First ever case report of co-occurrence of hobnail variant of papillary thyroid carcinoma and intrathyroid parathyroid adenoma in the same thyroid lobe

Highlights • First reported case of hobnail variant of papillary thyroid cancer and Intrathyroid parathyroid adenoma occurring within same thyroid lobe.• Next-generation sequencing of the mutation spectrum of hobnail variant of papillary thyroid cancer showed BRAFV600E mutation.• Studies that define other molecular abnormalities may be useful as therapeutic targets.

Intrathyroid parathyroid adenoma (ITPA) is rare, situated totally within the thyroid, surrounded by thyroid parenchyma, with an incidence of < 1% of all hyperparathyroidism cases [23,24]. Some authors categorize ITPA into complete type, completely wrapped by thyroid tissue; and incomplete type, wrapped in ≥ 50% by thyroid tissue. Parathyroid adenoma (PA) that are wrapped in ≤ 50%, lie under the pseudocapsule or sheath covering the thyroid gland are not considered ITPA [25,26]. ITPA usually results in primary hyperparathyroidism (PHPT) and presents with asymptomatic hypercalcemia on routine screening. However, atypical presentations include calcium homeostasis disturbances or normocalcemic PHPT. The classical manifestations of PHPT (bones, stones, abdominal moans, psychic groans) are frequently encountered in developing countries, and hypercalcemic symptoms could be present e.g. anorexia, nausea, constipation, polydipsia, and polyuria [27][28][29][30].
HPTC cases are documented [4,[9][10][11][12][13][14][15][16][17][18], and many ITPA cases have been published. However, only 2 cases have been reported as a simultaneous co-occurrence of PTC and ITPA [31,32]. To the best of our knowledge, there are no published reports of the simultaneous coexistence of HPTC and ITPA in the same thyroid lobe. This paper reports the first case of co-occurrence of HPTC and ITPA in the same thyroid lobe and further defines the clinical/molecular characteristics of HPTC that may be useful for prognostic stratification and may provide therapeutic targets. We report this case in line with the updated consensus-based surgical case report (SCARE) guidelines [33].

Patient presentation
A 61-year-old woman presented to the endocrinology clinic with generalized bone ache, polyuria and right neck mass for the last few months. There was no past history of kidney stones and she did not report other symptoms. She had a history of hypertension and dyslipidemia on treatment; otherwise, there were no contributory chronic medical diseases or past surgical intervention. There was no family history of thyroid cancer. On physical examination, she was vitally stable, good body built, very good physical performance, and neurological examination was unremarkable. She had a smooth, non-tender right neck swelling (3-4 cm) that moved with swallowing.

Blood
Revealed high corrected calcium (2.74 mmol/L), high intact PTH (111 pg/mL). Vitamin D was 44 ng/mL, and TSH and T4 were both normal. CBC, liver and kidney functions were unremarkable.

Ultrasound of the neck
Large complex nodule in right thyroid lobe (4.6 × 2.4 cm) with cystic component, focal dense area of calcification, and peripheral vascularity. Left thyroid lobe showed few small hypoechoic nodules, the largest was 3.5 mm. Isthmus was unremarkable. There were small non-significant lymph nodes along left upper jugular and both upper cervical regions. No mass was observed in bilateral parathyroid regions.

Parathyroid sestamibi scan
Early and late anterior images of neck and mediastinum taken at 20 min and 2 h after IV injection of 10 mCi radiotracer. Normal early thyroid tissue uptake and late physiological washout were observed with features of high uptake and retained focal activity related to the lower pole of the right thyroid lobe, suggestive of right inferior PA. There was also a big cold nodule projecting from the lateral border of the right thyroid lobe, suggestive of thyroid nodule (Fig. 1).

Cytopathology
Ultrasound-guided fine-needle aspiration of the complex nodule in right lobe was done. About 10 mL of straw-colored fluid was aspirated from the cystic part. The calcified solid part was also aspirated. The sample was processed as 4 fixed slides (pap stain) and 4 air-dried slides (DiffQuik). Microscopically, there were some Hurthle cells clusters with abnormal features, few follicular cells, macrophages, minimal colloid, and blood, consistent with atypical follicular lesion of undetermined significance (FLUS).

Surgical technique
The case was discussed at the thyroid multi-disciplinary meeting and it was decided to conduct right hemithyroidectomy with removal of the right inferior PA. Under general anesthesia, conventional neck exploration did not reveal the PA, and surgery proceeded with right hemithyroidectomy as planned, which was followed by a sudden drop of intraoperative rapid PTH (94.3% drop from pre-incision serum baseline, confirming excision of ITPA). Surgery was concluded. The patient recovered smoothly with no peri-operation complications. An experienced senior surgeon undertook the procedure.

Final histopathology of right thyroid lobe
H&E sections revealed the coexistence of encapsulated noninvasive HPTC measuring 2 cm with all margins uninvolved by carcinoma. No lymphovascular, perineural invasion or extrathyroidal extension were seen. AJCC staging was pT1b ( Fig. 2A and B). Also, an ITPA measuring 2 cm completely surrounded by thyroid tissue was identified (Fig. 3).

Immunohistochemical stains
In the HPTC, the carcinoma demonstrated decreased staining with thyroglobulin and positive staining with HBME-1, galectin-3, and CK19 (controls checked). In the ITPA, TTF-1, thyroglobulin and chromogranin immunostains were performed (controls checked). The parathyroid adenoma was positive for chromogranin but negative for TTF-1 and thyroglobulin, which highlighted only the thyroid tissue (Fig. 4A, B).
Sanger sequencing was performed on the amplified target DNA. It revealed a nucleotide change from Thymine (T) to Adenine (A) in codon number 600 in exon 15 of the BRAF V 600E gene. This change in nucleotide results in amino acid change from Valine (V) to Glutamic acid (E). This confirmed the BRAF V 600E mutation in our case (Fig. 5).

Follow up
The patient had no perioperative complications and was discharged after 2 days. The case was again discussed at the thyroid multi-disciplinary meeting, which decided that the patient's risk stratification category was low risk (although the case displayed one intermediate risk feature, i.e. aggressive tumor histology hobnail variant) [8]. Hence it was decided that the patient will be under close follow up with ultrasound every 6 months. Neck ultrasound at 6 months revealed that the left lobe was heterogeneous with multiple small hypoechoic nodules, the largest measuring 2.4 × 1.7 mm, and no suspicious lymph nodes. FNAC revealed colloid nodule with cystic degeneration. At 12 months she was completely asymptomatic, with no bone pain, normal calcium and PTH levels, and neck ultrasound and CT scan neck and thorax revealed no metastatic disease or mass, lymphadenopathy or lung metastasis. The patient was satisfied.

Discussion
HPTC is a recently described aggressive variant of PTC, comprising 0.3%-2.7% of PTCs [2,4,10]. IPTA is documented, but its co-occurrence with PTC is extremely rare [31,32]. HPTC cases have been reported in the literature [4,[9][10][11][12][13][14][15][16][17][18], and approximately 160 ITPA cases have been reported (2007-2018, Tables 1 and 2). However, only two cases were a simultaneous co-occurrence of PTC and ITPA (Table 3) [31,32]. The coexistence of HPTC and ITPA in the same  i Surgery started by left hemithyroidectomy and there was drop of iPTH just before and after hemithyroidectomy (1054 to 29.4 pg/mL), after confirmation of removal of hyperparathyroidism source, then left inferior parathyroidectomy was performed in same operation. j started with minimally invasive surgery and intraoperative scintigraphy, parathyroid adenoma discovered (intrathyroid), surgery proceeded to right hemithyroidectomy. k Patient 1: surgery started by minimally invasive video-assisted parathyroidectomy, failed to find PA. Surgery converted to conventional neck exploration and proceeded with hemithyroidectomy. Patient 2: first operation failed (Minimally Invasive Video-assisted Parathyroidectomy), patient did not improve, imaging localization performed before second operation, then intrathyroid and hemithyroidectomy performed by conventional technique. l Removed by surgical intervention, but author did not mention type of procedures performed. m Plan was for right inferior parathyroidectomy but patient asked for total thyroidectomy at same time. Authors did not explain why patient asked for that. n The article mixes ITPA (12) Table outlines  thyroid lobe has never been reported. This paper reports the first case.
In terms of the presentation, Table 1 shows that ITPA presents predominantly in females with mean age of 52.3 years. Our case was a female (61 years) with classical PHPT presentation (bone pain, polyuria) but no kidney stones. On examination, a right thyroid lobe swelling was discovered, with no compressive symptoms, cervical lymphadenopathy, or distance metastasis. Our patient's PHPT symptoms agree with Table 1, where more than 50 patients presented with classic hyperparathyroidism symptoms. However, Table 1 shows that some cases were asymptomatic [23,[34][35][36]. There are studies (72 cases) that did not report how patients presented [24]. In the symptomatic group, most studies did not specify the frequency of the symptoms [25,[37][38][39][40][41][42]. An exception is one study where the most frequent symptoms were bone pain (40%), fatigue (26%), and kidney stones (22%) [23]. Our case had right thyroid lobe swelling which supports Table 1, where 6 patients presented with neck swelling [36,39,[43][44][45].
The role pre-operative FNA cytological (FNAC) diagnosis in HPTC and ITPA are different. PTC diagnosis is based on The Bethesda System for Reporting Thyroid Cytopathology diagnostic categories [46]. However, FNAC diagnosis of HPTC is challenging, with no consensus on diagnostic criteria, although pre-operative diagnosis of HPTC is possible [2,21]. Others have reported on 10 HPTC cases, all had preoperative FNAC and all had hobnail feature [2]. Asioli et al. undertook FNAC for their 5 cases and all had hobnail feature ranging between 10%-50% [19]. Others performed FNAC (24 patients), where 21 had PTC with Bethesda category IV-V, and another 3 patients were Bethesda III, but they did not report whether the hobnail feature was present [17]. Similarly, others reported 2 cases with pre-operative FNAC diagnostic of PTC Bethesda VI, but again with no mention of the hobnail feature [16]. We are in partial agreement; in our case, preoperative FNAC showed Bethesda III cytology (FLUS). The above findings make it difficult to deduce the role of preoperative FNAC in the diagnosis of HPTC. Future research is required, and a definite HPTC diagnosis is usually confirmed on histopathology after thyroidectomy.
HPTC diagnosis by microscopic examination of the thyroidectomy specimen is also challenging. At low power, the tumor in HPTC usually forms papillary structures that have edematous and/or fibrotic stalks that can be associated with cystic changes [10,15], a feature present in other benign conditions e.g. hyperplastic thyroid nodules. At high power, the classic nuclear features of PTC such as nuclear grooving and pseudoinclusions are less prominent and less common in HPTC [19]. Moreover, cells in HPTC can occasionally have eosinophilic cytoplasm reminiscent of oncocytic cells [15,19]. For these reasons, HPTC can be overlooked or misdiagnosed as hyperplastic thyroid nodule. Since this PTC variant is more aggressive, careful examination of the lesions must be made with attention to the nuclear features of PTC such as nuclear enlargement, nuclear overlapping and nuclear membrane irregularity, in addition to the features characteristic for this variant e.g. discohesiveness of cells, micropapillary pattern and apical location of nuclei within the cell with surface bulge making this "hobnail" appearance.
Likewise, the role of preoperative FNAC diagnosis of ITPA seems not decisive. Whilst preoperative FNAC detected ITPA in 4 out of 5 IPTA patients; FNAC of 2 patients with suspicion that only one of them had ITPA, failed to prove ITPA in the other patient [23,43]. However, Fine Needle Aspiration intact parathyroid hormone (FNA-iPTH) from a suspected ITPA and intrathyroid cystic parathyroid adenoma (ITCPA) showed high iPTH in the ITCPA [39]. The role of FNAC in ITPA is inconclusive. Our preoperative FNAC of the right thyroid nodule revealed FLUS, failing to diagnose the ITPA.
Intraoperatively, rapid PTH monitoring is used to confirm PTA removal [47]. In our case, intraoperative rapid PTH monitoring confirmed ITPA excision, dropping suddenly from 228 to 17 pg/mL (92.5% drop at 5 min) to 13 pg/mL (94.3% drop at 10 min) after ITPA excision. Postoperatively, histopathology provides the definitive diagnosis when this is not accomplished before surgery. In our case, histopathology of right thyroid lobe provided the diagnosis of concurrent HPTC and ITPA.
In terms of the location, most ITPA were located in the lower part of the thyroid gland [23,24]. Table 1 shows 10 left and 13 right IPTAs [25,[34][35][36][37][38][39]44,45,48]. Others provided no documentation of the site/side affected [40][41][42]. We are unable to conclude whether the tumor behavior exhibits preference to a particular side of the thyroid gland. In cases of co-occurrence, our HPTC and ITPA were in the right side, in agreement with reports where PTC and ITPA were both in the right lobe (possibly a coincidence), but in contrast with others where each pathology was in a different lobe (Table 2) [31,32].
The most common mutation occurring in HPTC is BRAF V 600E mutation. This mutation is associated with a higher likelihood of extrathyroidal extension, lymph node metastasis, distant metastasis, recurrence and mortality [2]. Molecular analysis of 10 HPTC cases found BRAF V 600E mutation in 8 cases [2], and in 16 out of 17 cases [3]. Others reported similar results [2,6,19]. BRAF V 600E mutation was also present in our case, supporting the association between BRAF V 600E and HPTC.
In terms of management, the National Comprehensive Cancer Network guidelines indicate total thyroidectomy as primary treatment in PTC patients with any of: radiation history, distant metastasis, bilateral nodularity, extrathyroidal extension, tumor diameter > 4 cm, cervical lymph node metastases, or poorly differentiated features. However, there are no clear current guidelines for the treatment of HPTC [2]. Table 3 depicts that almost all HPTC patients received total thyroidectomy [2,3,6,10,[16][17][18][19][20]. However, one patient received hemithyroidectomy [6], and our case is the second patient to receive hemithyroidectomy. This was because our multidisciplinary team concluded that the risk stratification of the patient fitted well in the low-risk category (although it displayed a feature of intermediate risk, aggressive tumor histology, hobnail variant), hence hemithyroidectomy was decided as appropriate. Table 3 shows that most HPTC patients received variable extents of cervical lymph node dissection ranging from central lymph node dissection to radical neck dissection [2,3,6,10,16,19,20]. Radioactive iodine ablation is also documented, but it is unclear whether such ablation was also undertaken by others [6,[16][17][18][19][20]. As we undertook hemithyroidectomy, cervical lymph node dissection or radioactive iodine ablation were not required. There is a gap in the literature regarding the management of HPTC, as many of the above evidence did not clearly document treatment type and details in terms of the type of surgery and extent of cervical lymph node dissection. Future research could benefit from a focus on HPTC treatment and management to provide advice specific for this PTC variant when co-occurring with ITPA.
In terms of prognosis of HPTC, the literature reported high mortality rate, persistence of disease, and high recurrence rate (Table 3). One study reported 2 cases and both died (at 6 and 11 years) [16]. Others reported 10 HPTC patients, where half of the patients died [10]. Likewise, a study of 19 of 25 patients followed for ≈39 months reported that no patients died of the disease, but one third had persistent disease [17]. As for ITPA (Table 1), all patients became asymptomatic after surgery with normalization of Ca and PTH in the follow-up period (range 6-96 months) [23,[34][35][36][37][38][39][42][43][44][45]48]. However, others reported that after thyroid surgery for ITPA, few patients had transient hypocalcemia, and very few had permanent/prolonged hypocalcemia needing calcium supplement [23,45]. Our patient had excellent outcomes, no complications, and was asymptomatic at 12 months, with normal calcium and PTH. Her follow-up neck ultrasound of the remaining thyroid lobe found very small non-suspicious nodules. Subsequently, FNAC revealed benign changes of colloid nodule with cystic degeneration in the remaining left lobe, and her CT scan of the neck and thorax revealed no metastatic disease, mass or lymphadenopathy.

Conclusion
Hobnail variant PTC is extremely rare and its coexistence with intrathyroid PA in the same thyroid lobe has possibly never been reported before. The incidental finding of PTC during parathyroid surgery is rare, and requires a thorough investigation, particularly by imaging in search for any abnormal thyroid findings. The presence of PA should not in any way exclude the diagnosis of thyroid carcinoma. The monitoring of intraoperative rapid iPTH is vital to confirm the complete removal of the parathyroid adenoma. Hemithyroidectomy is appropriate for early stage (pT1b) HPTC coexisting with ITPA in the same thyroid lobe. Describing the cytomorphologic features to distinguish thyroid from parathyroid cells on FNA cytology samples and immunohistochemical stains needs to be considered.

Declaration of Competing Interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Sources of funding
This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.

Ethical approval
The Medical Research Centre at Hamad Medical Corporation, Doha, Qatar approved this case series.

Consent
Written informed consent for publication of their clinical details and/or clinical images was obtained from the patient.