History
74-year-old female who is status post thyroidectomy and status post subsequent radio- iodine¥ 2. She is currently being evaluated for recurrent disease because of elevated thyroglobulin of 14.
Findings
Breath hold images of the lung show a number of peripheral areas of atelectasis in the lingula and middle lobe which suggest a background of chronic bronchial disease.
There is also some postinflammatory linear scarring at the left base. However, no pul- monary nodules are apparent. One tiny pleural nodule is rectangular in shape and is probably a postinflammatory lesion. The 18-FDG whole body PET scintigraphy is neg- ative in the chest and abdomen. There is one moderately hypermetabolic focus in the neck (Figure 20.1.1). The neck dissection was on the right (Figure 20.1.2). The focus is on the left, just posterior to the inferior posterior ala of the thyroid cartilage (Figure 20.1.3). No recognizable lymph node is evident on the unenhanced CT. All of the tissue is the same density on CT at this level.
Impression
1. Single moderately hypermetabolic focus in the deep left neck, just posterior to the posterior inferior left thyroid cartilage ala, compatible with recurrent thyroid cancer.
2. Negative scintigraphy in the chest, abdomen and pelvis.
Pearls and Pitfalls
• Only 50% to 60% of papillary thyroid carcinomas and 64% to 67% of follicular cancers are iodine avid.4,5
• PET is not indicated for primary thyroid malignancy detection since FDG can accu- mulate in normal thyroid tissue (~30% of patients) and in certain benign thyroid diseases.3,7
• A PET study will usually be positive in patients with thyroglobulin level higher than 100 ug/L. PET will detect true positive disease in 11%, 50%, and 93% of patients with hTg levels of <10ug/L, 10 to 20ug/L, and >100ug/L.6,8
20 Thyroid Carcinoma
Lalitha Ramanna
Case 20.1
212
FIGURE20.1.2. FIGURE20.1.3.
• FDG is indicated for a known, differentiated, and recurrent papillary/follicular thyroid cancer patient with a negative I-131 scan. Increased expression of GLUT-1 is associated with the loss of radioactive iodine uptake in metastases.10,14
Discussion
Less than 1% of all cancer deaths are from thyroid cancer. Most differentiated thyroid cancers have an indolent course with low morbidity and mortality. 12% to 15% of the photopenic lesions seen on technetium-99m or radioiodine studies are malignant. Most malignant lesions are solid on ultrasonography. Fine needle aspiration is best for accu- rate diagnosis. The treatment of papillary and follicular thyroid cancers are surgery complemented with I-131 therapy.
Thyroid cancers are mostly in females with the age range of 45 to 50 years old. In countries where iodine intake is normal, 80% of the thyroid cancers are of papillary and follicular histology. In countries of low iodine intake, follicular and anaplastic his- tology are more common. Previous external irradiation is a risk factor for papillary carcinoma, whereas previous I-131 administration is not. Less than 10% of all thyroid cancers are undifferentiated anaplastic cancers, mostly occurring in elderly patients.
Medullary carcinoma constitutes the rest of the 10% cancers. These cancers have a high predisposition for MEN (multiple endocrine neoplastic) syndromes. The princi- ple treatment of papillary and follicular (Hürthle cell) carcinoma is surgery followed by I-131 radioablation 4 to 6 weeks postop and a whole body scan 4 to 7 days later.
External radiotherapy is reserved for elderly patients who have contraindications for surgery and I-131 treatment. Anaplastic cancers are treated with combination therapy of systemic chemotherapy and external radiotherapy. The treatment of medullary thyroid cancer is surgery, provided the presence of pheochromocytoma has been ruled out.
214 Part II Clinical Cases
History
49-year-old female who has a history of papillary thyroid cancer treated with radio- therapy and limited neck dissection. The patient is being seen for evaluation for metastatic disease.
Findings
There is focal hypermetabolism within the upper posterior triangle of the right neck (Figure 20.2.1) that corresponds to a lymph node on CT. Additionally two moderately positive hypermetabolic nodes are seen bilaterally in the superior mediastinum with the more prominent one on the right (Figure 20.2.2, right column). Inferiorly, there is hyperactivity in the thymus (Figure 20.2.2, left column) corresponding to prominence on CT; differential diagnosis includes thyroid metastases at this site.
Impression
Evidence for distant metastasis involving the right upper posterior triangle of the neck, the superior mediastinum, and possibly the anterior mediastinum.
Case 20.2
Pearls and Pitfalls
• In differentiated thyroid carcinoma, PET can detect recurrence and metastases with a sensitivity of 80% to 90%.11
• Removal of exogenous thyroid hormone in addition to TSH stimulation may improve the detection of unsuspected lesions.5,7
• The 3-year survival rate of a thyroid carcinoma patient with a positive PET scan is 60% as opposed to 98% survival of the same patient with a negative PET scan.1,6,7,8
• Thymic hyperplasia can cause false-positive PET results in the anterior mediastinum.
Discussion
Differentiated cancers, papillary or follicular, are highly treatable and usually curable.
Poorly differentiated cancers, medullary or anaplastic, are less common but aggressive, metastasize early, and have a poorer prognosis.
FIGURE20.2.1.
FIGURE20.2.2.
History
62-year-old male who has a history of undifferentiated thyroid carcinoma and esophageal cancer.A prior PET demonstrated several sites of hypermetabolism involv-
Case 20.3
ing the neck and lung. His most recent CT confirmed the right neck pathology. He was treated with radiotherapy and chemotherapy. Evaluation for metastases is requested.
Findings
There is hypermetabolism posterior to the left hyoid bone in the prevertebral space (Figures 20.3.1 and 20.3.2) that exhibits soft tissue asymmetry with lower density on the left side without evidence of a discrete mass on CT. This may represent residual normal thyroid tissue or metastatic disease. There is a band of activity in the paravertebral region on the right (Figure 20.3.3) that corresponds to nodular pleural 216 Part II Clinical Cases
FIGURE20.3.1.
FIGURE20.3.2. FIGURE20.3.3.
thickening on CT, compatible with metastatic disease vs. pleuritis. This latter finding has decreased from the prior PET examination. The left band of activity is no longer visible. There is no interval change in the thyroid bed activity left of midline, consis- tent with residual thyroid tissue vs. tumor. The focus of activity in the cecum (Figure 20.3.4) indicates possible pathology and endoscopic evaluation is recommended. The halo of activity around the glenohumeral joint (Figure 20.3.5) is likely the result of inflammatory joint disease. The abnormal lung activity from prior study has resolved.
Impression
1. Suspicious sites of mediastinal uptake involving the neck and chest with some inter- val improvement. The findings are most consistent with residual metastatic disease from either of the two known primary cancers.
2. The focal activity in the cecum is suspicious; endoscopic evaluation is advised.
Pearls and Pitfalls
• CT is superior to PET in the detection of small lung lesions less than one centimeter in diameter. Low FDG uptake makes even large lesions difficult to detect on PET.11,15
FIGURE20.3.4. FIGURE20.3.5.
• Highly differentiated thyroid cancer is usually I-131 positive and may display low FDG uptake; in contrast, poorly differentiated thyroid cancer will be I-131 negative and FDG positive.10
• Retinoic acids and somatostatin therapy are alternate treatments for undifferentiated thyroid cancer showing persistent serum thyroglobulin levels if I-131 therapy proves ineffective.2,9,13
Discussion
In medullary thyroid cancer (MTC), 71% to 80% of the cases present with loco- regional metastases and 20% have distant metastases. FDG may also be indicated for medullary thyroid cancer with rising calcitonin level, or CEA level, and a negative study on octreotide or MIBG scan.
218 Part II Clinical Cases
History
The patient has a remote history of thyroid carcinoma, status post thyroidectomy and I-131 ablation. He is now being evaluated for a soft tissue abnormality on CT near the thyroid bed, possibly representing recurrence.
Findings
There is active tissue in the right thyroid bed (Figures 20.4.1 and Figure 20.4.2). The tissue, which is moderately metabolically active, is below the thyroid cartilage and about 2 cm above the suprasternal notch. It is triangular in shape and adjacent to the trachea. It is modestly more active than the usual malignancy, but it should be noted that thyroid carcinoma is quite variable in its 18-FDG metabolic activity. It is so remote
Case 20.4
FIGURE20.4.1.
from surgery that it is considered unlikely to represent scar activity. No other meta- bolically active abnormality is evident on the torso exam.
Impression
Active soft tissue of about 2 cm dimension in the right paratracheal area in the right thyroid bed, status post remote bilateral thyroidectomy, suspicious for recurrent thyroid cancer.
Pearls and Pitfalls
• The sensitivity and specificity of PET for recurrent tumor ranges from 69% to 94%
and 42% to 95%, respectively.15
• 70% of the normal lymph nodes are detected with PET.7,12
• 50% of the changes in patient management are directly related to results obtained from PET imaging.6,7
• 20% of the differentiated thyroid cancers recur.
FIGURE20.4.2.
