Quantity and location of the tumor cells in a biopsy specimen

248 Objectives: Successful cancer genotyping, which is needed for precision medicine, is dependent on the collection of sufficient amount of cancer cell DNA from biopsy specimens. Specimens biopsied under 18F-FDG PET/CT guidance are radioactive and as previously shown their specific activity can be obtained by performing quantitative autoradiography. If the tracer uptake per cell is relatively constant, this specific activity could also be used for estimating the number of tumor cells and hence the amount of DNA in the specimens. Our goal is to develop a method for determining the number of tumor cells in the specimens independently in order to be able to deduce the tracer uptake per cell in situ. Methods: We present a method for counting the number of tumor cells in biopsy specimens. A specimen extracted under 18F-FDG PET/CT guidance using an 18G core biopsy needle from lung squamous cell carcinoma metastasis in the liver was subjected to quantitative autoradiography using digital autoradiography (imaging) plates. The biopsy was then embedded in paraffin and sliced into 5 μm thick sections parallel to the slide surface. All slices were stained using H&E and were counted using the TMARKER software after splitting the micrographs into slices, registering them and training the software with the help of an experienced pathologist. The coordinates for each counted cell, exported from TMARKER were used to produce the 3D distribution of tumor cells in the specimen. The tumor cells were binned into 117.8 x 405.8 x 24.3 µm voxels to allow simulation of the autoradiography measurement using the positron spectrum for 18F within the GATE Monte Carlo simulation platform. Results: TMARKER counted 2.3×105 tumor cells and 3.4×105 benign cells in 58 slices of about 1 cm long specimen. These numbers may have uncertainty of tens of percents due to losing part of the specimen during slicing. The ARG image simulated using the binned location of the cells and the measured ARG image are visually similar and their quantitative agreement is expected to be affected by specimen shrinking, deformation during processing and approximations in the simulation model. Since the autoradiography approach measures specific activity in kBq/mL, a conservative estimate for the uncertainty of the specimen volume of a factor of 2 due to potential loss of tissue and shrinking, and a 10% uncertainty for the measured specimen specific activity, gives a range for the whole specimen activity as determined from the autoradiography between 7.5 and 36 Bq. Accounting for an additional uncertainty from counting the cells (up to 10%), the estimate for the number of 18F-FDG molecules per cell for this specimen 1h 38min post injection is between 0.3 and 1.7. Conclusions: Combination of cell counting and activity measurements for radioactive biopsy specimens extracted under PET/CT guidance is a potential approach for estimation of 18F-FDG uptake per cell in situ. The quantification accuracy for uptake per cell with the described procedure is dependent on the amount of tissue lost during slicing as well as on specimen shrinking during processing and may be improved by performing well counter measurements. The described method can be extended to other radiotracers and the spatial information about the cells can be used to guide pathology and immunohistochemistry analysis. Acknowledgements: The authors want to thank the department of Medical Physics and Dr. Joseph Deasy for supporting this work, which is also supported in part through the NIH/NCI Cancer Center Support Grant P30 CA008748.