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The high-resolution oncologic transformable PET (HOTPET) camera can operate in different modes including breast mode and whole-body mode. In the whole-body mode, HOTPET has a transverse field-of-view (FOV) of 60 cm and an axial FOV of 13 cm. In the breast mode it has a transverse FOV of 39 cm and an axial FOV of 21 cm. The aim of this study was to compare the performance of the two modes of HOTPET for observation of small ( <; 10 mm) breast lesions to measure the effective gain in lesion detection by using a dedicated breast ring PET. We compared the two modes of camera by scanning a breast phantom with small lesion insert. The breast phantom was attached to an anthropomorphic torso phantom with lung and cardiac inserts. We scanned several small lesions with inner diameters in the range of 3.95 mm to 7.86 mm for several lesion activity contrasts. The ratio of the activity concentration in the lesions to the activity concentration in the breast background ranged from 1 to 10. Reconstructed images of lesions were compared visually and we also compared the image contrast of lesions and radial profiles of lesions. The 7.86-mm lesion was observable for lesion to breast background activity ratio of 2.2 or higher in HOTPET breast mode and for activity ratio of 3.1 or higher in the whole-body mode. The 4.95-mm lesion was observable for activity ratio of 3.1 in the breast mode and activity ratio of 6.9 in whole-body mode. The 3.95-mm lesion was visually observable for activity ratio of higher than 5 in the breast mode but it was not observable in whole-body mode even at the highest measured contrast. This study demonstrated that a whole-body PET camera such as HOTPET even with 2.9-mm spatial resolution may not be able to detect lesions smaller than 7 mm for lesion to background activity ratio 3.5, while a dedicated full ring breast PET similar to HOTPET breast mode could detect smaller lesions (≤ 5 mm) for similar uptake.