3010 - Accelerated Partial Breast Irradiation in the Supine vs. Prone Position: A Comparison of Dosimetry and Acute Toxicity

03:00pm - 04:00pm PT

Hall F

Screen: 23

POSTER

Presenter(s)

Andrew Wanna, MD - New York University Cancer Center, New York, NY

A. S. Wanna¹, D. Barbee², J. Kim³, C. Hardy Abeloos³, C. Oh⁴, A. Solan⁵, T. B. Daniels¹, O. Cahlon⁶, and N. K. Gerber⁶; ¹NYU Langone Health, New York, NY, ²Sun Nuclear Corporation, Melbourne, FL, ³NYU Grossman School of Medicine, New York, NY, ⁴Biostatistics, Department of Population Health, NYU Langone Health, New York, NY, ⁵NYU Grossman School of Medicine, New York City, NY, ⁶New York University Grossman School of Medicine, Department of Radiation Oncology, New York, NY

Purpose/Objective(s):

Our institution has offered accelerated partial breast irradiation (APBI) 3000cGy in 5 fractions since 2000. Patients are typically treated prone using 3D conformal technique or forward-planned IMRT, but a minority of patients are treated supine. This study compares dosimetry and acute toxicities between patients treated in the two positions. Our hypothesis is that ipsilateral breast, cardiac, and lung dosimetry will be improved in patients treated in the prone position.

Materials/Methods:

We retrospectively reviewed all patients with in-situ or invasive breast cancer treated with 3000cGy in 5 fractions APBI at our institution between January 2022 and December 2023. We obtained plan dosimetric data including planning target volume (PTV) volume, volume of PTV receiving 95% of the dose (V95%), ipsilateral breast volume receiving 15Gy (V15Gy) mean heart dose (MHD) and volume of ipsilateral lung receiving 10Gy (V10Gy). Acute toxicities were defined within 90 days of treatment completion and scored using the CTCAE 5.0 system. Differences between treatment groups were analyzed using Wilcoxon rank sum, Pearson's Chi-squared and Fisher’s exact tests.

Results:

A total of 168 patients were identified, 134 (79.8%) treated prone and 34 (20.2%) treated supine. Of the supine patients, 23 (67.6%) were treated with photon-only plans and 11 (32.4%) with mixed photon-electron plans. There were no statistically significant differences in age, BMI, tumor size, or breast laterality between the two groups. PTV volume was larger for prone patients (median PTV volume 157cc prone vs. 103cc supine, p=0.002). Target coverage was excellent in both positions (mean V95%: 99.5% prone vs. 99.1% supine, p=0.778). Ipsilateral breast V15Gy did not differ between groups (median 41.7% prone vs. 47.3% supine, p=0.214). Median MHD was significantly lower in prone patients (9.6cGy vs. 14.9cGy supine, p=0.033). This difference was more pronounced in right-sided patients vs. left-sided patients (right: 6.9cGy prone vs. 8.6cGy supine, p=0.037; left: 17.6cGy prone vs. 16.5cGy supine, p=0.710). MHD was also significantly higher in mixed photon-electron plans (22.5cGy) vs. supine photon-only plans (10.6cGy) p=0.009. Mean ipsilateral lung V10Gy was lower for patients treated prone (0.28% vs 1.44% supine, p=<0.001). Grade 1-2 dermatitis was more frequent in the supine cohort (59% vs. 38% prone p=0.029). There were no other significant differences in toxicities (70% any grade 1-2 toxicity prone vs. 85% any grade 1-2 toxicity supine, p=0.075) and no grade 3 toxicities. Fatigue was significantly higher in patients with BMI =30 vs. patients with BMI <30 (p=0.035).

Conclusion:

Though PTV volume was greater in prone patients, there were no differences in target coverage or ipsilateral breast dose between the two groups. Prone positioning was associated with lower heart and lung dose as well as lower rates of acute dermatitis, but heart and lung dose was minimal for patients treated both supine and prone.