In vitro assessment of hormone receptor status using a ligand-binding assay or immunohistochemistry in breast cancer patients predicts endocrine responsiveness with an accuracy of only 60%-70%. Assessment of an end product of estrogen receptor stimulation, such as the progesterone receptor, is assumed to provide a measure of functional receptor content and has proven to increase predictive accuracy. In analogy with the estrogen-dependent regulation of somatostatin receptor (SSTR) expression in endocrine-responsive human breast cancer cell lines, efficient antiestrogen treatment in patients may result in a downregulation of SSTR at the cell surface in breast tumors. In vivo imaging of this molecular event by means of sequential (99m)Tc-depreotide scintigraphy could enable selection of breast cancer patients susceptible to endocrine therapy.
Methods: Twenty patients with a diagnosis of advanced breast cancer in whom first- or second-line hormonal therapy was going to be initiated were included. Patients underwent sequential (99m)Tc-depreotide scintigraphy before and 3 wk after initiating hormonal treatment. Follow-up data were retrieved from routine clinical evaluation by means of physical examination, imaging (e.g., bone scan, CT, MRI) and blood analysis. Lesion-to-background ratios (L/BGs) were calculated on planar and SPECT images and a change of >25% between the baseline and follow-up scan was considered significant.
Results: At 6 mo after initiation of treatment, 8 patients had stable disease and were considered to be responding to hormonal treatment, whereas 10 patients had progressive disease and were considered to be nonresponders. The positive and negative predictive values of baseline (99m)Tc-depreotide scintigraphy for endocrine responsiveness were 73% (8/11) and 100% (7/7), respectively. Sequential scans were always both positive or both negative. The relative change in (99m)Tc-depreotide uptake between sequential scans significantly differed in responders compared with nonresponders (P= 0.017)-uptake decreased in the first group and increased in the latter. As such, baseline (99m)Tc-depreotide scintigraphy combined with the changes in tracer uptake between the baseline and follow-up scan predicted endocrine responsiveness with an accuracy of 100%.
Conclusion: Sequential (99m)Tc-depreotide scintigraphy could allow for separation of responders and nonresponders immediately or as early as 3 wk after initiation of treatment.