2700 - Risk Factors of Radiation Pneumonitis in Breast Cancer with Internal Mammary Chain Irradiation

S. M. Lee¹, B. S. Jang², J. H. Chang², K. Kim³, and K. H. Shin⁴; ¹Department of Radiation Oncology, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Korea, Republic of (South), ²Department of Radiation Oncology, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Korea, Seoul, Korea, Republic of (South), ³Department of Radiation Oncology, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Seongnam, Korea, Republic of (South), ⁴Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, Korea, Republic of (South)

Purpose/Objective(s): Radiation pneumonitis (RP) presents a clinical concern in breast cancer treatment, especially when radiotherapy (RT) field encompasses the internal mammary chain (IMC), increasing the potential for lung toxicity. This retrospective study evaluates the incidence and risk factors of RP in breast cancer patients with comprehensive RT including the IMC, comparing outcomes between those treated with 3D conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT). Materials/

Methods: A total of 438 breast cancer patients were treated with adjuvant RT to the IMC. Between 2013-2015, 63 patients received 3D-CRT (50.4 Gy in 28 fractions), while in 2021, 375 patients underwent IMRT (43.2 Gy in 16 fractions). Patients were first followed up 3-6 months post-RT and then biannually with chest CT. RP was diagnosed by qualified radiologists and assessed using the Common Terminology Criteria for Adverse Events v5.0. The studys primary endpoint was the development of any grade of RP within one year after the end of RT.
Results: The overall incidence of RP was significantly higher in the 3D-CRT group compared to the IMRT group (74.6% vs. 17.0%, p=0.01). Patients treated with 3D-CRT also exhibited a greater prevalence of symptomatic RP (18.5% vs. 5.6%, p=0.02). After adjusting for confounders with inverse probability treatment weighting and employing multiple logistic regression, adjuvant T-DM1 therapy (OR 3.28, 95% CI 1.30-30.2, p=0.02), elevated V30(%) of ipsilateral lung (OR 1.18, 95% CI 1.12-1.23, p=0.01), and HER2-receptor overexpression (OR 3.28, 95% CI 1.39-7.71, p=0.01) emerged as significant RP predictors. However, 3D-CRT did not (OR 0.13, 95% CI 0.00-5.76, p=0.298). A normal tissue complication probability model further revealed higher RP susceptibility in patients with HER2 overexpression or those receiving adjuvant T-DM1 therapy.
Conclusion: The study identifies HER2 receptor overexpression and the administration of adjuvant T-DM1 therapy, along with higher V30(%) of ipsilateral lung, as significant factors in RP risk assessment. Furthermore, this study also suggests the importance of careful dose-volume histogram parameter evaluation to mitigate RP risk, particularly when T-DM1 therapy is involved.

NTCP model for all grade of RP
Variable	Coeff.	Robust Std. Err.	P > z	OR
HER2 receptor
positive	1.188	0.436	0.006	3.280
Mastectomy (Vs BCS)	1.265	0.713	0.076	3.544
Adjuvant T-DM1
Yes	1.835	0.803	0.022	6.265
V30 lung	0.162	0.023	<0.001	1.176
Constant	-5.253	1.413	<0.001	0.005