2690 - Early Results of a Prospective Study of Preoperative Tumor-Bed Boost Followed by Oncoplastic Surgery and Adjuvant Whole Breast Radiotherapy for Early-Stage Breast Cancer

H. Jing¹, D. Chen², K. Men³, J. Wang⁴, and S. Wang¹; ¹Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China, ²Department of Radiation Oncology National Cancer Center/Cancer Hospital Chinese Academy of Medical Sciences (CAMS) and Peking Union Medical College (PUMC), Beijing, China, ³National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China, ⁴Department of Breast Surgical Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China

Purpose/Objective(s): Oncoplastic breast-conserving surgery (OBCS) improves cosmesis, while brings challenge in tumor-bed identification during radiotherapy. The adjuvant ultra-hypofractionated radiotherapy (aU-WBRT) further shortens treatment time. We present a phase II study aiming to evaluate the feasibility of a new treatment paradigm of neoadjuvant primary-tumor boost followed by breast-conserving surgery (BCS) or OBCS and then aU-WBRT for early-stage breast cancer (ESBC) (NCT05603078). Materials/

Methods: Pts=55yr, with a biopsy confirmed mono-centric breast cancer, cN0 were enrolled. Primary-tumor boost was given by one 10Gy under MR-Linac (1. 5T Unity, Sweden), followed by surgery in 2 weeks(w). The aU-WBRT (26 Gy/5f) was given in 6w from surgery. Chemo should start following RT toxicity recovery in 12w from surgery. Dose constraints for the boost: heart Dmax<3Gy, V2Gy<5cc, the skin Dmax<10.7Gy, ribs Dmax<10Gy. Dose constraints for the aU-WBRT: heart Dmean<2.5Gy, V7<5%, V1.5<30%; ipsilateral lung V20<8%, V8<15%, V5<25%; healthy breast Dmean<3Gy; brachial plexus Dmax=26Gy. The primary end point was the incidence of =G2 acute RT adverse events (AE) by 4w after aU-WBRT (CTCAE v4.03). According to the Simon’s two-stage design, 102 pts were needed to rule out an occurrence rate of 32%. Surgical complications by Clavien-Dindo (CD) Score were recorded.
Results: From 2022.5 to 2024.3, 40 pts were enrolled (table 1). Surgical complications were observed in 10 pts (25%), including 5 (12.5%) with CD G1-2 and the other 5 with CD G3 events. All but 4 pts (10%) started the planned aU-WBRT within 1w after the due date (=49 days). Twenty-five pts (62.5%) received chemo. With a mFU period of 9.8 months (2.4-18.9m), none had any sign of relapse. For the AEs, none but 4 pts (10%) reported G2 dermatitis, all developed after chemo begin. One pt (2.5%) suffered symptomatic pneumonitis 4.5m after aU-WBRT at her final chemo. One pt (2.5%) reported painful rib fracture 8m after aU-WBRT, and another (2.5%) reported asymptomatic rib fracture 13m after aU-WBRT. No heart, nerve or other G3 AEs occurred. For aesthesis, 29(72.5%), 8(20%), 2(5%) and one (2.5%) pt reported excellent, good, moderate and poor cosmesis, respectively.
Conclusion: Preoperative tumor boost followed by surgery and aU-WBRT is feasible. The optimal boost dose and the time-interval between the it and surgery have not been defined. This treatment algorism allows a chance of amendment during adjuvant RT based on the final pathology rather than a full-dose neoadjuvant RT. A longer-term FU is needed. Abstract 2690 – Table 1

		N	%			N	%
Index	Left	16	40	Node biopsy	Yes	28	70
	Right	24	60	cN	N0	40	100
Age (yr)	Median: 46.2	(33~55)		pN	N0	34	85
T before RT (mm)	Ave: 18.0	(9~35)			N1mi	1	2.5
T after RT (mm)	Ave: 15.6	(1~33)			N1	5	12.5
Boost to surgery (d)	Median: 11.5	(7~15)		Surgery to WBRT (d)	Median: 42	(37~71)
preRT subtype	Luminal(L) A	8	20	postRT subtype	L A	13	32.5
	L B1	16	40		L B1	12	30
	L B2	8	20		L B2	6	15
	HER2+	1	2.5		HER2+	2	5
	TN	4	10		TN	6	15
MP grade	1	5	12.5	Subtype agree	YES	30	75
	2	16	40		No	6	15
	3	16	40	Nodal RT		5	2.5
	4	3	7.5