A Three-Dimensional (3D) Analysis of the Conventional Supra-clavicular Field in Breast Cancer

Abstract

Purpose: To assess the 3D dose coverage of the supraclavicular fossa (SCF) and axillary apex (AA) following conventional dose prescription and blocking. Materials and Methods: 16 patients with breast cancer were evaluated using CT-based planning. The SCF, AA, and brachial plexus (BP) were outlined on the planning CT. Spinal cord and humeral head blocks were drawn on the 2D digitally-reconstructed radiograph without the targets displayed. The prescription point was placed 3cm deep, lateral to the sternocleidomastoid muscle, and superior to the medial third of the clavicle. From an oblique 6MV beam plan, dose-volume histograms were generated. Results: The fraction of SCF underdosed by 5% ranged from 2–85%. 2–5% of the SCF was underdosed in five patients, 13–20% in five, 24–30% in three and 68–85% in three. The portion of AA underdosed by 10% was 0–8% in five patients, 20–32% in four, 42–62% in three and 84–100% in five. The volume of BP receiving > 90% of the prescription dose varied from 7–93%. In 6/16 patients, >50% of the BP received >90% of the dose. 2D blocking was suboptimal for the SCF in 2 patients and for the AA in 6. Better target coverage and less volume above 100% of the dose was achieved with inverse planning. Conclusions: Although 2D SCF coverage was adequate in the majority of patients, the conventional field does not optimally cover the SCF in 3D. When the intention is to treat the AA, 3D planning may be required for adequate coverage. Intensity modulation may result in better coverage of the SCF and AA while reducing dose to the BP.