CT findings in primary thymic carcinoma

Computed tomography of five cases of thymic carcinoma was reviewed. The subtypes of thymic carcinoma were squamous (two cases), lymphoepithelioma-like (one case), undifferentiated (one case), and basaloid (one case). The patients ranged from 52 to 72 years of age. Computed tomography showed anterior mediastinal tumors infiltrating along the pleura or mediastinum with frequent internal necrosis or calcifications. The three nonsquamous cell cases developed extrathymic metastases to the mediastinal lymph nodes, lung, or chest wall. Although the CT features of thymic carcinomas were similar to those of invasive thymoma, the former showed more aggressive behavior. Among the four variants of thymic carcinoma, squamous carcinomas were less aggressive and had less tendency to develop necrosis than the others.     

Mediastinal computed tomography in myasthenia gravis

Thirty-six of 119 patients with myasthenia gravis examined by computed tomography for possible thymic mass had a thymectomy. The computed tomographic findings were compared to the findings of chest x-ray and linear tomography and then correlated with the surgical histopathologic results. Computed tomography detected all cases of thymoma or thymic enlargement due to hyperplasia. Chest x-ray correctly demonstrated 50% of thymomas present while linear tomography demonstrated 88%. Computed tomography was not able to distinguish between enlargement due to thymoma or hyperplasia unless calcification was present. The role of computed tomography is detailed and discussed in relationship to the other diagnostic modalities.     

Does CT of thymic epithelial tumors enable us to differentiate histologic subtypes and predict prognosis?

OBJECTIVE: The aims of our study were to describe the CT findings of thymic epithelial tumors and to correlate these findings with the histopathologic subtypes and prognosis. MATERIALS AND METHODS: The CT findings of thymic epithelial tumors were analyzed in 91 patients who had undergone surgery between May 1995 and June 2002. Two observers, who were unaware of the histopathologic classification made in accordance with World Health Organization (WHO) recommendations and the prognosis of the tumors, retrospectively reviewed the initial CT findings in terms of the contours and shapes of the tumors and the presence of necrosis, calcification, mediastinal fat or great vessel invasion, pleural seeding, contrast enhancement, and lymph node enlargement. These findings were compared with the simplified subgroups of WHO histologic classification (low-risk thymomas [types A, AB, and B1], high-risk thymomas [types B2 and B3], and thymic carcinomas [type C]) and with postoperative recurrence. RESULTS: The study found 31 low-risk thymomas (eight type A, 16 type AB, and seven type B1 tumors), 45 high-risk thymomas (25 type B2 and 20 type B3), and 15 thymic carcinomas (type C). Lobulated contour was more often seen in high-risk thymomas (26/45, 58%; p = 0.0456) and thymic carcinomas (10/15, 67%; p = 0.033) than in low-risk thymomas (9/31, 29%). Mediastinal fat invasion was more often seen in thymic carcinomas (5/15, 33%; p = 0.0133) than in low-risk thymomas (1/31, 3%). Great vessel invasion was seen only in thymic carcinomas (2/15, 13%; p = 0.0244). Tumors with a lobulated or irregular contour, an oval shape, mediastinal fat or great vessel invasion, and pleural seeding showed significantly more frequent recurrence and metastasis (all, p < 0.05). CONCLUSION: Although CT is of limited value in differentiating histologic subtypes according to the WHO classification, CT findings may serve as predictors of postoperative recurrence or metastasis for the thymic epithelial tumors.     

MSCT对胸腺上皮肿瘤WHO简化病理分型的鉴别诊断价值

目的探讨多层螺旋CT(MSCT)对低危型、高危型胸腺瘤及胸腺癌的鉴别诊断价值。方法回顾性分析经病理(穿刺或手术)证实的67例胸腺上皮肿瘤,基于WHO病理分型简化为低危型胸腺瘤、高危型胸腺瘤及胸腺癌三组,并对其CT征象进行统计分析。结果67例TETs中,低危型胸腺瘤30例(A型3例、AB型20例、B1型7例),高危型胸腺瘤22例(B2型11例、B3型11例),胸腺癌15例。高危型胸腺瘤、胸腺癌较低危型胸腺瘤易表现为边缘分叶或不规则(P均<0.05)及易出现对心包侵犯(P均<0.05);胸腺癌较低危型胸腺瘤易出现增强后密度不均、囊变坏死及胸膜转移(P均≤0.003);胸腺癌较低危型、高危型胸腺瘤易出现纵隔淋巴结肿大(P均≤0.002);纵隔大血管侵犯在三组间两两比较均有统计学差异(P均<0.05)。结论MSCT对TETs的WHO简化病理分型鉴别诊断具有重要价值。     

恶性胸腺瘤的CT与MRI诊断

目的：分析侵袭性胸腺瘤及胸腺癌的CT及MRI表现，探讨其鉴别诊断要点。方法：复习经病理组织学证实的18例侵袭性胸腺瘤及9例胸腺癌的CT、MRI所见，重点观察肿瘤的大小、密度、边缘改变，对周围纵隔结构的侵犯。结果：18例侵袭性胸腺瘤、9例胸腺癌CT、MRI均表现为前纵隔不规则软组织肿块。前者肿瘤直径大于8cm者15例（83％），病灶内密度及信号不均匀12例。心血管受侵16例，后者肿块直径在8cm以上者7例（77.8％),肿块内密度及信号不均6例，心知管受累8例，纵隔肺门淋巴结转移1例，肺内转移3例，双侧肾上腺及肝脏转移1例。结论：虽然侵袭性胸腺瘤和胸腺癌的影像学表现相似，但胸腺癌更具有侵袭性，胸外转移常见，病理组织学检查有助于两者的鉴别。     

18F-FDG PET/CT of thymic epithelial tumors: usefulness for distinguishing and staging tumor subgroups.

The purpose of our study was to assess the usefulness of integrated PET/CT using 18F-FDG for distinguishing thymic epithelial tumors according to the World Health Organization (WHO) classification. METHODS: Thirty-three patients (age range, 34-68 y; mean age, 54.6 y) with thymic epithelial tumors, who underwent both integrated PET/CT and enhanced CT, were included. The clinicopathologic stages, maximum standardized uptake values (SUVs), and uptake patterns of tumors on integrated PET/CT images, and various enhanced CT findings, are described according to the simplified (low-risk [types A, AB, and B1] and high-risk [types B2 and B3] thymomas and thymic carcinomas) subgroups of the WHO classification. Discriminant analysis was performed to determine the relative capabilities of integrated PET/CT and enhanced CT findings to differentiate tumor subgroups. RESULTS: Tumors included 8 low-risk thymomas, 9 high-risk thymomas, and 16 thymic carcinomas. The maximum SUVs of high-risk thymomas (P < 0.001) and low-risk thymomas (P < 0.001) were found to be significantly lower than those of thymic carcinomas. Homogeneous 18F-FDG uptake within tumors was more frequently seen in thymic carcinomas than in high-risk thymomas (P = 0.027) or low-risk thymomas (P = 0.001). The uptake pattern (homogeneous vs. heterogeneous) on integrated PET/CT images and the presence of mediastinal fat invasion on enhanced CT images were found to be useful for differentiating tumor subgroups. In addition, integrated PET/CT helped detect lymph node metastases, which were not identified on enhanced CT in 2 patients. CONCLUSION: Integrated PET/CT was found to be useful for differentiating subgroups of thymic epithelial tumors and for staging the extent of the disease.     

Thymic epithelial tumors: comparison of CT and MR imaging findings of low-risk thymomas, high-risk thymomas, and thymic carcinomas

OBJECTIVE: To assess the CT and magnetic resonance (MR) imaging findings of thymic epithelial tumors classified according to the current World Health Organization (WHO) histologic classification and to determine useful findings in differentiating the main subtypes. MATERIALS AND METHODS: Sixty patients with thymic epithelial tumor who underwent both CT and MR imaging were reviewed retrospectively. All cases were classified according to the 2004 WHO classification. The following findings were assessed in each case on both CT and MRI: size of tumor, contour, perimeter of capsule; homogeneity, presence of septum, hemorrhage, necrotic or cystic component within tumor; presence of mediastinal lymphadenopathy, pleural effusion, and great vessel invasion. These imaging characteristics of 30 low-risk thymomas (4 type A, 12 type AB, and 14 type B1), 18 high-risk thymomas (11 type B2 and seven type B3), and 12 thymic carcinomas on CT and MR imaging were compared using the chi-square test. Comparison between CT and MR findings was performed by using McNemar test. RESULTS: On both CT and MR imaging, thymic carcinomas were more likely to have irregular contours (P < .001), necrotic or cystic component (P < .05), heterogeneous contrast-enhancement (P < .05), lymphadenopathy (P < .0001), and great vessel invasion (P < .001) than low-risk and high-risk thymomas. On MR imaging, the findings of almost complete capsule, septum, and homogenous enhancement were more commonly seen in low-risk thymomas than high-risk thymomas and thymic carcinomas (P < .05). MR imaging was superior to CT in the depiction of capsule, septum, or hemorrhage within tumor (all comparison, P < .05). CONCLUSION: The presence of irregular contour, necrotic or cystic component, heterogeneous enhancement, lymphadenopathy, and great vessel invasion on CT or MR imaging are strongly suggestive of thymic carcinomas. On MR imaging, the findings of contour, capsule, septum, and homogenous enhancement are helpful in distinguishing low-risk thymomas from high-risk thymomas and thymic carcinomas.     

胸腺瘤的CT诊断

目的评估CT诊断良恶性胸腺瘤的价值.材料和方法分析31例经外科和病理证实的胸腺瘤的CT表现.结果良性胸腺瘤10例,除1例与左肺动脉局部有粘连外,其余边界均清晰.侵袭性胸腺瘤12例,胸腺癌9例,除1例鳞癌外,CT均为不规则肿块,血管间隙均受侵,前者心包受侵8例,胸膜受侵7例,后者心包受侵5例,胸膜受侵5例,肺转移4例,双肾上腺转移1例,胸骨破坏1例.结论CT能准确鉴别良性恶性胸腺瘤,显示恶性胸腺瘤的浸润范围,并对侵袭性胸腺瘤和胸腺癌进行鉴别.     

Invasive and noninvasive thymoma: distinctive CT features

PURPOSE: The purpose of this work was to evaluate the CT features of thymoma and to determine the most helpful findings in differentiating invasive from noninvasive thymoma. METHOD: The CT scans from 27 patients with invasive thymoma and 23 with noninvasive thymoma were independently assessed by two observers without knowledge of their invasiveness. The presence and distribution of various CT findings were independently analyzed. RESULTS: Invasive thymomas were more likely to have lobulated (16/27, 59%) or irregular (6/27, 22%) contours than noninvasive thymomas (8/23, 35% and 1.5/23, 6%, respectively) (p < 0.05). Invasive thymomas had a higher prevalence of low attenuation areas within the tumor (16/27, 60%) than noninvasive thymomas (5/23, 22%) (p < 0.001) as well as foci of calcification (14.5/27, 54% vs. 6/23, 26%; p < 0.01). CONCLUSION: The presence of lobulated or irregular contour, areas of low attenuation, and multifocal calcification is suggestive of invasive thymoma.     

Pleomorphic carcinoma of lung: comparison of CT features and pathologic findings

PURPOSE: To retrospectively evaluate computed tomographic (CT) features of pleomorphic carcinoma of the lung and to compare these features with pathologic findings. MATERIALS AND METHODS: Ten patients (10 men, three women; mean age at diagnosis, 64.1 years; range, 43-75 years) with pleomorphic carcinoma treated from June 2000 to January 2003 were selected from two institutions. Two radiologists retrospectively reviewed CT features, which included size and location of tumor, presence of calcification, attenuation values and internal architecture of the mass, and invasion of pleura and chest wall. Attenuation values of the mass on CT scans were compared with pathologic findings in tumors in available gross specimens. Follow-up CT scans were not routinely obtained except in two patients with progressive pleural effusion and rapid growth of the tumor as seen on serial chest radiographs. RESULTS: On unenhanced CT scans, attenuation of the tumor was similar to that of the surrounding muscle. Calcification within the tumor was visible in one patient. Invasion of chest wall was noted in two patients. Seven patients had pleural invasion. Tumors were located at the lung periphery in nine patients. On contrast material-enhanced CT scans, lesions with the longest diameter larger than 5 cm showed central low-attenuation areas with substantial enhancement in the tumor periphery; in comparison, lesions with the longest diameter smaller than 5 cm showed homogeneous enhancement. Size of two lesions with the longest diameter larger than 5 cm increased rapidly after a follow-up of shorter than 3 weeks. Low-attenuation areas on contrast-enhanced CT scans were found to correspond to areas of myxoid degeneration, necrosis, or hemorrhage in pathologic specimens. CONCLUSION: Findings of this study suggest that pleomorphic carcinomas of the lung preferentially manifest as large peripheral lung neoplasms with a central low-attenuation area and frequently invade the pleura and chest wall.     

不典型胸腺瘤的影像诊断与病理对照

目的 探讨不典型胸腺瘤的影像表现.方法 16例不典型胸腺瘤均经病理证实,分析其影像学(包括X线、CT及MRI)表现,并与病理结果对照.结果 不典型胸腺瘤均为前纵隔软组织密度肿块,分叶状12例,钙化9例.肿瘤邻近脂肪间隙消失13例,直接侵犯纵隔胸膜及前胸壁7例,包绕大血管7例,有3例肿瘤边缘光滑,周围脂肪组织清晰,甚似良性胸腺瘤.8例可见位于同侧的胸膜种植结节,胸腔积液6例,心包积液3例.结论 不典型胸腺瘤具有一定的影像学特征.     

胸腺瘤的CT和平片分析

目的　探讨胸腺瘤的X线与CT诊断价值。方法　分析 30例经手术病理证实的胸腺瘤X线和CT表现 ,并与病理对照。结果　 18例非侵袭性胸腺瘤 ,胸片仅发现 15例 ,CT扫描 10例 ,均为前纵隔软组织肿块 ,边界清楚 ,密度均匀。 12例侵袭性胸腺瘤 ,肿瘤侵犯邻近器官 ,CT显示纵隔胸膜受侵 3例 ,心包受侵 2例 ,大血管结构受侵 2例 ,胸膜种植 3例 ,胸腔积液 2例 ,腹腔受侵和纵隔淋巴结转移各 1例 ;胸片仅能显示 1例肿瘤侵犯纵隔胸膜和 1例胸膜种植。结论　CT扫描在胸腺瘤的发现和判别侵袭性方面明显优于X线平片并且是可靠预测胸腺瘤侵袭性的检查方法。     

Computed tomography and magnetic resonance imaging of mediastinal tumors

We describe the clinical, pathological, and imaging findings of mediastinal tumors with focus on thymic hyperplasia, thymic epithelial tumors, and germ cell tumors, malignant lymphoma, and various cystic masses. Chemical shift magnetic resonance imaging (MRI) is useful in characterization of the normal thymus and differentiation of hyperplastic thymus and thymic tumors. In contrast to noninvasive thymomas, invasive thymomas and thymic carcinomas show a more aggressive growth pattern. Local invasion and pleural spread are characteristic of invasive thymoma and mediastinal lymphadenopathy and distant metastasis suggest thymic carcinomas. Mature teratoma typically shows various computed tomography (CT) attenuation, and MR signal intensity depending on its contents and fat tissue and bone within the lesions are its characteristic findings. Seminomas typically have homogenous internal CT attenuation and MR signal intensity with minimal contrast enhancement. Nonseminomatous malignant germ cell tumors characteristically show prominent internal degenerative changes and invasion to the adjacent structures. In mediastinal lymphomas, a residual mass is common after treatment and MRI provides important information in distinguishing viable tumors from residual benign masses. Some mediastinal cysts may reveal high attenuation similar to solid lesions on CT depending on their contents and MRI can be useful in the differentiation of cystic masses from solid lesions.     

侵袭性胸腺瘤的CT表现及病理对照

目的：分析侵袭性胸腺瘤的CT表现，评价CT对其临床分期的准确性。资料与方法：回顾分析32例经手术病理证实的侵袭性胸腺瘤的CT所见，通过对病变的密度、周围纵隔结构的侵犯、胸膜肿植以及纵隔外转移情况的观察进行分期，并和病理分期进行比较。结果：侵袭性胸腺瘤的CT表现主要有：分叶状或不规则肿块；密度不均匀；局灶性钙化；邻近结构的脂肪层消失；侵及心包、胸膜、肺及其他器官。CT对临床分期判断准确性为87．5％，特异性为90％，敏感性为83．3％。结论：CT对侵袭性胸腺瘤术前分期及病变范围的显示准确，对确定手术方案，评价治疗效果及判断复发具有重要意义。     

MR imaging of thymoma--comparison with CT, operative, and pathological findings]

Twenty-six patients with thymoma, who had magnetic resonance (MR) imaging and computed tomography (CT) before surgery, were studied. Twenty-six thymomas were classified into 11 non-invasive thymomas (Masaoka's clinical stage I) and 15 invasive thymomas (stage II, III, and IV). On MR imaging compared with histological findings, low signal intensity rim of the tumor was corresponded to fibrous capsule of the tumor, and linear and/or reticular low signal intensity lines in the tumor were corresponded to the fibrous septae dividing thymoma into lobules. The detectability of these findings by MR imaging was superior to that by CT. Margin of the tumor was smooth in non-invasive thymoma rather than invasive thymoma. The diagnostic accuracy of invasion to vessel, and pleura or lung on MR imaging and CT was compared with operative and histological findings. MR imaging was same as CT in its ability to detect tumoral invasion to vessel, and slightly superior to pleura or lung. In conclusion MR images clearly show the findings corresponding to pathologic specimens, and MR imaging combining with CT is useful to differentiate non-invasive thymoma to invasive thymoma.     

Mediastinal imaging in myasthenia gravis: correlation of chest radiography, CT, MR, and surgical findings

Chest radiographs and CT and MR images of the mediastinum were studied in 16 patients with myasthenia gravis who underwent thymectomy (two with a final diagnosis of thymoma, seven with hyperplasia, and seven with a normal thymus). The anterior mediastinum was analyzed on imaging studies for thymic morphology and the presence of mass lesions, and the findings were then correlated with the results of surgical resection and pathologic examination. The chest radiographs detected an anterior mediastinal mass in two patients consistent with thymoma on subsequent CT and MR examinations. Chest radiographs in the other 14 patients were normal. In seven patients with a final diagnosis of thymic hyperplasia, both CT and MR demonstrated normal thymic morphology in five, an enlarged thymus in one, and a small thymus in one that was easily identified on CT but was difficult to define on MR. In the other seven patients with a normal thymus on pathologic examination, both CT and MR showed an involuted thymus in four, a normal thymus in two, and an enlarged thymus in one. While both CT and MR were superior to chest radiography for studying the thymus, CT provided better spatial resolution and thymic definition in a much shorter scanning time than MR did. This study suggests that CT should remain the procedure of choice when further imaging of the thymus is needed after the initial chest radiographs in patients with myasthenia gravis.     

^18F-FDG符合线路显像在胸腺上皮肿瘤诊断中的应用价值

目的探讨^18F-脱氧葡萄糖（FDG）符合线路显像在胸腺上皮肿瘤（TET）诊断中的价值。方法回顾性分析37例TET患者的^18F—FDG符合线路显像结果,图像分析采用视觉分析及半定量方法[肿瘤／正常肺组织放射性比值（TLR）]。按WHO TET病理学分类结果将患者分成3组（高生存率组,包括A、AB、B1型;中生存率组,包括B2、B3型;低生存率组,包括胸腺癌）;同时将^18F—FDG显像结果与增强CT影像进行对比,TLR与免疫组织化学检查所测Ki67标记指数（细胞增殖指标）进行相关性分析。组间TLR比较采用方差分析。结果（1）^18F—FDG符合线路显像阳性率91．9％（34／37）;（2）3个不同生存率组TLR间差异有统计学意义（高、中、低生存率组TLR依次为：1．42±0．27,2．13±0．74,3．00±1．19,F=9．99,P〈0．05）;（3）TLR与Ki67标记指数有明显相关性（r=0．613,P=0．002）;（4）^18F-FDG显像发现2例增强CT未发现的病灶,病灶分别位于前胸壁和右锁骨上淋巴结;（5）共有4例伴发重症肌无力。结论^18F—FDG符合线路显像有助于WHO TET病理学分类和发现更多的TET侵犯、转移灶;可以用^18F—FDG在TET的浓聚程度反映细胞的增殖活力。     

Large cell neuroendocrine carcinoma of the lung: radiologic and pathologic findings

PURPOSE: The purpose of this work was to determine the radiologic and pathologic findings of large cell neuroendocrine carcinoma (LCNEC). METHOD: We retrospectively evaluated chest radiographs, CT scans, and pathologic findings of five patients with pathologically confirmed LCNEC. They were confirmed by percutaneous needle biopsy (n = 2) and by surgery (n = 3). The average age of patients was 60 (51-70) years, and all five were smokers (mean 30 pack-years) and men. Radiologic findings were reviewed for the pattern of lesion, location, and associated findings by two radiologists under consensus. Pathologic findings were reviewed by two pathologists. RESULTS: In all five patients, tumors were represented as a peripherally located nodule or mass without associated secondary pneumonitis or distal atelectasis radiographically. On CT scan, masses were oval or round and well demarcated with lobulated margin in all cases, their sizes ranged from 2 to 5 cm, and they did not show internal calcification and necrosis. On contrast-enhanced CT, three cases showed moderate enhancement more than the chest wall muscle. Lymphadenopathy was observed in ipsilateral hilar and mediastinal areas in three cases. Distant metastasis to liver was noted in one case. One case of LCNEC was Stage IV, two were Stage IIIa, and two were Stage Ia at the time of diagnosis. CONCLUSION: Although the epidemiology of LCNEC is more similar to that of small cell carcinoma than atypical carcinoids, in its strong association with smoking, rapid progression, and poor prognosis, our five cases of LCNEC show peripherally located pulmonary nodule or mass with or without regional lymphadenopathy, which are findings similar to those of atypical carcinoids rather than small cell carcinoma.     

不典型胸腺瘤和胸腺癌的临床与CT表现比较

目的 探讨不典型胸腺瘤(B3型胸腺瘤)和胸腺癌(C型胸腺瘤)在临床和CT表现上的异同.方法 搜集经手术切除、按2004年WHO胸腺上皮肿瘤分类标准分类,并有完整CT资料的B3型(30例)和C型(17例)胸腺上皮肿瘤,对其包括预后在内的临床资料和CT图像做回顾性分析.2组患者年龄和肿瘤横断面最长径的比较采用独立样本t检验;对于性别、有无重症肌无力、手术方法及2组间肿瘤边缘、形态、有无坏死和钙化、肿瘤的强化形式和程度以及对周围纵隔脂肪、胸壁、大血管、纵隔胸膜、心包和胸壁的侵犯情况,是否存在纵隔淋巴结肿大、胸膜种植转移、胸水和远处转移等采用Fisher精确概率法检验.对于2组患者预后情况则采用Kaplan-Meier法进行生存分析.结果 (1)临床:B3型患者[平均年龄(43±15)岁]较C型[(56±11)岁]年轻(t=-2.905,P=0.006);90.0％ (27/30) B3型合并有重症肌无力症状,而C型仅5.9％( 1/17),差异有统计学意义(P=0.000);B3型肿瘤的完整切除率(80.0％,24/30)高于C型(11.8％,2/17,P=0.001);70.6％( 12/17)C型患者于术后2年内死亡,B3型中仅20.0％ (5/30)在术后2～9年内死亡;5年生存率C型为14.7％,B3型为94.7％,差异有统计学意义(P =0.000).(2)CT表现:C型肿块的长径为(6.4±2.1) cm,大于B3型的(4.6±2.3)cm(t=-2.778,P=0.008);C型边缘多为不规则形状( 13/17,76.5％),而B3型则多为分叶状(18/30,60.0％,P=0.019);C型(15/17例,88.2％)较B3型(17/30,56.7％)更容易发生肿瘤坏死(P =0.049);在增强扫描中与B3型比较,92.8％ (13/14)C型表现为不均匀强化(P=0.042);同时C型更易发生大血管侵犯(15/17,P=0.001)和远处转移(8/17,P=0.028).结论 B3型和C型胸腺瘤在临床、预后及CT表现有一定差异,有助于作出区别.     

Use of fluorine-18 fluorodeoxyglucose positron emission tomography in the detection of thymoma: A preliminary report

Thymomas are lacking in malignant cytological features. Their staging is defined by the invasiveness of the tumour. This study aimed to analyse the uptake patterns of fluorine-18 fluorodeoxyglucose (FDG) in thymomas of different stages. FDG positron emission tomography (PET) scan was performed in 12 patients suspected of having thymoma and in nine controls. Qualitative visual interpretation was used to detect the foci with FDG uptake higher than that of normal mediastinum. Tumour/lung ratio (TLR) was calculated from the counts of ROIs over the mass and over comparable normal lung tissue in thymoma patients. Mediastinum/lung ratio (MLR) was calculated from the counts of ROIs over the anterior mediastinum and lung in controls. The PET scan patterns of distribution of foci with FDG uptake and TLRs were correlated with the computed tomography (CT) or magnetic resonance imaging (MRI) findings, and staging of the thymomas. Thymectomy was performed in ten patients and thoracoscopy was done in two patients. The results revealed ten thymomas (two stage I tumours, two stage II, four stage III and two stage IV, according to the Masaoka classification), and two cases of thymic hyperplasia associated with myasthenia gravis. Myasthenia gravis was also noted in four thymoma patients. FDG studies showed (a) diffuse uptake in the widened anterior mediastinum in patients with thymic hyperplasia, (b) confined focal FDG uptake in the non-invasive or less invasive, stage I and II thymomas, and (c) multiple discrete foci of FDG uptake in the mediastinum and thoracic structures in stage III and IV advanced invasive thymomas. The thymomas had the highest TLRs, followed by the TLRs of thymic hyperplasia and the MLRs of control subjects (P <0.005). No significant difference was found between thymomas in different stages or between thymomas with and thymomas without myasthenia gravis. In comparison with CT and/or MRI, FDG-PET detected more lesions in patients with invasive thymomas and downgraded the staging of thymoma in four patients. Our preliminary results suggest that FDG-PET is useful in the assessment of the invasiveness of thymomas, and may have the potential to differentiate thymomas from thymic hyperplasia.