Modified sector-integration method for predicting the output factors ofelectron beams including extended source to surface distance

Abstract

A modified sector-integration method is presented that can predict the outputfactors of irregular shaped electron fields even in the case of extended sourceto surface distance (SSD). The model takes as input measured output factors forcircular inserts of various radii. These circular fields were measured at SSDsof 100, 105 and 110 cm to determine the effective source distance as a functionof radius (ESD(r)). For an arbitrary electron field at any SSD, the shape isdivided into small sectors, and the contribution calculated from the radius andESD(r).The calculated output factors were verified by direct measurements of varioustypes of electron fields mainly based on clinical use. The energies modelled were8, 10 and 12 MeV for applicator sizes of 10 cm × 10 cm and 14 cm × 14 cm(defined at 95 cm). The calculated values agreed with the measured data within 1%for the various rectangular cutouts including extended source to surface distance.We retrospectively modelled 97 patient inserts of irregular shape, and foundagreement within 2% of measured values.