The use of Valencia applicators at extended source surface distances for irregular surface contours

The use of Valencia applicators across irregular surface contours results in significant dose heterogeneities at the prescription depth. Investigations were carried out using extended source surface distances (standoff) to reduce these dose heterogeneities. Relative output factors were measured with a Roos chamber and Gafchromic EBT3 film was used to measure profiles and percentage depth doses at numerous standoff distances. The use of 20 mm standoff was found to reduce the relative difference in delivered dose across a 4 mm change in surface contour by 10.6% for the H2 applicator and 10.0% for the H3 applicator. The radiation became more penetrating with standoff and the field size larger, as expected. The results show that standoff can be used with the Valencia applicator to reduce dose heterogeneities across changes in surface contour.