343 - Upright Breast Radiotherapy: A Simulation Study of Anatomical Geometry and Dose-Volume Characteristics Using Paired Upright and Supine CT

04:35pm - 04:45pm ET

Room 156

Presenter(s)

Kohei Oguma, MS - Keio University School of Medicine, Tokyo 160-8582, Tokyo

K. Oguma¹, N. Sanuki², Y. Yamada², M. Sawada³, M. Yamada², Y. Yokoyama², A. Takeda², and M. Jinzaki²; ¹Department of Radiology, Keio University School of Medicine, Tokyo 160-8582, Japan, ²Department of Radiology, Keio University School of Medicine, Shinjuku-ku, Japan, ³Keio University Graduate School of Medicine, Shinjuku-ku, Tokyo, Japan

Purpose/Objective(s):

To quantitatively evaluate the dosimetric feasibility of upright whole-breast irradiation from a treatment-planning perspective using paired upright and supine CT imaging.

Materials/Methods:

CT imaging data acquired between 2017 and 2018 from female volunteers who had undergone same-day upright and supine CT scans under inspiratory breath-hold were retrospectively analyzed. The left whole breast was contoured as the clinical target volume, the left lung and heart were contoured as organs at risk. Posture-related anatomical changes in volume, geometry, and CT values were quantitatively assessed. Three-dimensional conformal tangential left whole-breast irradiation plans (42.56 Gy in 16 fractions) were generated on both CT datasets using identical planning criteria. Dose–volume histogram parameters were compared using paired statistical analyses.

Results:

A total of 33 volunteers were included. The median (interquartile range) age was 45 (35–54) years, and the body mass index (BMI) was 20.9 (18.4–22.0) kg/m2. In the upright position, inferior breast displacement was observed (P < .001) and was more pronounced in participants with higher BMI, larger breast, and older age. In addition, lung volume increased with decreased mean CT values, while heart volume decreased with compaction. Target dose coverage was equivalent between upright and supine positions (P = .458). Left lung dose was significantly reduced in the upright position, with a median reduction of 3.8% in V20 and a lower mean left lung dose (supine: 4.4 [3.9–5.3] Gy vs. upright: 2.2 [1.6–3.3]; P < .001). The overall irradiated body volume was also reduced by 21.6%. Heart dose metrics remained comparable between the supine and upright positions under inspiratory breath-hold.

Conclusion:

This planning comparison study using paired supine and upright CT images demonstrated that upright whole-breast irradiation achieved target coverage and heart dose equivalent to conventional supine treatment, while consistently reducing left lung dose and overall irradiated body volume.