Comparative Measurements of Homogeneity and Dose Distribution Cobalt-60 and 9-MeV X-Ray Beams in the Mamma and the Chest Wall

Abstract

In radiotherapeutic practice the homogeneity of the dose distribution has an important influence on the tumor killing rate. After mastectomy or conservative operation in patients with breast cancer, irradiation of the mamma and thoracic wall requires substantial attention to treatment planning due to varying topographic features in the thoracic area. Problems concerning dose distribution and homogeneity often result from an uneven course of the body surface or a different volume of the mamma after lumpectomy or quadrantectomy. This leads to a number of questions, for example: What radiation quality should be used in irradiation of the chest wall? Is there a change in the build-up effect upon altering the angle of incidence? While there are no physical problems in depth dose distribution when administering a vertical beam to the body surface, clinical experience shows that the build-up effect does not seem to be that expected from tangential rotation or opposed field techniques for mammary or chest wall tumors. In general, this margin effect is not a consideration in radiation treatment planning systems. We therefore attempted to find a qualitative scale for isodoses of cobalt-60 and 9-MeV X-rays in patients with tumors near the body surface and to draft practical guidelines for clinical routine, especially for linear accelerators, in this energy range.