Application of radiation effect models in combined external and intracavitary radiotherapy of carcinoma of the uterine cervix

Dose-volume histogram analysis of high dose rate intracavitary brachytherapy for uterine cervix cancer

PURPOSE

We retrospectively analyzed the relationship between dose distribution and local control using a dose-volume histogram (DVH) in patients with cancer of the uterine cervix treated by definitive radiotherapy including intracavitary brachytherapy.

METHODS AND MATERIALS

Twenty-five patients with squamous cell carcinoma of the uterine cervix who underwent definitive radiotherapy between August 1987 and April 1994 were selected for the present study. They included 15 patients with local control and 10 patients with local recurrence. In principle, these patients were treated with 50 Gy of external beam pelvic radiotherapy and a point A dose of 24 Gy, in four fractions, of intracavitary brachytherapy. The DVHs of tumor volumes were calculated by superimposing three-dimensional (3D) dose distributions on computed tomography (CT) images taken before brachytherapy.

RESULTS

Differential DVHs revealed a tendency for the portion of the total tumor volume to which the delivered dose was low to be larger in patients with local recurrence. The tumor volumes and the absolute dose volumes of which the absorbed dose was less than 24 Gy [DV (< 24 Gy)] were significantly larger in patients with local recurrence than those in local control patients (p = 0.02 and 0.03, respectively). The percent DV (<24 Gy) was not significantly different in the two groups. In patients with larger tumor volume, the absolute DV (<24 Gy) was also larger and a strong linear correlation was noted between them.

CONCLUSIONS

The analysis of dose distribution of brachytherapy using DVH was useful to evaluate the quality of dose distribution quantitatively. The absolute dose volume was considered more important than the percent dose volume for evaluation of the clinical outcome. Our study suggested that unfavorable dose distribution for the tumor volume in brachytherapy was one of the reasons of poor local control in patients with large tumor volume.

HDR versus LDR gynecological brachytherapy revisited

Despite the obvious breakthrough of high dose-rate (HDR) afterloading systems on the gynecological brachytherapy market, questions still remain regarding the transfer of available expertise gained throughout the last 80 years with low dose-rate (LDR) radium and cesium, especially regarding the conversion of LDR total dose into equivalent HDR dose per fraction and total dose. Calculation of biologically equivalent schedules requires a knowledge of repair capacity and repair kinetics of tumors and normal tissues, both of which influence the biological effect of any radiation dose. The clinical experience with HDR is, however, accumulating and it is acknowledged that the new technique entails an acceptable therapeutic index as compared to the classical LDR. There is thus a state of apparent 'equivalence' between the two treatment modalities. This state is influenced by many factors in which, in contrast to what is frequently claimed, radiobiological factors do not play the most important role. It is probably its high-tech environment which makes HDR an acceptable alternative. Treatment at LDR, indeed, has proven to be quite tolerant to a lack of absolute precision, something that would be disastrous with HDR techniques. Because HDR intracavitary brachytherapy has not been compared in controlled trials with the best existing LDR brachytherapy, but only retrospectively with heterogeneous LDR clinical data, it cannot be claimed to be equivalent, but simply feasible.