Incidental dose to internal mammary nodes in post-operative radiation therapy for breast cancer

Abstract

Aims

The incidental dose to the internal mammary nodes (IMN) is understudied in patients treated with newer radiation therapy (RT) techniques. The aim of this study was to quantify the incidental IMN dose in a series of breast cancer (BC) patients receiving post-operative RT to the chest wall/breast and regional nodes (level III-IV).

Methods

We retrospectively analyzed data from 95 high-risk BC patients treated between 2015 and 2022. Patients received RT (50 Gy/25fr or 40.05 Gy/15fr) to the breast/chest wall and nodal levels III-IV after mastectomy or breast conserving surgery (BCS). Exclusion criteria were IMN irradiation and pre-operative systemic therapy. One radiation oncologist contoured the CTV_IMN according to ESTRO guidelines and divided it into four sub-regions based on intercostal spaces (ICS): IMNupper, ICS1, ICS2, ICS3. Dosimetric parameters collected were Dmean, V90, V95, D90, and D95. The Dmean was correlated to tumor laterality and location, type of surgery and reconstruction, RT technique (3D-CRT, IMRT, helical RT) and boost.

Results

Mean Dmean to IMN was 71.4 % (range 19.6–118.6) of the prescription dose. Among sub-region, ICS2 and ICS3 received significantly higher doses than ICS1 and IMN upper (p = 0.04). V90 of over 90 % was achieved in only 4/95 patients, 3 were treated with helical RT, and the other with IMRT. The mean V95 and V90 were 15.4 % and 26.2 % respectively. Univariate analysis showed that mastectomy (p = 0.002), omission of boost (p = 0.001), and helical RT (p < 0.0001) were associated with significantly higher IMN Dmean. No significant correlation emerged with laterality, tumor location and type of reconstruction.

Conclusions

In our series, incidental IMN doses were highest after mastectomy and with helical RT delivery, possibly due to more medial margin in chest wall delineation and the helical dose distribution. Nevertheless, incidental doses to the IMN were below recommended doses, thus highlighting the need for IMN contouring when identified as targets.