ESTRO 2025 - Abstract Book

S489

Clinical - Breast

ESTRO 2025

1 Radiation Oncology, Medisch Spectrum Twente, Enschede, Netherlands. 2 Radiation Oncology, Yonsei University College of Medicine, Seoul, Korea, Republic of. 3 Radiation Oncology, Michigan Healthcare Professionals, Farmington Hills, USA. 4 Radiation Oncology, Chonnam National University Medical School, Gwangju, Korea, Republic of Purpose/Objective: As the prognosis of early-stage breast cancer patients continues to improve, reducing serious long-term toxicity has become crucial. Previous publications showed that the risk of radiation-induced lung cancer is substantial [1-3]. These studies analyzed historical techniques, phantom measurements, or estimated lung doses. The current study aims to estimate secondary lung cancer risk associated with breast radiotherapy based on actual clinically delivered lung doses. We assessed differences in lung cancer risk based on treatment volume and dose schedules, for several contemporary dose delivery techniques. Material/Methods: Four cohorts consisting of South Korean early-stage breast cancer patients were analyzed, treated with either stereotactic partial breast irradiation (S-PBI), 3D-conformal PBI (3D-PBI), VMAT whole breast irradiation (VMAT-WBI) or 3D-conformal WBI (3D-WBI). Using the clinically delivered mean lung dose (MLD) from CT-based dose planning, we calculated secondary lung cancer risk for each individual patient using the RadRAT online calculator with inputs for age, MLD and treatment year [4]. To minimize bias from differences in age and follow-up time, we repeated the analysis with age standardized to 50 years and treatment year to 2024. Additionally, we rescaled all treatment plans to 26 Gy in 5 fractions to ensure a fair comparison among treatment techniques. Results: In total, 1,690 patients were included: 806 treated with S-PBI, 43 with 3D-PBI, 456 with VMAT-WBI, and 385 with 3D WBI. The mean MLD was highest for 3D-WBI at 2.60 Gy, followed by mean MLDs of 1.86 Gy for VMAT-WBI, 1.52 Gy for 3D-PBI, and

1.37 Gy for S PBI. Median estimates for lifetime excess secondary lung

cancer risks were 2,700,

2,510, 4,030, and 6,080 cases per 100,000 patients in the S-PBI, 3D PBI, VMAT-WBI, and 3D-WBI cohorts,

respectively. Differences between all groups were statistically

significant except between S-PBI and 3D-PBI. These differences remained significant after standardizing for age, treatment year, and dose schedule (figure 1). In the standardized cohort analysis, relative risks of developing lung cancer after radiation treatment were 1.80 for S-PBI, 1.73 for 3D-PBI, 2.20 for VMAT-WBI, and 2.20 for 3D-WBI,