Gap corrections for brachytherapy

With discrete radiation sources, it is a deliberate treatment policy to provide gaps between two implantations of radioactive sources. Gap correction formulae have been arrived at to evaluate the ultimate biological effect on tissue. These formulae do not maintain the identity of the CRE and TDF concepts and hence Supe and Supe have chosen for external beam treatments appropriate gap corrections which maintain the equivalence of the concepts. During the continuous radiation therapy the tissue cells have a tendency to clump together in the sensitive phase and hence the radiation dose is more effective in this mode of treatment. Consequently the mathematical expressions for the CRE and TDFs for continuous radiation therapy differ from those for external beam therapy. The formula for complex treatment schedules of external beam therapy will not apply to those in brachytherapy, have been arrived at. In order to simplify the evaluation of the effective biological effects Tables for value of (T1+G1/T1)-0.155 and CRE0.71 have been prepared. The formulae developed for the resultant CREs and TDFs for the combination of brachytherapy treatments with gaps are different than those for combinations of external beam therapy. The powers of CRE and the terms of the type (T1+G1/T1) have changed appreciably.