Between these two thyroid malignancies, PTC is the most common type, characterized by low malignancy, high incidence rate, good prognosis and above 90% 10-year survival rate. Although PTL is rare, the treatment is mainly chemotherapy and radiotherapy, and the effect of surgery is limited [17, 18]. The 5-year survival rate of stage-IE thyroid lymphoma is as high as 75–89%, while the annual survival rate of stage-IIE thyroid lymphoma is reduced to 25–40% [2, 19]. The prognosis of thyroid lymphoma depends on the stage, so early diagnosis is crucial. PTL has the features of older onset age, markedly hypoechoic, internal linear echogenic strands, posterior echo enhancement, lack of calcification, rich vascularity, low CT density and homogeneous mild to moderate enhancement, while PTC retains the features of hypoechoic, irregular border, calcification, easy capsular invasion and heterogeneous significant enhancement. Ultrasound combined with CT is conducive to the diagnosis of early PTL and papillary carcinoma.
This study retrospectively reviewed the clinical and imaging features of 50 PTL patients and 100 PTC patients, and found that there was no significant difference in gender ratio between patients with PTL and PTC, both occurring commonly in the female. Comparatively, PTL (aged 63.06 ± 10.14 years) was greater than PTC (aged 41.63 ± 15.06 years) in the age of onset (63.06 ± 10.14 years old) with a statistical significance (P < 0.001), which is similar to the prior reports by Acar et al.  and Ota et al., and was also higher than PTC in the incidence with asymmetric enlargement and Hashimoto's thyroiditis. Studies have shown that most PTL originate in the setting of HT, with which the patients carry a 40–80 folds higher risk of thyroid lymphoma than the healthy population, and HT is the recognized risk factor for PTL[5, 21]. Of the PTL group (78.0%, 39/50) in this study, their pathology confirmed the evidence of HT, which is consistent with previous results reporting 30–90% PTL patients with HT.However, according to the study of Mukasa, the incidence rate of PTC was higher than that of PTL in the patients with HT and Graves, which is inconsistent with the present result and suggested to be the influence of sample size and selection bias. Based on the significant correlation between HT and thyroid malignancy, some scholars[23, 24] proposed that the former was a precancerous lesion, which might be controversial and is expected to conduct in-depth investigation into monitoring the HT development so as to early detect, diagnose and treat PTL and PTC.
In the current study, the nodule size of PTL and PTC group was correspondingly 44.70 (± 15.50) mm and 31.86 (± 9.40) mm, and PTL lesions were obviously larger, with an average size of about 45mm, which is similar to the reports of GU et al. .Besides, this study also found that the most common echo type of PTL and PTC lesions was the markedly hypoechoic and the hypoechoic respectively, and that the proportion of markedly hypoechoic was significantly higher in PTL than in PTC (78% vs. 4.3%). Xia et al.  argued that the markedly hypoechoic type in PTL as one of its typical features, might be attributed to the high consistency of its tumor cell morphology and size. The present study demonstrated that the proportion of internal linear echogenic strands was higher in PTL than in PTC, while previous studies  showed that this echo’s hypoechoic structure might result from the degree of fibrous tissue proliferation. Additionally, in our study most nodules’ posterior echo enhancement was significantly higher in PTL than in PTC (84%, 42/50). Logistic regression analysis indicated that this enhancement was an independent predictor of PTL, which is consistent with the result of Ota et al., who concluded that posterior echo enhancement was a sonographic characteristic reliably distinguishing PTL from other diseases, and might be related to the large number of lymphocyte infiltration in PTL. Their subjects’ pathology revealed that lymphoma cells grew densely and homogeneously with same shape and size, which produced very small acoustic impedance difference and attenuation and finally posterior echo enhancement. Generally, posterior echo enhancement is prone to occurring in cystic nodules or benign solid tumors but rarely in the type of malignant tumors. However, once markedly hypoechoic nodule is found in the thyroid, the posterior echo will be significantly enhanced, a highly visible symptom of PTL.
With respect to calcification, the rate of PTL group in this study was only 16% (8/50), significantly lower than that of PTC group (78.0%, 78/100). Logistic regression analysis showed that lack of calcification was an independent predictor of PTL, which is consistent with the result of the research by Gu et al. that lack of calcification was a characteristic manifestation of PTL. In contrast, no subject with calcification in PTL was found in their study, but 8 in the present research, which might be influenced by the small sample size of their study and their situation of most PTL complicated with HT.
In terms of CT features, nodules of PTL and PTC mostly showed a low density during the non-contrast phase, the average CT value of PTL was lower than that of PTC, and the density was more homogeneous. However, after enhanced scanning, PTL tended to have a mild to moderate homogeneous enhancement, while PTC mostly showed a heterogeneous significant enhancement, with a statistically significant difference. The results show that the CT features of PTL in this study are consistency with those offered in previous literature, while these features of PTC are somewhat different from those obtained in the previous findings, such as by Kim et al., who argued that the CT of PTC mostly performed with homogeneous low density, and showed a significant homogeneous enhancement after enhanced scanning, which might be related to the larger PTC nodules we chose.
Several limitations are identified in this study: (1) as a retrospective study, relatively long in the sampling period and subjective in the evaluation of sonographic and CT features; (2) small sample size and sample selection bias. Besides, the study also indicates that further improvements include collection of larger samples in the later stage, combination of new technologies in sonographic elasticity, radiography, iconography, etc.