complication as against 62.5% of those receiving a rectal BED of (140 < 16 0) Gy(3) (chi(2) = 46.43; P < 0.001). Conclusion: We suggest that at a dose rate of 220 +/- 10 cGy/h at point A t he brachytherapy dose reduction factor should be around 30%, as suggested b y radiobiological data, to keep the morbidity as low as possible without co mpromising the local control rates. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.