Room: ePoster Forums
Purpose: to study the physical factors contributing to true surface or skin dose leading to acute skin reactions in breast radiotherapy.
Methods: The biologically relevant depth for acute skin reactions in radiotherapy is 70 Âµm. The dose at this depth is difficult to either measure or calculate and can be quite different than the dose at a depth of as little as 1 mm. For breast radiotherapy with medial and lateral tangential beams, the skin dose depends on both the contribution from the entrance beam and the exit beam. The skin dose has been estimated in a breast model hemi-ellipse accounting for field size, beam energy, obliquity, lack of backscatter, fractionation, size and shape of the hemi-ellipse. Dose distributions at the skin surface have been computed as a function of the polar angle from the center of the hemi-ellipse.
Results: The exit dose always dominates the entrance dose for all realistic parameters. As a result, the surface dose is higher for 18 MV than 6 MV over the entire surface for all reasonable sizes and shapes of the hemi-ellipse. The ratio of the surface dose to the mid-depth dose ranges from about 35% at polar angle 0Â° up to 70% at polar angle 80Â°. The dose rises sharply at angles above 30Â°. The surface dose rises moderately at all angles as the size of the hemi-ellipse increases. The biologically effective dose for erythema and moist desquamation is about 2 - 3 Gy higher at all polar angles for conventional fractionation (2.00 Gy x 25 fractions) than for hypofractionation (2.66 Gy x 16).
Conclusion: Understanding the physical factors that lead to high surface dose in breast radiotherapy may help to predict those patients at risk for acute skin reactions and modification of treatment parameters may lead to mitigation.