Implication of uterine configuration on dosimetry to point a using the Amersham afterloading system

Abstract

Ninety-four biopsy-proven cases of cervix carcinoma who underwent brachytherapy with the flexible Amersham afterloading system are examined in this study. The uterine configuration showed a wide range with respect to the length, angle of version (indicated by angle alpha) and the degree of right or left shift of the midline at the internal os (indicated by angle beta). The manufacturers of the afterloading system have given certain precalculated dose rates to A which they derived from computer calculations for the various configurations of the system, for a particular loading strength of the Caesium 137 source trains, and for a particular range of angle alpha and beta. The study involves mathematical calculations of point A dose rates using post application radiographs, and a mean value of point A dose rate is derived to compare with other variables. A distortion of the uterus in the form of flexion (as opposed to version), resulting in bending of the uterine tandem is noted in 3 cases. There are 22% of cases with gross degree of retroversion (angle alpha more than 180°) for which the precalculated tables are lacking. The angle beta, however, is in the normal range for all the cases but one. Nineteen cases (20.2%) showed point A dose rates which are beyond the specified dose rates of the manufacturers. Analysis of these 19 cases showed 6 cases with angle alpha more than 180°, eight cases with more than a 10° shift of the uterus to right or left of midline (angle beta) and the rest five cases have fairly normal values of these angles. Analysis of these five cases showed that three of them showed more than 8 CGy/hr difference in dose rates to individual points A right and left. An undue curvature of the uterine tandem to one side in the distal half, with a straight proximal half (near the anterior flange) is noted in these three cases. The angle alpha and beta are normal here since the nearest 2 cm of uterine tandem to the anterior flange is straight, but the distal end lying in a closer position to one side, contributed more dose to point A of that side. This factor of flexion might have resulted both in contributing to a difference in dose rates to points A (Right and Left), as well as undue difference in mean point A dose rates beyond the specified accuracy limits of the manufacturers. The flexion deformity might have been caused by uterine distortion which is a result of post external radiotherapy fibrosis of the uterus, post partum fibrosis, and post infection fibrosis. The cause of dosimetric differences in the last 2 out of 94 cases could not be explained at present. The importance of dosimetric factors and their implications are discussed, and a highly individualistic attitude in brachytherapy is highlighted.