2722 - Ultra-Hypofractionated One-Week Locoregional Radiation for Early Breast Cancer: Dose Distribution to Target Volumes and Organs-at-Risk

I. Ratosa^1,2, A. Montero^2,3, R. Ciervide², B. Alvarez², M. Garcia Aranda², J. Valero Albarrán², X. Chen², M. Lopez², D. Zucca², O. Hernando², E. Sanchez², M. A. de la Casa², R. Alonso², P. Fernandez Leton⁴, and C. Rubio²; ¹Division of Radiation Oncology, Institute of Oncology Ljubljana, Ljubljana, Slovenia, ²Department of Radiation Oncology, HM Hospitales, Madrid, Spain, ³Facultad de Ciencias de la Salud, Universidad Camilo José Cela, Madrid, Spain, ⁴Department of Medical Physics, HM Hospitales, Madrid, Spain

Purpose/Objective(s): Ultra-hypofractionation (UHF) is considered a viable alternative treatment plan for breast-only postoperative radiotherapy (RT) in patients with breast cancer (BC), but its use in irradiating locoregional lymph nodes (RNI) remains investigational. We here report treatment planning data for a cohort of patients who underwent breast/chest wall and RNI. Materials/Methods: Between the years 2020 and 2023, 276 patients with early BC received a 1-week schedule of 26 Gy in 5.2 Gy/fraction to the breast/chest wall and RNI. Patients receiving breast-conserving surgery also received a simultaneous integrated boost (SIB) to a cumulative dose of 29–31 Gy in 5 fractions. Patients underwent CT-based simulation in the supine position, using either free breathing or deep inspiration breath hold (DIBH) with the CatalystTM (C-RAD AB, Uppsala, Sweden) system. Clinical target volumes (CTVs) and organ-at-risk (OAR) contours followed ESTRO guidelines. PTVp_breast was cropped 3 mm below skin to generate PTVp_eval; all other PTVs were generated by adding a 5 mm isotropic margin to CTVs. RT treatment plans were done with the treatment planning system. The treatment plans had been optimized as follows: V95%=95%, V107%=2 cm³, and D_max<110%. Obligatory OAR constraints were ipsilateral lung V12<25%, contralateral lung D_mean<5.6 Gy, V3.6<10%, heart V12<5%, contralateral breast V3.6<30%, brachial plexus D_max<32Gy and D3cm³<30Gy. Statistical analysis involved descriptive statistics, Kruskal-Wallis, and chi-squared tests.

Results: Patients were treated with the 3D-CRT (n=135, 48.9%) or VMAT (n=141, 51.1%) technique. Thirty patients (10.9%) received DIBH. All patients who received IMN irradiation were treated with the VMAT technique. In all PTVs, optimal distribution was reached: PTVp_breast_eval V95% =97.3% (range: 90–99.8%), PTVn_L1–2 V95% =99.2% (range: 93.9–100%), PTVn_L3–4 V95% =99.2% (range: 93.4–100%), and PTVn_IMN V105% =98.60% (range: 95.2–100%). Treatment planning parameters for OAR are presented in Table.

Conclusion: For patients receiving UHF to the breast or chest wall with RNI, our study confirms the feasibility of treatment planning using either 3D-CRT or VMAT.

Abstract 2722 – Table 1

	3D-CRT (n=135) Median (range)	VMAT (n=141) Median (range)	Total (n=276) Median (range)	p value
Ipsilateral lung Dmean [Gy]	5.9 (2.2–7.7)	5.8 (2.7–7.7)	5.9 (2.2–7.7)	0.100
Ipsilateral lung V12Gy [%]	18.4 (6.3-24.9)	14.8 (6.7-27.9)	16.1 (6.3-27.9)	<0.0001
Contralateral lung Dmean [Gy]	0.4 (0–1.8)	1.5 (0.4–2.3)	0.8 (0–2.3)	<0.0001
Contralateral lung V3.6Gy [%]	0 (0–20.8)	4.7 (0–24)	0.1 (0–24)	<0.0001
Heart Dmean[Gy]	0.7 (0.2–5.6)	2.2 (0.9–4.4)	1.3 (0.2–5.6)	<0.0001
Heart V12Gy [%]	0 (0–3.1)	0 (0–15.1)	0 (0–15.1)	<0.0001
Contralateral breast V3.6Gy [%]	0.1 (0.1–20.8)	17.5. (0.3–73.5)	9.7 (0–73.5)	<0.0001
Brachial plexus D1 cm3 [Gy]	27.1 (26.5–27.7)	27.1 (26.7–28)	27.1 (26.5–28)	0.908