Radiobiological quality of high dose rate interstitial brachytherapy treatments of carcinoma of the cervix

Abstract

Aim

The aim of the present study was to apply the Figure of Merit (FOM) concept to High Dose Rate (HDR) interstitial brachytherapy treatments delivered for carcinoma of the cervix at the Kidwai Memorial Institute of Oncology, Bangalore, in order to evaluate the radiobiological quality of the implants.

Materials/Methods

Twenty seven patients received external beam radiotherapy, as 50 Gy in 25 fractions over five weeks, followed by interstitial brachytherapy by the HDR template technique on a Gammamed HDR machine. Doses were delivered 5mm from the periphery of the implant along the transverse plane. Points in the bladder and rectum, receiving the maximum dose rate, were used for the evaluation of total dose received by these critical organs. BED and FOM values were evaluated for all patients.

Results

Two patients had a rectal FOM value greater than 10. A FOM bladder value of higher than 10 was observed in six patients. Only in one patient were FOM values for both the rectum and bladder found to be above 10. The relationship between FOM values for the rectum and the bladder indicated that most FOM values were clustered near the initial portion of the scattergram obtained. For one patient the rectal FOM value was above 10, while the FOM for the bladder was significantly smaller. In another patient, having a high FOM value for the bladder, the FOM value for the rectum was also above the critical value of 10.

Conclusions

Good physical optimization can lead to good radiobiological optimization, as observed in our study. Radiobiology can not improve a bad quality implant. HDR interstitial brachytherapy, which gives better geometrical sparing for critical normal organs such as the rectum and bladder, should be the treatment of choice for patients with carcinoma of the cervix. In optimizing treatment for a patient, the maximizing of FOM should be supplemented by a comparison of reference BED values and new treatments for the tumour, rectum and bladder.