1031 Collimators in asymmetric mode and 3D planning. Applications in breast cancer

Abstract

Introduction The possibility of use linear accelerators with two pairs of opposing collimators operating independently (asymmetric mode), allows beam modifications in habitual treatment planning. These modifications can be verified in 3D planning. We reported a variation in breast cancer treatment planning to avoid inhomogeneities at toracoclavicular fields’ junction. Technique 1.—Patient positioned in an adjustable inclined plane to maintain the chest wall in line with the treatment couch. This position avoids collimator rotation in the tangential fields. 2.—Treatment planning in a conventional simulator or CT. The 3D dosimetry with multi-image CT display allows us to know dose distribution in the whole target volume. 3.—Treatment delivered using a LILAC with two pairs of asymmetric collimators and three isocentrics beams. The isocenter was at the junction beams. (a) Superior longitudinal half beam in asymmetric mode, (with secondary field blocks to avoid humeral articulation) to treat the axilosupraclavicular fosse. (b) Inferiors longitudinals half beams in asymmetric mode, to cover the breast or the chest wail with two tangential fields. (i) They can be also in asymmetric mode in transversal collimator (a quarter of field) to avoid lung radiation. Conclusion Our dosymetric 3D study verifies the perfect dose homogeneity in treatment fields junction using collimators in asymmetric mode. This radiation technique allows to do diary treatment without moving the patient or the treatment couch. The progressive introduction of informatic control systems in treatment radiotherapy (performing collimators size, gantry angulation and other radiation parameters) makes easy to reproduce this technique diary.