Abstract 33P
Background
The internal mammary nodes (IMNs) play a crucial role in the lymphatic drainage of the breast. There is often variability among physicians in actual practice regarding IMN irradiation, as it can lead to higher doses to the heart and lungs. This can vary depending on the chosen RT technique and the inclusion of nodal irradiation. Understanding these variations is essential for optimizing treatment plans to minimize unnecessary radiation exposure and associated risks.
Methods
This study retrospectively analyzed the treatment plans of breast cancer patients who underwent hypofractionated adjuvant RT(42.56Gy/16#) at our center between 2019 and 2021. Incidental IMN dosing (contoured based on RTOG atlas) was compared between 3D conformal RT (3D-CRT) and volumetric modulated arc therapy (VMAT), as well as based on the inclusion of nodal irradiation.
Results
Of the 104 patients whose plans were reviewed, 72(69.2%) were treated with 3DCRT, and the rest with VMAT (30.8%). 27(26%), 27(26%), and 50(48%) patients received RT to the Whole Breast (WB)/CW[S1], WB/CW + SCF [S2], and WB/CW + SCF + Axilla[S3] respectively. The mean and median IMN Mean doses are 35.53Gy and 36.44Gy each. The mean rank sums of IMN doses differentiated based on the RT technique and irradiated sites have been shown below Table: 33P
Mean rank sums of IMN doses
| Characteristic | IMN Min | IMN Max | IMN Mean | V95% | V90% | V50% | |
| RT Technique | 3DCRT | 42.01 | 50.6 | 53.26 | 59.13 | 56.58 | 48.97 |
| VMAT | 76.09 | 56.77 | 50.78 | 37.59 | 43.31 | 60.45 | |
| p-value | 0.0001 | 0.339 | 0.703 | 0.001 | 0.038 | 0.073 | |
| Sites irradiated | S1 | 33.26 | 45.69 | 53.41 | 61.35 | 58.44 | 50.28 |
| S2 | 51.7 | 44.8 | 40.67 | 39.54 | 41.93 | 41.13 | |
| S3 | 63.32 | 60.34 | 58.4 | 54.72 | 55 | 59.84 | |
| p-value | 0.000 | 0.038 | 0.048 | 0.023 | 0.095 | 0.031 |
Conclusions
Although mean IMN doses are below therapeutic dosage levels, 3DCRT has shown a better V95% and V90% IMN coverage compared to VMAT. S3 irradiation is associated with a higher mean IMN dose and V50% coverage, while S1 irradiation alone is associated with a better V95% coverage. Lower incidental IMN dosing might be sufficient to control subclinical disease in combination with currently available systemic therapy. And until prospective trials show a significant benefit of planned IMN coverage, patients with central or median disease with bulky axillary nodes should be treated with 3DCRT instead of VMAT for better IMN coverage.
Clinical trial identification
Editorial acknowledgement
Legal entity responsible for the study
The authors.
Funding
Has not received any funding.
Disclosure
All authors have declared no conflicts of interest.