Design and assessment of a scintigraphy-guided biplane localization technique for breast tumours: a phantom study

The aim of this study was to design and assess a scintigraphy-guided stereotaxic localization technique for breast phantoms. We wished to develop and validate scintigraphically aided biopsy of non-palpable breast masses identified only by sestamibi scintimammography. A biopsy table was built for patients to lie prone with the breast in a dependent position. The breast can be compressed in the cranial and caudal directions by fenestrated paddles. Three freely adjustable radioactive reference lines, placed along the x, y and z axes, each containing about 30 MBq (800 micro Ci) 99Tcm, were mounted on sliding rules on the external frame which surrounded the phantom. The breast phantom was a semi-square-shaped sponge. Background activity was provided by a sponge cloth containing 37 MBq (1 mCi) 99Tcm solution. Non-palpable lesions were carved out of the same sponge and wrapped in thin plastic film, and labelled with about 11 MBq (300 microCi) 201T1. The lesions, 3-15 mm in size, were placed at random at phantom depths of 2-6 cm by an individual different from the person performing the localization. Scintigraphy-guided three-dimensional localization of the lesion was performed by acquiring two orthogonal images and superimposing the reference bars over the lesion image and thus identifying the exact x, y and z coordinates of the lesion. Using these coordinates, a 22 gauge needle, containing about 37 MBq (1 mCi) 99Tcm within its lumen, was stereotactically placed into the phantom, and the tracer contained in the needle injected into the lesion. Needle placement was primarily guided by the exact coordinates, but also by real-time visualization of the radioactive needle. Pre- and post-localization images were acquired and regions of interest (ROIs) defined. Also after the tracer was injected into the lesion, images of the phantom with and without the lesion were obtained to calculate the percentage of tracer injected outside the lesion. The results of 30 consecutive localization attempts included 25 exact localizations with less than 20% tracer injected outside the lesion, 2 "near misses' with 37-44% injected outside the lesion, and 3 "misses' with 60, 85 and 100% of the tracer being injected outside the lesion. The missed localizations were all in lesions at least 4 cm deep, and all had partially superimposed ROIs, which indicates the needle came very close to the lesion. To conclude, our scintigraphy-guided biplane localization technique for breast phantom lesions successfully localized 90% of all lesions.