Role of 99mTc-sestamibi SPECT in accurate selection of primary hyperparathyroid patients for minimally invasive radio-guided surgery

Abstract

Dear Sir, We read with interest the recent paper by Rubello et al. entitled “Role of Tc-sestamibi SPECT in accurate selection of primary hyperparathyroid patients for minimally invasive radio-guided surgery” [1]. Minimally invasive radio-guided surgery (MIRS) is widely employed for primary hyperparathyroidism caused by a solitary parathyroid adenoma. Since an intraoperative gamma probe is used, significant sestamibi uptake by the adenoma is important [2, 3]. Obviously, as the authors underline, radiation attenuation problems may cause underestimation of the sestamibi uptake by adenomas in an anterior planar image, resulting in doubt as to the appropriateness of MIRS in some cases. In this regard the authors report an interesting finding, namely that the parathyroid to background (P/B) ratio derived from SPECT correlates well with the intrasurgery P/B ratio; this means that using SPECT it is possible to predict the intraoperative measurement obtained with the gamma probe, thus allowing better patient selection for MIRS. However, we believe that this finding is not going to have a significant impact on the selection of patients for MIRS since only patients with a high probability of having a solitary adenoma are candidates for MIRS [4]. In our experience most of these patients demonstrate a P/B ratio significantly higher than the threshold, referred to by the authors, of 1.2 in the anterior planar image and thus do not require any quantitative confirmation. In the case of deeply located adenomas, despite the relatively reduced sestamibi intensity in the anterior planar image, they will still be well recognised as parathyroid lesions and any doubt about the appropriateness for MIRS in those cases can be dispelled just by knowing that the adenoma is located deeply. In this regard we visually estimate the depth and at the same time take into account the attenuation effect on sestamibi uptake from adenomas by using oblique planar imaging in all cases, rather than the time-consuming and rather complicated SPECT technique. Oblique planar imaging is easy to perform and very valuable even when the adenoma is behind the thyroid gland [5, 6]. In the presented study the ratio derived from SPECT was useful in only 1/22 patients although 7/22 adenomas were deeply located. The quantification might possibly be useful in estimating the appropriateness of the sestamibi uptake for MIRS in a few borderline cases. However, trying to include these patients will inevitably increase the probability of multiple gland disease since it is well known that frequently only the largest gland is visible in hyperplasia, especially in the early stages, when the size of the other glands remains below the sensitivity of the techniques used [7, 8]. The result will then be that more operations will be changed to bilateral exploration assuming that rapid PTH measurement is available. In this regard, SPECT, because of its slightly better sensitivity compared with planar imaging, would be most useful not in selecting more patients but rather in excluding some on the basis of the identification of more than one parathyroid lesion in cases of multi-gland disease. Nevertheless, ultrasound can contribute this important information even better than SPECT. In conclusion, we believe that in the selection of eligible patients for MIRS, quantification is not needed and that performance of SPECT exclusively for this purpose is not justified. Eur J Nucl Med Mol Imaging (2007) 34:607–608 DOI 10.1007/s00259-006-0332-y