A dosimetric analysis of Morris, Fletcher, and Henschke systems for treatment of uterine cervix carcinoma.

The role of intracavitary irradiation in the treatment of uterine cervix carcinoma is well established, and over the years a number of different systems for intracavitary irradiation have been developed. To compare the clinical efficacy of different systems and to develop guidelines for the design of applicators with new sources such as americium-241, we present a dosimetric comparison of three systems: (a) the Morris system, a modified Stockholm technique; (b) the Henschke system; and (c) the Fletcher system. Using a computerized planning system, dose distributions with different configurations of each system were calculated. For each case, doses to point A, B, and a set of reference points representing bladder and rectum were also calculated. Also, the 60 Gy reference volumes, as defined by ICRU Report No. 38, 1985, were calculated for six different treatment regimens. These treatment regimens employ widely different combinations of whole pelvis external beam dose, split pelvis external beam dose, and intracavitary irradiation dose to achieve similar clinical outcomes for the treatment of various stages of cervix carcinoma. From this analysis we observe the following: (a) The Morris system produces a higher dose rate to point A (70 to 90 cGy/hr) compared to the Fletcher or Henschke system (50 to 70 cGy/hr); (b) the doses to point B relative to point A dose are about the same for all three systems at 28 to 32%; (c) the doses to reference rectum and bladder points relative to point A dose for clinically equivalent configurations are about the same for Fletcher and Henschke systems (58-65%) not including the effects of shields in the vaginal ovoids, and somewhat higher for the Morris system (72-79%); (d) the volume treated to a given dose rate by each intracavitary system alone is about the same; and (e) the 60 Gy volume depends critically upon the external beam whole pelvis dose, rising steeply as the external beam whole pelvis dose approaches 30 Gy. Since different groups have used widely different prescriptions of external beam whole pelvis dose, ranging from 0 to 50 Gy depending upon stage, the 60 Gy volumes for these various dose prescriptions are strikingly different. Because the Morris system uses lower values for the external beam whole pelvis dose than the others, its 60 Gy volume for the advanced Stage IIB and IIIB is 2 to 4 times lower than others. This choice makes the Morris system more conservative than others, probably resulting in slightly lower cure rates for the advanced stage disease.