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Simple Summary: This multicenter study (n = 1133, inclusion criteria: 3.5 cm maximum tumor size and preoperative indication for a boost) provides detailed data on acute and long-term toxicities in breast cancer patients undergoing breast-conserving surgery combined with an anticipated intraoperative boost with low-energy X-rays followed by whole breast irradiation. Toxicity assessments, based on LENT SOMA criteria, were performed annually up to 10 years of follow-up. IORT boost was completed in 90% and EBRT in 97% of cases. No grade 3 or 4 acute toxicities were observed, with mild acute side effects reported in a small proportion of patients. Chronic toxicities were seen in 16.2% of patients with teleangiectasia, 14.3% with grade ≥ 2 fibrosis, 3.4% with grade ≥ 2 pain, and 1.1% with hyperpigmentation. The results show that the therapy is safe and feasible in terms of toxicity and confirms intraoperative boost as a standard method of boost application in a large prospective cohort. In the context of breast cancer treatment optimization, this study prospectively examines the feasibility and outcomes of utilizing intraoperative radiotherapy (IORT) as a boost in combination with standard external beam radiotherapy (EBRT) for high-risk patients. Different guidelines recommend such a tumor bed boost in addition to whole breast irradiation with EBRT for patients with risk factors for local breast cancer recurrence. The TARGIT BQR (NCT01440010) is a prospective, multicenter registry study aimed at ensuring the quality of clinical outcomes. It provides, for the first time, data from a large cohort with a detailed assessment of acute and long-term toxicity following an IORT boost using low-energy X-rays. Inclusion criteria encompassed tumors up to 3.5 cm in size and preoperative indications for a boost. The IORT boost, administered immediately after tumor resection, delivered a single dose of 20 Gy. EBRT and systemic therapy adhered to local tumor board recommendations. Follow-up for toxicity assessment (LENT SOMA criteria: fibrosis, teleangiectasia, retraction, pain, breast edema, lymphedema, hyperpigmentation, ulceration) took place before surgery, 6 weeks to 90 days after EBRT, 6 months after IORT, and then annually using standardized case report forms (CRFs). Between 2011 and 2020, 1133 patients from 10 centers were preoperatively enrolled. The planned IORT boost was conducted in 90%, and EBRT in 97% of cases. Median follow-up was 32 months (range 1–120, 20.4% dropped out), with a median age of 61 years (range 30–90). No acute grade 3 or 4 toxicities were observed. Acute side effects included erythema grade 1 or 2 in 4.4%, palpable seroma in 9.1%, punctured seroma in 0.3%, and wound healing disorders in 2.1%. Overall, chronic teleangiectasia of any grade occurred in 16.2%, fibrosis grade ≥ 2 in 14.3%, pain grade ≥ 2 in 3.4%, and hyperpigmentation in 1.1%. In conclusion, a tumor bed boost through IORT using low-energy X-rays is a swift and feasible method that demonstrates low rates in terms of acute or long-term toxicity profiles in combination with whole breast irradiation. [ABSTRACT FROM AUTHOR]