Primary thyroid lymphoma is a rare extranodal non-Hodgkin lymphoma developing in the thyroid, accounting for 1-5% of all thyroid malignancies. The majority of cases (60-80%) are diffuse large B-cell lymphoma (DLBCL), with mucosa-associated lymphoid tissue (MALT) lymphoma being the second most common type. There is a strong association with Hashimoto thyroiditis — patients with chronic autoimmune thyroiditis have a 40-80 fold increased risk of thyroid lymphoma. Chronic lymphocytic infiltration in Hashimoto leads to MALT tissue formation and lymphomatous transformation. Typically presents in women aged 60-70 years as a rapidly growing neck mass. Treatment is based on chemotherapy (R-CHOP); surgery is generally not required.
Age Range
50-80
Peak Age
65
Gender
Female predominant
Prevalence
Rare
Thyroid lymphoma development is directly related to the chronic autoimmune process in Hashimoto thyroiditis. In Hashimoto, the thyroid parenchyma is heavily infiltrated by chronic T and B lymphocytic infiltration — this process leads to formation of MALT (mucosa-associated lymphoid tissue), which is not normally present in the thyroid. Prolonged antigenic stimulation causes clonal expansion in B lymphocytes and accumulation of oncogenic mutations. MALT lymphoma begins as a low-grade process and may transform to high-grade DLBCL in some cases. Lymphomatous infiltration homogeneously displaces thyroid tissue — this produces the markedly hypoechoic (pseudocystic), homogeneous mass appearance on ultrasonography. Because lymphoma cells are very densely packed, they markedly attenuate sound waves and produce posterior acoustic enhancement — mimicking cystic lesions (pseudocyst sign). On CT, it appears as a homogeneous hypo/isodense mass with mild-to-moderate enhancement after contrast. Marked diffusion restriction on DWI is characteristic due to the high cellularity of lymphoma cells.
Markedly hypoechoic, nearly anechoic, homogeneous mass with posterior acoustic enhancement on Hashimoto thyroiditis background — an appearance mimicking a cystic lesion but actually representing solid lymphomatous infiltration. This pseudocystic appearance is the ultrasonographic correlate of the very dense, homogeneous packing of lymphoma cells. Absence of fluid on aspiration attempt and presence of internal vascularity on Doppler confirm the solid nature.
Markedly hypoechoic, nearly anechoic homogeneous mass on ultrasonography — pseudocystic appearance. The mass is not a true cyst but the very dense, homogeneous infiltration of lymphoma cells mimics a cystic lesion. Posterior acoustic enhancement accompanies (cause of confusion with cysts). While Hashimoto parenchyma is heterogeneous and generally hypoechoic, the lymphoma focus stands out as even more hypoechoic than the surrounding parenchyma ('dark island' appearance). Margins are generally well-defined but may be lobulated.
Report Sentence
A ... mm markedly hypoechoic, homogeneous, pseudocystic-appearing mass is seen in the [right/left] thyroid lobe within a parenchyma showing Hashimoto thyroiditis background, with posterior acoustic enhancement; thyroid lymphoma should be primarily considered.
Linear, streaky vascular structures within the mass on color Doppler — 'stranding' pattern. This pattern represents normal thyroid vessels entrapped within the mass — lymphomatous infiltration displaces thyroid tissue but does not completely destroy the vessels, which are preserved as linear flow patterns. This finding may help distinguish from carcinomas (ATC) that completely destroy vessels.
Report Sentence
Linear vascular flow patterns within the mass are seen on color Doppler (stranding sign), suggesting vascular preservation in lymphomatous infiltration.
Homogeneous, hypo/isodense mass in the thyroid region on contrast-enhanced CT. Shows mild-to-moderate homogeneous enhancement after contrast — homogeneous unlike the heterogeneous enhancement and widespread necrosis of ATC. Thyroid capsule may be disrupted but aggressive extrathyroidal invasion as seen in ATC is generally not observed. Accompanying cervical lymphadenopathy is common — lymph nodes are homogeneous, round, and generally do not contain necrosis.
Report Sentence
A homogeneous, mildly-to-moderately enhancing mass is seen in the thyroid region without necrosis; with accompanying cervical lymphadenopathy, thyroid lymphoma should be considered in the differential diagnosis.
Very marked diffusion restriction on DWI — very bright signal at high b-values, very low values on ADC map (typically <0.8 × 10⁻³ mm²/s, often 0.5-0.7 × 10⁻³ mm²/s). Lymphoma shows the lowest ADC values among all thyroid malignancies — this finding is diagnostically very valuable and helps distinguish from other thyroid tumors.
Report Sentence
The thyroid mass demonstrates very marked diffusion restriction on diffusion-weighted imaging, with ADC value of ... × 10⁻³ mm²/s, which is very low and consistent with lymphoma.
The mass shows intermediate-to-low signal intensity on T2-weighted images and is generally homogeneous. This reflects shortened T2 relaxation time due to the high cellularity of lymphoma cells. Homogeneous T2 signal is distinguishing from the heterogeneous necrotic pattern of ATC and the high T2 signal of simple cysts.
Report Sentence
The thyroid mass shows homogeneous, intermediate-to-low signal intensity on T2-weighted sequences, consistent with high cellularity.
The thyroid mass shows very high FDG uptake on FDG PET-CT (SUVmax typically >10-15). Lymphoma shows the highest FDG uptake among all thyroid tumors. PET-CT is also critical for staging by evaluating accompanying nodal and extranodal involvement. While Hashimoto thyroiditis shows diffuse low-to-moderate FDG uptake, the lymphoma focus stands out with very intense focal uptake.
Report Sentence
The thyroid mass demonstrates very high FDG uptake on FDG PET-CT (SUVmax: ...); this level of intense uptake is consistent with thyroid lymphoma and nodal/extranodal involvement should be evaluated for staging.
Criteria
Comprises 60-80% of primary thyroid lymphomas. High-grade, aggressive lymphoma. Composed of large, pleomorphic B lymphocytes. CD20 positive. Rapidly growing mass, compressive symptoms (dyspnea, dysphagia) common.
Distinct Features
Larger mass on imaging, faster growth. Very high FDG uptake on PET-CT. R-CHOP chemotherapy is standard treatment. 5-year survival 50-70%.
Criteria
Comprises 20-30% of primary thyroid lymphomas. Low-grade, indolent lymphoma. Composed of small-to-medium B lymphocytes. Very strong association with Hashimoto. Slow growth, localized disease common.
Distinct Features
Smaller mass, slower growth. Moderate-to-high FDG uptake on PET-CT. Radiation therapy or chemotherapy in treatment. 5-year survival >90%. Risk of transformation to DLBCL (5-10%).
Criteria
Coexistence of both DLBCL and MALT components. Represents high-grade transformation of MALT lymphoma. Two distinct cell populations observed on histopathology.
Distinct Features
Prognosis similar to pure DLBCL — aggressive component determines behavior. Treatment follows DLBCL protocol (R-CHOP). Imaging findings similar to DLBCL.
Distinguishing Feature
ATC is very heterogeneous with extensive necrotic areas, aggressive extrathyroidal invasion, and chaotic vascularity. Lymphoma is more homogeneous (pseudocystic), shows linear stranding vascularity, necrosis is less prominent. Median survival 3-6 months in ATC, much better prognosis in lymphoma.
Distinguishing Feature
Hashimoto nodule is generally smaller, benign in character, and grows slowly. Lymphoma is a rapidly growing, markedly hypoechoic (pseudocystic) mass showing posterior acoustic enhancement. On DWI, lymphoma shows very low ADC while Hashimoto nodule shows higher ADC.
Distinguishing Feature
Papillary carcinoma shows microcalcifications, irregular margins, taller-than-wide shape — calcification is rare in lymphoma. Papillary carcinoma mostly grows slowly; lymphoma presents as rapidly growing mass. Pseudocystic appearance and posterior enhancement in lymphoma are distinguishing.
Urgency
urgentManagement
medicalBiopsy
NeededFollow-up
specialist-referralIn suspected thyroid lymphoma, tissue diagnosis should be obtained by core biopsy (FNA is insufficient — adequate tissue for flow cytometry is needed). Cytology by FNA can be misleading — lymphocytic infiltration may not be distinguished from Hashimoto. Immunohistochemistry (CD20, CD3, Ki-67) and flow cytometry should be performed on core biopsy. FDG PET-CT is required for staging. R-CHOP is standard treatment in DLBCL. Local radiotherapy or single-agent rituximab may suffice in MALT lymphoma. Surgery is generally not needed — chemotherapy and/or radiotherapy is adequate. Compressive symptoms (airway obstruction) may require emergent steroids + chemotherapy.
Thyroid lymphoma treatment is chemotherapy + radiotherapy (CHOP regimen), not surgery. MALT lymphoma may be treated with radiotherapy alone. A rapidly growing mass in patients with Hashimoto thyroiditis should raise suspicion for lymphoma. Diagnosis is made by core biopsy or surgical biopsy.