SAT563 Follicular Thyroid Cancer Masquerading as Chest Wall Mass With an Aggressive Course

Disclosure: I. Goyal: None. E. Punni: None. L. Adhikari: None. Introduction:Follicular thyroid cancers (FTC) are more aggressive than papillary thyroid cancer and usually have more advanced tumor stage at the time of presentation. Musculoskeletal metastases are rare though, but when present can cause various complications like pathological fractures and nerve compression. Clinical case:A 75-year-old female was seen in orthopedics clinic for a left infraclavicular chest wall mass. MRI chest showed a large destructive mass invading the manubrium. Biopsy showed follicular cells consistent with ectopic thyroid tissue. CT neck/chest showed 2x1.8 cm right thyroid lobe nodule and left sternoclavicular joint metastasis. She underwent total thyroidectomy and resection of portions of manubrium, left clavicle and left 1(st) and 2(nd) rib. Pathology showed FTC in right thyroid lobe, measuring 2.6 cm in greatest dimension with angioinvasion, and FTC of manubrium, adjacent connective tissue and sternoclavicular joint and ribs, measuring 6.3 cm in greatest dimension. Pathological TNM staging was pT2N0M1. Her 6-week post-op thyroglobulin (Tg) level was 28.7 ng/mL (Beckman Coulter Immunometric Assay, reference value <0.1 athyrotic). She was started on levothyroxine supplementation to achieve a TSH <0.01 uIu/mL. She was subsequently treated with 200 mCi of radio-active iodine (RAI) as adjuvant therapy. Post-therapy whole body scan showed increased activity in the right strap muscle and iodine-avid osseous metastasis in right hemisacrum. She further received radiation therapy to neck and sternal regions, and the right sacroiliac joint. Following this, patient was monitored with serial Tg panel and neck ultrasound (US). Her initial post-op US neck showed a soft tissue fullness in right thyroidectomy bed, concerning for residual disease but this was no longer seen after 3 months. Her Tg level downtrended from 28.7 ng/mL to 12.6 ng/mL and further to 1.8 ng/mL at 1-year follow-up. However, her course was complicated by development of several oral ulcers resulting from high-dose RAI therapy which were treated symptomatically with hydration, topical analgesics and Listerine mouth washes. The patient reported ongoing bowel incontinence since her initial cancer diagnosis. MRI pelvis showed rim-enhancing 4.5cm lesion centered at right sacral ala and extending into adjacent S1/S2 neural foramina. She was re-referred for palliative radiation therapy.Conclusion:This case of FTC is unique due to its uncommon initial presentation. Skeletal metastases to sacrum resulted in bowel incontinence in our patient. When present, skeletal metastases need treatment with high-dose (200 mCi) RAI. RAI therapy can cause xerostomia, hypogeusia and sialadenitis due to reductions in salivary flow. Clinicians should be aware of the early and late-onset complications of RAI therapy, discuss these with the patient before therapy and manage the complications adequately if these occur. Presentation Date: Saturday, June 17, 2023