Dosimetric analysis of a simplified intensity modulation technique for prone breast irradiation

Purpose/Objective: Prone-position breast radiotherapy has been described as an alternative technique to improve dose homogeneity for women with large, pendulous breasts. We report the feasibility and dosimetric analysis of a simplified intensity modulated radiation therapy (IMRT) technique, previously reported for women in the supine treatment position, to plan prone-position breast irradiation. Materials/Methods: We designed a custom prone-breast board with an adjustable aperture for the breast and a platform height and width allowing clearance through a CT simulator. Eleven patients with clinical stage TisN0-T1bN1 breast cancers undergoing breast-conserving therapy were studied. Patients were positioned on the prototype board with the treatment breast placed through the aperture and the contralateral breast placed on a custom support wedge. CT images were obtained using 3mm slices. Planning target volumes (PTV) were defined as the entire breast delineated on the CT data set, extending to within 5mm of the skin surface and 1cm from the posterior field edge. Treatment plans were developed using both conventional tangents and a simplified intensity modulated tangential beam technique based on optimization of intensity distributions across the breast. All treatment plans were done by a single, experienced planner. The plans were compared with regard to dose-volume parameters. Results: Patients ranged in age from 39-79 years (median 61 years). Patients underwent excisional biopsy alone (n=2), excisional biopsy and sentinel lymph node biopsy (n=6), or excisional biopsy and axillary lymph node dissection (n=3). Two of the patients undergoing lymph node biopsy or dissection had one positive node. The final pathologic diagnoses were ductal carcinoma in situ (n=2) and infiltrating ductal or lobular carcinoma (n=9). Two patients received adjuvant chemotherapy with Adriamycin, Cytoxan, and Taxol. The median separation at the posterior field edge was 16.5cm (range 11.5cm-23cm). The median separation at the isocenter was 9.5cm (range 7.5cm-14cm) and the median breast volume was 814cc (range 399cc-2023cc). Breast depths, as measured from the chest wall to the most gravity dependent skin surface, ranged from 8cm to 16cm (median 11cm). For field widths (2 cm skin flash added to breast depth) greater than 14.5 cm, the IMRT beam was split into two fields with a 2cm overlap corrected by an intensity junction in the overlap region. Dose heterogeneity within the breast PTV was significantly greater for the conventional tangent plans. Eight of eleven patients (73%) received maximum doses of 110% or higher using the conventional tangents versus only one of eleven patients (9%) using the IMRT plan. The isodose level encompassing 5% of the PTV (D05), an indicator of the high dose region within the PTV, was reduced from an average of 109% with conventional tangents to 105% with IMRT. The greatest improvement was seen in the patient with the largest breast separation (23cm at posterior field edge) and depth (16cm). This patient required beam splitting to cover the entire field using IMRT. For this patient, using IMRT reduced the D05 from 117% to 109% and the Dmax decreased from 123% to 112%. For all patients, the maximum dose within the PTV was reduced from an average of 113% with conventional tangents to 107% with intensity modulation (Table). The overall planning and treatment time for the simplified IMRT technique were comparable to that of the standard technique. Conclusions: An IMRT approach is feasible for prone-position breast radiotherapy and improves dose homogeneity, particularly in women with larger breast sizes. Further follow-up is necessary to determine if improvements in dose homogeneity impact on acute toxicity and cosmetic outcome in this cohort of women who have historically suffered from poor cosmesis after breast conserving therapy. Tabled 1IMRTConventionalRatio (IMRT/Conv)Mean Dmax107.2%±1.8%113.0%±4.4%95.0%±2.2%Mean D05105.3%±1.4%109.1%±3.1%96.6%±1.6% Open table in a new tab