2689 - Early Results of a Phase II Trial of Ultra-hypofractioNated Adjuvant Radiotherapy ± sImultaneous Integrated Boost for Low-risk Breast Cancer Patients (UNIQUE)

H. Jing¹, Y. Zhang¹, Y. LI², 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, ²State Key Laboratory of Molecular Oncology and Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences (CAMS) and Peking Union Medical College (PUMC), Beijing, China

Purpose/Objective(s): Ultra-hypofractionated whole breast RT by 5 daily fractions (5F_WBRT) achieved similar oncological outcomes without unexpected toxicity, revealed in the FAST-Forward trial. However, simultaneous tumor bed boost (SIB) in the context of 5F_WBRT has not been validated. We present a phase II study to evaluate the feasibility of 5F_WBRT±SIB in early-stage breast cancer (ESBC) (NCT05762900). Materials/

Methods: Eligibility: patients(pts) who diagnosed with breast cancer, have undergone definitive surgery with negative margin (R0) and axillary nodal evaluation, staged pT1-2N0-1M0. All of the pts were treated within 8 weeks(w) from the last chemo or surgery. The dose was 5.2Gy for 5f to the whole breast and 6Gy/f to the tumor bed. Dose constraints were: the heart Dmean<3Gy, V7<5%, V1.5<30%; the ipsilateral lung V20<8%, V5<25%; the healthy breast Dmean<3Gy; the brachial plexus Dmax<24Gy. The primary end point is the occurrence of both =G2 acute breast adverse event (AE) and =G3 lymphopenia within 12w (CTCAE 4.0). According to the Simon’s two-stage design, 65 pts are needed to rule out an occurrence rate of 77%.
Results: From 2022.7 to 2024.2, 151 pts were recruited. At the full follow-up (FU) of the 1^st 65 pts, only 7 (10.8%) developed the pre-defined toxicities. We went on recruiting for 198 pts, from which a 5-yr rate of any =G2 late AE of 30% could be ruled out. The mFU time was 5.7m (0.8-19.7m), and 108 pts (71.5%) were followed >12w. None has any sign of relapse. The median RT time was 7 days (5-10d). Three pts (2.0%) experienced acute breast edema during the course and waited until the subside. None reported symptomatic heart, lung or nerve toxicity. For the 149 pts who received BCS, 131(86.8%), 15(9.9%) and 2(1.3%) reported excellent, good and moderated cosmesis, respectively. In 138 pts who had blood tests from the initiation to 6 months after the RT, 68 pts (49.3%) had lymphocytopenia (LP), with 46(33.3%), 20(14.5%) and 2(1.4%) grade 1, 2 and 3 LP, respectively, and 92.0% of them recovered in later tests.
Conclusion: Adjuvant u-WBRT±SIB in ESBC is feasible, with a total of 21 pts (13.9%) had =G2 nonhematological AEs or =G3 LP, and 15.9% of =G2 LP, which are much less than what were reported in 15-25 fractions. Nevertheless, longer FU is need to rule out unexpected long-term AEs. And prospective trials and larger sample sizes are warranted. Abstract 2689 – Table 1 Abstract 2689 – Table 2

		N	%			N	%
Index	Left	71	47	Patho	Ductal	136	90.1
	Right	77	51		Lobular	10	6.6
	both	3	2	T(mm)	Ave:15	(1~48)
Age(yr)	Median: 51	(33~82)		pN	N0	139	92.1
pT	Tis	21	13.9		Ni+/Nmi	2	1.4
	T1mi	1	0.7		N1	10	6.6
	T1	107	70.9	IHC	ER+	117	77.5
	T2	22	14.6		PR+	106	70.2
LVI	+	20	13.2		HER2+	27	17.9
Grade	1	28	18.5		TN	23	15.2
	2	63	41.7	Chemo	Neo	3	2
	3	56	37.1		Adjuv	85	56.3
Surgery	BCS	150	99.3	SIB		136	90.1
Tech	IMRT	6	4	Nodal RT		10	6.6
	VMAT	137	90.7		SCV	7	4.6
	TOMO	8	5.3		Axillary	5	3.3
					IM	4	2.6

	G1(%)	G2(%)
Breast pain in 2w	62(41.1)	3(2)
Breast pain at the last FU	41(27.2)	0
Breast edema in 2w	58(38.4)	14(9.3)
Breast edema at the last FU	34(22.5)	1(0.7)
Dermatitis	141(93.4)	0
Breast shrinkage at the last FU	10(6.6)	2(1.3)