Rectal follicular thyroid-like carcinoma: A case report and review of literature

Abstract

BACKGROUND

Follicular thyroid carcinoma (FTC) is the second most common subtype of thyroid malignancy, with distant metastases most often to the bones, lungs, brain, and liver, and only rarely to other sites. Rectal follicular thyroid-like carcinoma is a rare condition characterized by infiltration of FTC within the rectal wall. There are almost no literature reports.

CASE SUMMARY

We report a case of rectal thyroid-like follicular carcinoma in a 61-year-old woman. The patient presented with intermittent rectal bleeding, and a colonoscopy revealed a mass in the mid-rectum. She underwent laparoscopic resection of the lesion, and was diagnosed with rectal thyroid-like follicular carcinoma by postoperative pathology. After a laparoscopic partial rectal resection, she was discharged on postoperative day 7. At 6 months postoperatively, the patient was still alive.

CONCLUSION

Rectal follicular thyroid-like carcinoma may arise from malignant struma ovarii, highlighting the need to consider ovarian origins in atypical metastases of FTC.

Key Words: Malignant struma ovarii; Follicular thyroid carcinoma; Rectal neoplasms; Neoplasm metastasis; Case report

Core Tip: We report a rare case of rectal follicular thyroid-like carcinoma. This case highlights the necessity of including an ovarian origin in the differential diagnosis when encountering metastases of follicular thyroid carcinoma in atypical locations. Our findings aim to serve as a useful reference for clinicians in the diagnosis and management of similar cases in the future.

INTRODUCTION

Follicular thyroid carcinoma (FTC) accounts for 10%-15% of all thyroid malignancies. It typically metastasizes hematogenous to the bones, lungs, brain, and liver, while spread to other organs is uncommon[1,2]. However, when distant organs show metastasis of thyroid carcinoma, malignant struma ovarii (MSO) should also be considered in the differential diagnosis. Struma ovarii is a highly differentiated monodermal teratoma composed predominantly (> 50%) of thyroid tissue, and accounts for < 3% of all ovarian teratomas[3,4]. MSO represents approximately 5% of struma ovarii and is characterized by malignant thyroid tissue within the ovary. Distant metastasis of MSO is exceedingly rare, with few reports in the literature[5,6]. Here, we present a case of rectal follicular thyroid-like carcinoma, and review the clinical, pathological, and therapeutic implications of FTC and struma ovarii.

CASE PRESENTATION

Chief complaints

A 61-year-old woman presented with a 1-week history of intermittent rectal bleeding.

History of present illness

The patient was admitted to Union Hospital due to intermittent rectal bleeding. Colonoscopy revealed a 2.0 cm × 3.0 cm elevated lesion with surface ulceration located 10 cm from the anal verge (Figure 1). Endoscopic ultrasound demonstrated poor delineation of the mucosal and muscularis propria layers, appearing as a mixed echogenic solid mass.

Figure 1 Colonoscopy findings. Colonoscopy revealed a 2.0 cm × 3.0 cm elevated lesion with surface ulceration located 10 cm from the anal verge.

History of past illness

The patient had a history of right oophorectomy and adnexectomy for ovarian teratoma 20 years earlier.

Personal and family history

The patient denied any family history of malignant tumors.

Physical examination

The abdomen was flat and soft. Surgical incision scars were seen on the abdomen. Digital rectal examination revealed blood staining of the finger cots.

Laboratory examinations

Laboratory examination revealed no abnormalities.

Imaging examinations

Pelvic magnetic resonance imaging (MRI) showed focal thickening of the upper rectal wall with a nodular mass protruding into the lumen (Figure 2A). Contrast-enhanced abdominal computed tomography (CT) confirmed localized rectal wall thickening with mild enhancement (Figure 2B).

Figure 2 Imaging examination findings. A: Pelvic magnetic resonance imaging showed focal thickening of the upper rectal wall with a nodular mass protruding into the lumen; B: Contrast-enhanced abdominal computed tomography confirmed localized rectal wall thickening with mild enhancement.

FINAL DIAGNOSIS

Based on CT, endoscopy, and other relevant examinations, the patient was diagnosed with rectal thyroid-like follicular carcinoma.

TREATMENT

After an informed discussion, a laparoscopic partial rectal resection was performed for the tumor. Pathological examination revealed infiltration of thyroid tissue within the mucosa and muscularis propria of the rectal wall, with features of malignancy, consistent with metastatic FTC (Figure 3A).

Figure 3 Pathological examination findings. A: Pathological examination revealed infiltration of thyroid tissue within the mucosa and muscularis propria of the rectal wall, with features of malignancy, consistent with metastatic follicular thyroid carcinoma; B: Fine-needle aspiration cytology of the right thyroid lobe indicated a Bethesda II benign nodule.

OUTCOME AND FOLLOW-UP

The postoperative course was uneventful, and rectal bleeding resolved. Postoperative thyroid function tests were within normal limits. Neck ultrasound demonstrated a solid nodule in the right thyroid lobe, measuring approximately 16.7 mm × 12.6 mm× 9.1 mm (American College of Radiology Thyroid Imaging Reporting and Data System 4). It was classified as Bethesda II according to the fine needle aspiration cytology, indicating a benign nodule (Figure 3B). The patient was discharged on postoperative day 7. Follow-up MRI of the hepatobiliary system, retroperitoneum, and pelvis two months postoperatively revealed no evidence of tumor metastasis. At 6 months postoperatively, the patient was still alive.

DISCUSSION

FTC typically occurs in middle-aged and elderly women, with incidence increasing with age. The prognosis is generally less favorable in the elderly[7]. FTC spreads predominantly via the hematogenous route, most commonly to bone, lung, brain, and liver, while metastasis to other sites is uncommon[8,9]. Struma ovarii is itself rare, representing < 3% of ovarian teratomas, and less than 5% of these cases are malignant[4,5]. MSO usually involves a unilateral ovary and is most common in premenopausal women, with an average age of 45[10]. Retrospective data suggest that MSO typically metastasizes within the pelvis and abdomen, with rare spread to the liver, lungs, bones, or brain, and no clear correlation between metastatic sites and histological morphology[11-13]. Some studies suggest that molecular changes such as epidermal growth factor receptor or B-Raf proto-oncogene mutations may be related to malignant transformation and distant metastasis of struma ovarii, although the precise mechanisms remain uncertain[5,13-15].

When FTC is found in distant tissues, establishing the primary site is critical. In ovarian metastasis of FTC, the lesion consists exclusively of thyroid carcinoma cells, whereas SO contains other teratoma components[16]. Struma ovarii lacks the ability to invade and metastasize. However, distant metastases of MSO only contain malignant thyroid tissue, which is consistent with the characteristics of cancer cells in distant metastases of thyroid cancer[11]. Therefore, when FTC cells appear in the rectal wall, the primary tumor site must be confirmed. For patients diagnosed with struma ovarii, thyroid examination remains necessary. In this case, thyroid function tests were normal. The ultrasound showed a right-lobe nodule classified as American College of Radiology Thyroid Imaging Reporting and Data System 4, and fine-needle aspiration yielded Bethesda II (benign). Given the patient’s history of ovarian teratoma resection 20 years earlier, the rectal lesion was most consistent with MSO.

The clinical presentation of FTC is often dominated by local compression or metastatic disease. Bone metastases may manifest with pain, palpable masses, or pathological fractures, whereas lung metastases can present with cough, sputum production, or dyspnea[7]. The clinical manifestations of MSO are nonspecific. Retrospective studies indicate that many patients are asymptomatic, with tumors incidentally discovered during routine imaging or surgery[17]. When symptoms occur, they often resemble those of other ovarian tumors, such as abdominal pain, pelvic mass, ascites, or abnormal uterine bleeding, as well as symptoms related to distant metastasis[17]. Additionally, 5%-8% of patients may experience thyroid dysfunction[11,18].

For thyroid disease, ultrasound is the preferred imaging modality for evaluating nodules, and fine-needle aspiration cytology remains the diagnostic gold standard[19]. Evaluation of thyroid function is necessary. It typically includes thyroid-stimulating hormone (TSH), free thyroxine, free triiodothyronine, anti-thyroid peroxidase antibodies, and anti-thyroglobulin (Tg) antibodies[20]. In MSO, laboratory findings are nonspecific. When associated with thyrotoxicosis, laboratory findings of MSO may include suppressed TSH, elevated Tg, and increased free triiodothyronine/free thyroxine levels. This is similar to the findings in patients with primary hyperthyroidism[21]. Imaging studies suggest the presence of ovarian struma, but cannot provide a differential diagnosis. Struma ovarii usually appears as a solid mass with smooth margins and multicyclic components. On CT, cystic regions appear hyperdense, with approximately half showing contrast enhancement of the cyst wall[22]. On MRI, solid portions enhance significantly, while cystic contents typically show low to intermediate signal on T1 and low signal on T2[23]. Distant metastases from MSO share imaging features with thyroid carcinoma metastases, appearing as irregular or rounded soft-tissue nodules of variable size. Definitive diagnosis of MSO, therefore, relies on histopathological evaluation of surgical specimens.

At present, there is a lack of consensus regarding the standard treatment protocol for MSO. Surgical resection remains the mainstay, ranging from ovarian cystectomy to salpingo-oophorectomy, hysterectomy, and excision of metastatic foci, depending on patient age and fertility considerations[24]. Combination chemotherapy has shown limited benefit, distinguishing MSO from other malignant germ cell tumors[25]. In addition to surgical treatment, some authors advocate thyroidectomy and postoperative I-131 therapy, treating it as thyroid cancer even when the thyroid is unaffected[26,27]. However, due to insufficient long-term follow-up data, the long-term efficacy of this treatment for MSO remains unknown[28].

Prognosis of FTC is poorer than that of papillary thyroid carcinoma and depends on patient age, tumor size, stage, completeness of resection, response to radioiodine, and the extent of vascular invasion[29]. In contrast, struma ovarii generally carries a favorable overall prognosis. No clear correlation has been demonstrated between surgical strategy and progression-free survival[13]. Reports suggest that maintaining TSH in the lower normal range may improve patient survival[30]. Additionally, elevated serum Tg and Tg antibody serve as useful markers for recurrence and metastasis, similar to thyroid cancer[30]. Therefore, regular surveillance in accordance with thyroid cancer guidelines is recommended for MSO patients.

CONCLUSION

We report a rare case of rectal thyroid-like follicular carcinoma, most likely arising from malignant transformation of struma ovarii. This report reviews the clinical manifestations, pathological features, treatment strategies, and prognostic outcomes associated with both FTC and struma ovarii. Our findings aim to serve as a useful reference for clinicians in the diagnosis and management of similar cases in the future.