A new semiquantitative method for comparing brain tumor uptake of Tc-99m sestamibi and Tl-201

Purpose: We describe a new method for measuring brain tumor uptake of Tl-201 and Tc-99m sestamibi (MIBI) that permits the semiquantitative comparison of tracer uptake to yield comparable 'tumor bulk' ratios. We tested this method in patients treated recently and remotely with chemotherapy to determine if this method could identify differences between these two patient groups. Methods: Eleven patients with high-grade astrocytoma underwent Tl-201 and Tc-99m MIBI SPECT. Each patient received 5 mCi Tl-201 intravenously followed by SPECT using a dual-head gamma camera. This was immediately followed by an intravenous injection of 20 mCi Tc-99m MIBI and repeated SPECT. Four patients had recent therapy (from 1 day to 6 weeks before SPECT) and seven had remote treatment (>1 year before SPECT). Regions of interest were outlined in the tumor area using a computer- automated program to include all counts above background activity. Tumor activity counts were obtained from this region of interest. The tumor region of interest was mirrored to the contralateral uninvolved cerebral hemisphere to obtain background control count activity. A hypothetical volume of the number of pixels with background count activity necessary to constitute the tumor count activity (tumor bulk) was calculated using the ratio of total tumor counts (Ct), subtracting background (Cb), and dividing by the average counts per pixel in the control region (Cab). This was multiplied by the number of pixels (P), the pixel volume (Vp), and summed over all sections (i) involved with tumor. This method yields the equation tumor bulk = Vp · (i)/Σ {[Ct(i) - Cb(i)]/[Cab(i)]} · Pi Results: The mean Tc-99m MIBI to Tl-201 tumor bulk ratio was 1.03 (range, 0.81 to 1.12) in four patients who had recently received chemotherapy. The mean Tc-99m MIBI to Tl-201 tumor bulk ratio was 1.55 (range, 1.46 to 1.64) in seven patients who had remote therapy. The difference in the Tc-99m MIBI to Tl-201 tumor bulk ratio between the two groups was significant (P = 0.0001). Patients who received recent chemotherapy had relatively lower Tc-99m MIBI uptake compared with Tl-201. In remotely treated patients, uptake of the Tc-99m MIBI was greater compared with Tl-201. Conclusion: This method allows semiquantitative comparison of different tracer uptake values independent of tracer dose and reduces the variability in drawing a region of interest when measuring tumor uptake. Among the patients studied, those who had recent chemotherapy showed a low Tc-99m MIBI to Tl-201 ratio. This method of measuring 'tumor bulk' can provide a useful index of viable tumor size in evaluating early tumor response and during ongoing chemotherapy.