Dosimetric Comparison of Supine vs. Prone Position Utilizing Traditional Linac-Based Stereotactic Body Radiation Therapy for Accelerated Partial Breast Irradiation

Abstract

<h3>Purpose/Objective(s)</h3> Our institution recently completed a pilot trial using stereotactic body radiation therapy (SBRT) for accelerated partial breast irradiation (APBI) on a traditional Linac delivering 30 Gy in 5 fractions. In this study, we compared dosimetry of patients treated in supine versus prone positions. <h3>Materials/Methods</h3> Patients enrolled on our recent pilot trial and additional post-accrual patients using the same SBRT technique were included. All patients had early-stage breast cancer, met the APBI ASTRO suitability criteria, and were treated on a traditional Linac. Treatment position, breast laterality and volume, BMI, and organs at risk (OAR) dosimetry were recorded. Prone treatments were delivered on a prone breast board using sliding window IMRT, and supine plans used either IMRT or partial arcs. All prone and 15 of 18 supine plans used free breathing technique (3 utilized deep inspiration breath-hold). CTV was 1 cm expansion from GTV. PTV was a 3 mm expansion from CTV and excluded skin surface+3 mm. Laterality was controlled for heart dosimetry. A small subset of patients was simulated in both supine and prone positions with associated radiation plans available for comparison. For unpaired data, unadjusted comparisons of means were performed using two-group t-test, and analysis of covariance was used for adjusted comparisons. For paired data, unadjusted comparisons were performed using the paired t-test, and adjusted comparisons used mixed models repeated measures analysis. Statistical tests were two-sided. <h3>Results</h3> This study included 35 patients with 36 treated plans. One patient had bilateral breast cancer. Of the 36 treated plans, 18 were supine (11 right-sided breast cancer, 7 left-sided) and 18 prone (8 right-sided, 10 left-sided). Eight patients had comparison plans in both positions. For patients with left-sided breast cancer, heart mean dose supine vs prone was 0.49 and 1.14 Gy, respectively (p < 0.001), and continued to be significant when controlling for BMI and treated breast volume. Mean heart dose was not different for right-sided patients supine vs prone, 0.36 vs 0.47 Gy, respectively (p = 0.870). Ipsilateral lung V1.5 Gy was higher for supine vs prone, 39.44 vs 29.89% (p=0.017). Max rib dose was higher for supine compared to prone, 28.29 vs 16.12 Gy (p<0.001). For the subset of patients with plans in both positions, mean heart dose continued to be lower for left-sided patients treated supine vs prone (p = 0.001). Ipsilateral lung V1.5 Gy and max dose to ribs were lower for prone with p values of 0.003 and <0.001, respectively. <h3>Conclusion</h3> Supine treatment provided heart dosimetry benefit for left-sided breast cancer patients compared to prone. Benefit was independent of BMI and breast volume. Prone position offered lower dose to ipsilateral lung and ribs. The results from this study can help determine the best treatment position based on patient-specific OAR concerns.