2287 - Impact of Compressed Breast Thickness and Breast Density in Mammography on Setup Error for Partial Breast Irradiation

S. Fujimura¹, S. Kito¹, Y. Kanemaru¹, K. Saitou¹, S. Itou¹, H. Suzuki¹, K. Nakamura¹, and K. Murofushi²; ¹Department of Radiation Oncology, Department of Radiology, Tokyo Metropolitan Cancer and Infectious Disease Center Komagome Hospital, Tokyo, Japan, ²Department of Radiation Oncology, Tokyo Metropolitan Cancer and Infectious Diseases Center, Komagome Hospital, Tokyo, Japan

Purpose/Objective(s): The study explored the relationship between breast density, compressed breast thickness (CBT) in mammography (MMG) before surgery, and setup errors of the tumor bed after breast-conserving surgery (BCS). The objective was to determine which patients are likely to experience large setup errors under partial breast irradiation (PBI) for early-stage breast cancer. Materials/

Methods: This study included 66 patients who underwent whole breast irradiation (42.56 Gy/16 Fr) using a general linac with a six-axial moving couch from April to December 2023. Additionally, these cases had MMG taken preoperatively. Two qualified radiographers classified MMG as dense (extremely dense and heterogeneously dense) and non-dense (scattered and fatty). CBT cases of =43.5 mm and <43.5 mm were classified as high (H) and low (L), respectively. The position of each clip placed at the surgical cavity wall (2–6 for each patient, 255 clips for all patients) was determined relative to the rib using cone beam computed tomography (a total of 262 data sets of 1, 2, 9, and 16 Fr), and the vector lengths were calculated. The root mean square (RMS) of all clips in each Fr (RMSfr) was calculated, and the course of the variation between each Fr was assessed. Furthermore, the RMS of RMSfr (RMSall) was calculated for all cases. Cases were categorized as dense or non-dense and CBT-H or CBT-L; multiple comparison tests for the median RMSfr and RMSall were conducted using Bonferroni’s correction.
Results: The median RMSalls of the dense_CBT-L, dense_CBT-H, non-dense_CBT-L, and non-dense_CBT-H groups were 3.6, 4.0, 3.3, and 5.0 mm, respectively. The median RMSall difference between the CBT-H and CBT-L groups was 0.6 mm, whereas non-dense_CBH_H, including the distinction of breast density, increased to >1.0 mm compared with the other groups. Multiple comparison tests revealed a significant difference in the median RMSalls among the four groups (p-value = 0.0065). The table shows the number of cases and the statistics of the four RMSfr for groups in each fraction. The p</em>-values of the combination of CBT and breast density for 1, 2, 9, and 16 Fr were 0.98, 0.71, 0.15, and 0.06, respectively. The median RMSfr in the non-dense_CBT-H group was larger than that in the other groups, which increased from 3.0 to 5.4 mm with increasing fractions. Additionally, the median RMSfr in 16 Fr was the highest at 5.4 mm, which is 1.6 mm higher than the other groups.
Conclusion: Errors in the tumor bed setup for PBI after BCS are more pronounced in large and non-dense breasts on MMG. Abstract 2287 – Table 1

	n	50th percentile (25th?75th percentile) (mm)
		1 Fr	2 Fr	9 Fr	16 Fr
Dense_CBT-L	21	2.7 (1.5?3.6)	2.7 (1.8?4.0)	3.3 (2.4?4.7)	3.7 (2.5?6.1)
Dense_CBT-H	11	2.5 (1.3?5.1)	3.1 (2.4?4.4)	3.0 (2.4?5.6)	3.5 (2.3?5.9)
Non-dense_CBT-L	12	2.5 (2.0?3.7)	2.6 (1.8?4.1)	3.5 (1.8?4.1)	3.2 (1.7?3.9)
Non-dense_CBT-H	22	3.0 (2.3?5.6)	3.6 (1.9?5.0)	4.4 (3.1?5.4)	5.4 (3.4?6.9)
Total	66	2.7 (2.0?4.2)	3.0 (1.9?4.4)	3.5 (2.5?4.7)	3.8 (2.8?6.0)