Introduction/Background Lymphocytes are synthesized in the bone marrow mostly found in flat bones such as the Sternum. However, the dose received by the sternum in radiotherapy (RT) of breast cancer (BC) is not considered and there are no defined dose limits.

The purpose of this study was to evaluate the dose received by the sternum and to investigate the correlation between the delivered dose and the risk of lymphopenia.

Methodology A total of 33 patients treated for BC were included. They all had chemotherapy and hypofractionated RT.

The internal mammary chain was irradiated in 9,1% of cases.

Complete blood counts were done and collected before and on average 4 months after the end of RT. The dosimetric variables analyzed for the sternum were the mean dose (Dmean), maximum dose (Dmax), V5Gy, V10Gy, and V20 Gy.

Results The average age was 46 years old. Right BC was noted in 45,5% while

51,5% had left BC, and 3% had bilateral BC.

The average volume of the sternum was 52,24 cc.

The mean Dmean/Dmax to the sternum was 3,72 Gy/25,41 Gy

The average V5Gy, V10Gy, V20Gy were respectively 12,49%, 6,9% and 3,56%.

Lymphopenia occurred in 39,4% of patients after the end of RT.

Grade 2 lymphopenia was observed in 6,1% of cases. No lymphopenia above grade 2 was noted.

The Dmean (p=0.018) and low-dose radiation parameters : V5 (p=0.042), V10 (p=0.02) and V20 (p=0.021) to the sternum were found to be correlated to the risk of lymphopenia.

The Cox regression analyses showed that Dmean > 4,9 Gy, V5Gy>19,6%, V10 Gy>11%, and V20Gy>5,8% was significantly correlated lo lymphopenia.

Conclusion Given the impact of lymphopenia on survival, we suggest considering the sternum as an OAR with the following dose constraints: Dmean < 4Gy, V5Gy < 19%, V10 Gy < 11% and V20Gy < 5%.

Disclosures No conflict of interest to disclose.