ESTRO 2021 Abstract Book

S930

ESTRO 2021

done by a radiation oncologist (RO) and checked by another RO and a medical physicist. Positive lymph nodes were manually contoured as GTV-LN on the CT of the corresponding PET-CT data set and transferred to planning CT after fusion. It was evaluated whether lymphatic CTVs include GTV-LNs. Results Between October 2018- May 2020 33 patients were treated with chest wall or breast and peripherical lymphatic irradiation after NACT. PET-CT data could not be obtained in 8 of 33 patients. Two patients were excluded due to suspicious mediastinal LN positivity on PET-CT. There were 105 radiologically defined positive lymph nodes in 23 patients. The arm positions were up in 22 scans and down in 1 patient. The arm position has a significant influence on fusion quality. 13 patients had seroma due to axillary surgery. Only 61.9 % (n:65) of the whole lymph nodes were totally-covered in 21.7% (n: 5) of patients. A total of 2.7 % (n:3) of level I lymph nodes in 2 patients and one supraclavicular lymph node are totally under-covered. Totally 38 % of the LN metastases were outside the boundaries of the RTOG contouring atlas. The most common geographical missing is seen in the lateral region (Table 1).

Lymph Node Coverage

Patients (n)

Lymph Node (n)

Location of undercoverag e

Totally covered

5

65

Partially covered

Level I Level II Level III

8 1 2 0 2

10

lateral lateral cranial

3 3 0 3

Supraclavicular

-

Totally undercovered

Level I

lateral

Level II

0

0

-

Level III

0

0

-

Conclusion The extent of regional nodal areas in the RTOG atlas may insufficient in covering positive lymph nodes. For patients with nodal involvement undergoing neoadjuvant chemotherapy, PET-CT image fusions can be helpful to be sure that positive lymph nodes are in the treatment volume. PO-1117 Incidental irradiation of the internal mammary in breast radiotherapy. L. Cardoso 1 , M.L. Del Valle 1 , L. Gómez 1 , S. Pena 2 , Á. Del Castillo 2 , A. Casado 1 , P. Valencia 1 , M.I. Garavis 1 , M. Herrera 1 , P. Alonso 1 , M. Martín 2 , Á. Sanz 3 , D. Rodríguez 1 , P. Diezhandino 4 1 Hospital Clínico Universitario de Valladolid, Radiation Oncology , Valladolid, Spain; 2 Hospital Clínico Universitario de Valladolid, Medical Physics, Valladolid, Spain; 3 Hospital Rio Hortega, Medical Oncology , Valladolid, Spain; 4 Hospital Clínico Universitario de Valladolid, Radiation Oncology, Valladolid, Spain Purpose or Objective Regional nodal irradiation in breast cancer has shown to improve oncological outcomes. The specific impact of radiotherapy of the internal mammary nodes (IMN) remains uncertain. Our objective is to analyse the incidental irradiation of IMN in breast cancer patients treated with radiotherapy, estimated in a specific reference point. Materials and Methods We did a retrospective dosimetry review in 20 patients treated with whole breast radiotherapy (WBRT) after conservative surgery in our institution from July 2020 to January 2021. Ten patients (the control group) were treated with elective irradiation of IMN, and ten patients (the study group) received only WBRT. For both groups, two parameters were selected for the analysis. The dose received in the middle third of the IMN in the third chondrosternal junction, at the central point of the volume delineated as internal mammary following RTOG contouring atlas (reference point), and the internal tangential field (IT) gantry angle. All patients were treated with tangential fields optimized using the field in field technique. No specific field to IMN was used.

Figure 1. Elective irradiation of IMN