Primary results of ANZ 1002: Post-operative radiotherapy omission in selected patients with early breast cancer trial (PROSPECT) following pre-operative breast MRI.

Abstract

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Background: Selective use of radiotherapy (RT) after surgery for early breast cancer (EBC) has been an elusive goal. The role of breast MRI in localised EBC is controversial. We aimed to determine if preoperative MRI could identify patients with EBC in whom the ipsilateral invasive recurrence (IIR) rate was sufficiently low without RT, such that RT might be safely omitted. Here we report primary and secondary outcomes, and imaging/biopsy findings for occult lesions. Methods: PROSPECT is a prospective single-arm study. Criteria for omission of RT included age at least 50, nil/minimal or mild Background Parenchymal Enhancement (BPE) on MRI, unifocal pT1N0 cancer, not Triple Negative (TNBC), no LVI. All patients who underwent PROSPECT MRI were included in the analysis. Imaging findings on MMG, US and MRI were documented and all biopsies were recorded. Pathology of lesions identified by MRI was described. The primary outcome was the IIR at 5 years of those treated without RT. An IIR rate of 5% or less was considered acceptable. The protocol specified primary analysis occurred after the 100 ^th patient reached 5 years follow up in May 2021. Results: Between 9/2011 and 5/2019, 443 patients had MRI after diagnosis. BPE was nil/minimal or mild in 344 patients. MRI detected 194 occult lesions in 144 (33%) patients; 139 (72%) were ipsilateral. 61 MMG/US occult malignant lesions - 36 invasive and 25 DCIS - were identified in 48 patients (11% of total cohort). Of 38 ipsilateral lesions in 32 patients (7% of total cohort), 23 were DCIS, 4 were T1a, 7 T1b and 4 T1c. 201 patients were treated on trial without RT. The median age was 63 years (range: 50 to 84), median tumour size 11 mm (range: 2 to 20), grade 1 (104), grade 2 (86) or grade 3 (11). The rate of IIR at 5 years was 1% (1/101). There were 2 IIRs at 4.6 and 7.7 years follow up, 1 regional recurrence, and 1 patient with both a regional and distant recurrence with 1 breast cancer death. There was 1 contralateral (CL) breast cancer, 1 CL DCIS, 2 other cancer diagnoses and 1 death from other causes. Of 242 patients undergoing MRI but not in the main study, median age was 63 range: 50-80, median tumour size, 13mm (range: 4-145). 9 underwent mastectomy (2% of total cohort). Followup is complete for 235. There was 3 IIR, 3 CL primary and no distant metastases or breast cancer deaths. Conclusions: Breast MRI in selected, low risk patients identified occult malignancy in 11% of patients. At a median of 5 years follow up the IIR and other breast cancer events was very low. This suggests that local recurrences may be due to occult breast cancers, and MRI may allow the identification of truly localised cancers for which radiation may be safely omitted. The event rate for the entire cohort was very low, suggesting that identification of occult malignancy in apparently unifocal EBC is beneficial. Confirmatory trials are needed. Clinical trial information: 12610000810011.