Shoulder-sparing Inverse Planning IMRT versus Forward Planning IMRT for Post-mastectomy Chest Wall and Regional Nodal Irradiation: A Dosimetric Comparison Study

Introduction: Post-mastectomy radiotherapy (PMRT) using moderate hypofractionation has become the standard of care for chest wall and regional nodal irradiation. However, shoulder pain and stiffness affect up to one-third of survivors, especially those with axillary dissection. These late effects can impair the activity of daily living and overall quality of life. Prior dosimetric studies, including our own earlier work, have largely overlooked the dose to the adjacent shoulder musculature (Shoulder-OAR). Recognising this gap, the present study evaluates the potential of Tangent Inverse Planning IMRT (T-IMRT) to better spare the Shoulder-OAR compared with conventional forward-planned Field-in-Field 3DCRT (FIF 3DCRT).

Methodology: Ten left-sided PMRT patients treated with moderate hypofractionation (40 Gy in 15 fractions, DIBH) were replanned using both techniques. Target volumes and conventional organs at risk were contoured using standard guidelines, and additional shoulder and back OARs were delineated based on recent guidelines. Dosimetric parameters were compared using the Mann–Whitney test.

Results: T-IMRT achieved target coverage with lower doses to the shoulder and back:

• Shoulder V30Gy: 28.8 ± 10.1(FIF) vs 4.6 ± 4.5(T-IMRT):p < 0.001
• Shoulder V20Gy: 37.0 ± 8.7 vs 15.3 ± 5.8:p < 0.001
• Back-OAR- V30Gy: 19.10±8.99 vs. 3.08±2.45:p<0.00024
• Back-OAR- V20 was  24.91±10 vs. 14.38±8.13:p<0.0152).

The low dose to the left lung (V8Gy): 35.85±7.96 and 54.3±11.64; p<0.002) in FIF 3DCRT and T-IMRT, respectively. Cardiac and lung doses remained comparable, though low-dose lung exposure slightly increased due to IMN inclusion.

Conclusion: IMRT can significantly reduce radiation exposure to the shoulder muscles, which may help preserve long-term arm mobility and improve quality of life. Most dosimetric studies, including our previous work, have largely overlooked this area. There is an urgent need for a collaborative, multi-institutional effort to validate the use of shoulder and back organ-at-risk (OAR) sparing in treatment planning and to correlate these dosimetric benefits with functional outcomes.

Competing Interests: The authors declare no competing financial interests.

Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Licence: © Author(s), 2026. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, and unrestricted adaptation and reuse, including for commercial purposes, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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