Parathyroid lipoadenomas are rare and are incidentally detected during thyroid cancer surgery in this case. In this paper, 40 literatures with "parathyroid lipoademona" as search term on PubMed [1, 2, 4, 5, 7–42] and 65 cases of parathyroid lipoadenoma (including this case) were counted. After excluding 18 cases with missing clinical data, 47 cases had complete clinical data, including 30 females and 17 males, with a female-to-male ratio of about 1.8:1, that is, parathyroid lipoadenoma accounted for a higher proportion in females than in males. In addition, it has been reported in the literature that parathyroid lipoadenoma may account for a higher proportion in males than parathyroid adenoma [1]. The age of onset of parathyroid lipoadenomas varies from 24 to 88 years, mostly located in the neck, but up to 15% of parathyroid lipoadenomas are ectopic or located in the mediastinum, much higher than common parathyroid adenomas[2].
In 1958, Ober and Kaiser[3] first described parathyroid lipoadenoma in a middle-aged man with a gradually enlarging neck mass that was painless and without manifestations of hyperparathyroidism.A large lipoma was found under the sternum on neck exploration.Because the tumor was nonfunctional, they named it "parathyroid hamartoma".The concept of "parathyroid lipoadenoma" was first introduced in 1962 by Abul Haj [4] et al.Parathyroid lipoadenoma is a rare type of parathyroid adenoma with an incidence of 0.5–1.6% of primary hyperparathyroidism and contains fat, principal cells, and eosinophils, of which fat content is > 50%, unlike adenomas with fatty infiltration, and the fat component is an integral part of parathyroid lipoadenoma[5].In 2022, the World Health Organization defined parathyroid lipoadenoma as "a tumor with simultaneous increases in parathyroid parenchyma and adipose tissue, and more than 50% of the glandular volume consists of adipose tissue"[6].In the currently reported cases of parathyroid lipoadenoma, the origin of the adipose tissue component is unknown, and most of them are small brown lesions with the largest reported weight of 480 g.Some researchers have speculated that the factors of increased fat composition may be the same as those of increased parathyroid chief cells [7, 8].It has been suggested that fat may be present in adenomas by high BMI, and some speculations suggest that some adipogenic factors may be produced by adenoma epithelial or stromal cells, which in turn stimulate adipocyte proliferation[9].Christofer Juhlin proposed that parathyroid lipoadenomas may be associated with hypertension[5].
Parathyroid lipoadenomas can be classified as functional and non-functional, with approximately 64 percent of previously reported cases symptomatic, 28 percent asymptomatic, and 7 percent suspicious[10], and four parathyroid lipoadenomas found on autopsy for which function is uncertain. Among them, the clinical features, laboratory tests and parathyroid adenomas of functional parathyroid lipoadenomas are similar, that is, patients show recurrent urinary stones and hip or shoulder fractures, but the increase in parathyroid hormone and blood calcium may not be as obvious as parathyroid adenoma, and a very small number of patients have asymptomatic hypercalcemia.
The difficulty of parathyroid lipoadenoma is mainly diagnosed by preoperative imaging localization.29 of 64 patients (45%) had lesions localized using imaging studies.Ultrasonography was performed in 24 cases, and lesions or suspected lesions were found in 14 (58%) of them.Computed tomography(CT) was reported in 12 cases, of which 9 (75%) detected or suspected lesions.Technicium-99m sestamibi scintigraphy was reported in 15 cases, of which 8 (53%) reported detection or suspicion of lesions.In one of them, an X-ray was performed and the tumor was found to be located in the mediastinal cavity.Positron emission tomography (PET) scans were used in 6 cases, of which 3 (50%) were reported as lesions.Because of the high adipose tissue content, lipomas tend not to be detected by preoperative imaging techniques.On ultrasound, typical parathyroid adenomas are homogeneous hypoechoic lesions, however, fat-rich variants of parathyroid adenomas sometimes present as hyperechoic lesions [9], resulting in reduced accuracy, and ultrasound examination of this patient showed no parathyroid lesions; they can be localized as lipomatous masses on CT, and Johnson et al[12] reported cases in which CT scans of parathyroid lipoadenomas had CT values of − 60 to − 90 HU, consistent with lipoma density, proposing the possibility of parathyroid lipoadenomas on such CT scans in similar cases of non-local primary HPT.However, CT may also be confused with lipomas, lymph nodes, or other fat-rich lesions; the accuracy of dual-phase Sestamibi scanning of the parathyroid depends on the proportion of parathyroid chief cells, which are more than 50% fat in parathyroid lipoadenomas and have less principal cell content than normal parathyroid tissue, resulting in reduced accuracy of Sestamibi scanning; however, there are cases [13] indicating the usefulness of using 99Tcm-MIBI imaging in localizing the diagnosis, reoperation, and persistent hyperparathyroidism, and they propose speculation that the lesion may be shown based on the space-occupying appearance, or that the target may have a low signal ratio to the background.Cases showed a good target-background signal ratio in Tc-99m sestamibi imaging, which may be due to the relatively large size of the lesion, as well as the fact that the main part of the lesion consists of parathyroid chief cells; the sensitivity of syngeneic SPECT for parathyroid lipoadenoma was significantly reduced [12].However, Sabri et al.[1]concluded that although SPECT is not as effective as parathyroid adenoma in the diagnosis of parathyroid lipoadenoma, it may be superior to ultrasound.Cholinergic PET-CT imaging is superior to ultrasound and CT in the localization of parathyroid adenoma, but there is no relevant report for parathyroid lipoadenoma.
Primary hyperparathyroidism is most commonly caused by solitary parathyroid adenomas, accounting for more than 80–85%; less commonly, parathyroid hyperplasia, parathyroid carcinoma, and multiple parathyroid adenomas[2]. Parathyroid lipoadenoma is a very rare cause of primary hyperparathyroidism with an incidence of 0.5–1.6% of primary hyperparathyroidism. In parathyroid lipoadenomas, the sensitivity of imaging is significantly reduced by low parathyroid cell concentration and excessive fat content. Abundant adipocytes in parathyroid tissue can be found in the following conditions: normal tissue, lipohyperplasia, lipoadenoma, and cancer infiltrating the surrounding adipose tissue. It is difficult to differentiate these diagnoses by intraoperative rapid frozen section. Therefore, definitive diagnosis of parathyroid lipoadenoma should include pathologic diagnosis of parathyroid hyperplasia within adipose tissue, as well as a significant decrease in hormone following intraoperative removal of the parathyroid gland. Increases in parathyroid gland weight (> 40–60 mg) may also represent parathyroid lipoadenomas in appropriate morphologic and clinical settings [6]. Parathyroid lipoadenomas should be considered when no lesions are reported on ultrasound but clearly identified by CT or SPECT. Parathyroid carcinoma infiltrating in adipose tissue may be misdiagnosed as parathyroid lipoadenoma, so careful pathological examination is still required for differential diagnosis.