Comparing ¹⁸F-FDG positron emission tomography (PET) and breast magnetic resonance imaging (MRI) to predict pathological complete response (pCR) and 3-year invasive disease-free survival (3-y iDFS) in patients (pts) with HER2+ early breast cancer (EBC): An unplanned exploratory analysis of PHERGain trial.

586 Background: The PHERGain phase II trial (1) (NCT03161353) demonstrated the feasibility of chemotherapy (CT)-free treatment using a PET-based, pCR-adapted strategy in HER2+ EBC pts treated with dual HER2 blockade (trastuzumab pertuzumab; HP). Due to limited PET availability, breast MRI is warranted as an alternative tool for assessing early treatment response. Methods: Briefly, group B pts in PHERGain were initially treated with HP, and CT was subsequently introduced according to on-treatment PET results. Adjuvant therapy was determined based on prior neoadjuvant treatment (NAT) and pCR. All pts completed up to 18 HP cycles. PET and MRI were performed before randomization and after 2 treatment cycles, with an additional MRI performed before surgery. PET response was centrally assessed per EORTC criteria. Locally assessed MRI response was defined as a ≥30% decrease in the sum of diameters of the target lesions. MRI reduction was defined ad hoc as any shrinkage in the diameter of target lesions after 2 cycles without new lesions or progressive disease in non-target lesions. Results: After 2 HP cycles, 79.6% (227/285) of group B pts were PET responders, 47% (134/285) had an MRI response, and 82.5% (235/285) achieved an MRI reduction. A high association was observed between PET response and MRI reduction. Similar rates of pCR and 3-y iDFS were observed between pts with a PET response or an MRI reduction. PET-nonresponders without MRI reduction had the worst 3-y iDFS despite receiving standard NAT with CT and HP (Table). Conclusions: The complex design of this study does not allow a direct comparison between PET and MRI. Although PET is the recommended imaging technique for early treatment response, these data suggest that MRI-assessed tumor shrinkage could alternatively guide NAT after the first 2 HP cycles in HER2+ EBC pts following the PHERGain strategy when PET is not available. 1. Pérez-García JM, Lancet Oncol 2021. Clinical trial information: NCT03161353 . [Table: see text]