Immune-sparing potential of isocentric lateral decubitus positioning in adjuvant whole-breast radiotherapy.

Background: Lymphocytes are among the most radiosensitive cell types, and low-dose exposure during radiotherapy has been associated with measurable immune suppression. In breast cancer treatment, systemic immune exposure is partly driven by irradiation of highly vascularized organs such as the heart, lungs, and liver. The axillary lymph nodes also play a critical role in initiating antitumor immune responses. Isocentric lateral decubitus (ILD) positioning, by modifying the beam orientation, may reduce incidental exposure to these immune-related structures. This study aimed to evaluate the potential of ILD to spare both circulating immune cells and axillary lymph nodes compared to standard supine positioning during whole-breast irradiation (WBI).

Methods: Eight patients with localized breast cancer treated with breast-conserving surgery and adjuvant WBI (without nodal irradiation) were included in this retrospective dosimetric study. Each patient underwent CT simulation and treatment planning in both supine and ILD positions. The estimated dose to circulating immune cells (EDIC) was calculated using a model based on mean doses to the lungs, heart, liver, and whole body. The level I axillary region was retrospectively contoured, and both the mean dose and D95 were extracted. Dosimetric comparisons between positions were performed using paired Wilcoxon signed-rank tests.

Results: Isocentric lateral decubitus positioning significantly reduced the EDIC compared to the supine position (median 0.56 Gy vs. 1.12 Gy; p < 0.01). Lung, heart, and liver doses were also significantly lower. The axillary dose was reduced, with the mean dose decreasing from 10.2 to 3.8 Gy (p = 0.016) and D95% from 0.82 to 0.35 Gy (p = 0.039).

Conclusion: Isocentric lateral decubitus positioning significantly reduces both systemic and regional immune irradiation during adjuvant WBI. These findings suggest that ILD may help to preserve immune function, particularly in immunogenic subtypes such as triple-negative breast cancer, and support its consideration in treatment planning when nodal irradiation is not indicated.