Abutment of adjacent intraoperative radiotherapy electron fields to treat large tumor volumes

Abutment of unmodified Intraoperative Radiotherapy (IORT) electron fields to irradiate large volumes can lead to dose inhomogeneities at the junction site of the matched fields. Although precise field matching is difficult to achieve in the IORT setting, we have fabricated special cones and dosimetrically evaluated their use. Based on these results, we have established guidelines for the routine use of multiple IORT fields that require field matching. This study examines the abutment of adjacent IORT electron fields for various cone sizes and beam energy combinations, using film dosimetry and a scanning densitometer. Results indicate that for abutting two adjacent 9 x 9 cm square cones, an optimum field separation at the depth of the 90% isodose line (d90) can be obtained as follows: For 6, 9, and 12-MeV electron beams, abut the inner wall of the second IORT cone to the inner wall of the first IORT cone. For 16- and 20-MeV electron beams, abut the inner wall of the second IORT cone to outer wall of the first IORT cone.