ESTRO 2021 Abstract Book

S931

ESTRO 2021

Figure 2. Whole breast radiotherapy alone.

Results Median age in the control group was 58.5 years and 59 years in the study group. Fifty percent of patients presented left-sided breast cancer in each group. In the control group, all patients underwent axillary lymphadenectomy except one with sentinel lymph node biopsy (SLNB), while in the study group all patients underwent SLNB. The median percentage of dose received at the reference point in the control group (with elective irradiation of the IMN) was 100.94% of the prescribed dose. In the group of patients with only WBRT, the median percentage dose received was 91.45%. There were no significant differences between the median gantry angle in the IT (55.0º in the control group, and 56.5º in the study group, p = 0.481). Conclusion Incidentally dose at the reference point in the middle third of IMN in patients treated with WBRT alone is close to therapeutic range (>95%). There are no significant differences in IT gantry angulation between both groups; it suggests that, the dose received in IMN may be more affected by anatomical factors than by treatment design. PO-1118 Incidental dose to axilla in breast cancer patients undergoing post-operative radiotherapy K. Rastogi 1 , A. Munshi 2 , R. Chauhan 2 , K. Bansal 2 , V. Pandey 2 , T. Ganesh 2 1 Manipal Hospital, Radiation Oncology, New delhi, India; 2 Manipal Hospital, Radiation Oncology, New Delhi, India Purpose or Objective The purpose of this study was to evaluate incidental irradiation of adjoining axillary levels (I-III) in breast cancer patients undergoing post-operative RT and to compare axillary dose received in BCS and mastectomy patients. Materials and Methods We retrospectively analyzed incidental axillary dose in sixty cases of breast cancer treated with surgery and adjuvant RT. Axillary levels I, II and III were delineated on planning computerized tomography (CT) scans and all three axillary levels were evaluated for mean dose, V20, V40, V45 and V47. The dose received by each axillary level was also correlated with the type of surgery performed, planning target volume (PTV), axillary volume, body mass index (BMI), thoracic depth and width, body depth and width. Results The baseline characteristics were balanced between BCS and post mastectomy patients. Mean volume of axilla was 84.8 cc and 74.5 cc in BCS and mastectomy cases respectively. Two patients were excluded from final analyses since they received RT to internal mammary nodal chain as well. Mean dose to axilla total, level I, II and III was 14.5 Gy, 18.2 Gy, 6.6 Gy and 2.2 Gy respectively for BCS cases and 27.2 Gy, 33.8 Gy, 18.4 Gy and 4.5 Gy respectively for mastectomy cases. V20, V40, V45, V47.5 for whole axilla was 28.1%, 4.6%, 0.9%, 0.2% in BCS patients and 63.2%, 35.8%, 16.9% and 4.5% in mastectomy patients. We observed that patients with high BMI, larger breast separation and higher PTV received higher incidental dose to axilla.