Abstract P3-12-25: Dosimetric impact of patient rotation during prone breast radiotherapy
Authors: Cheng K , Hoopingarner S , Wright C , Daly M , Fragoso R , Zhao X 
Published: May 2019

Abstract
Purpose
Prone positioning has been used as a viable alternative to conventional supine position for patients receiving breast radiation therapy. However, little research has been done exploring the axial rotation of patients toward the treated breast when “sinking” into the opening of the breast board and its potentially negative effects on dosimetric outcomes, which may include increased heart and lung dose. The physician may need to move the posterior border away from the chest wall to reduce heart and lung dose.
Methodology
49 consecutive female patients with left sided early stage breast cancer treated at University of California Davis Medical Center were assessed from 2015 to 2018 (age range: 42-84 years, median age: 62 years). All patients underwent prone whole breast therapy with conventional external beam radiation therapy (EBRT) at doses of 50 Gy (n = 12) or hypofractionated at 42.56 Gy (n = 37). Treatment plans and dose volumes were retrospectively analyzed for each patient. Standard tangents were designed for each patient using clinical landmarks of the midaxillary line and midsternal line, which were then compared to the delivered tangent beams. The angle created between a vertical line centered on center sternum and a line drawn from center sternum to center spinal cord served to define degree of axial rotation. Breast depth was defined by the longest horizontal length from outer rib to edge of breast on sagittal view. Patients were divided into subgroups by degree of rotation and absolute breast depth. A two tailed paired Student's t-test was used for analysis.
Results
Overall mean heart and lung dose were 82.2 cGy and 50.43 cGy for the entire cohort, respectively. For standard tangents, patients with degree of rotation &lt; 5 degrees in the prone position (n = 23) had significant lung sparing as compared to patients with degree of rotation &gt; 5 cm (n = 26) (mean lung dose: 61.8 cGy vs 129.6 cGy, p = 0.00329). This was also seen for cardiac sparing (mean heart dose: 105.9 cGy vs 183.9 cGy, p = 0.000235). Even with reduction of posterior border for treatment delivery, there remained a significant increase in mean heart and lung dose with increased rotation (p = 0.038, p = 0.046). Although not statistically significant, for patients with &gt; 5 degrees of rotation there was a trend toward increased reduction of the posterior border of the tangent (13 mm vs. 7.5 mm, p = 0.13). A significant predictor of increased rotation was breast depth &gt; 10 cm (p = 0.01). Patients with absolute breast depth &gt; 10 cm (n = 23) in the prone position had significant lung sparing as compared to patients with absolute breast depth &lt; 10 cm (n = 26) (mean lung dose: 58.6 cGy vs 40.8 cGy, p = 0.042).
Conclusion
To our knowledge, this is the first dosimetric comparison of prone breast therapy exploring the degree of patient roll into the prone-breast setup cavity. This study demonstrates a significant increase in mean lung and heart dose when patient rotation is greater than 5 degrees. Given this, the posterior border may have to be reduced to prevent a higher than intended dose to the heart and lung. Proper attention during simulation is important to allow for optimal dose distribution and special attention should be paid to women with smaller breast size.
Citation Format: Cheng K, Hoopingarner S, Wright C, Daly M, Fragoso R, Zhao X. Dosimetric impact of patient rotation during prone breast radiotherapy [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-12-25.