Discussion :
Invasive [Malignant] Thymoma
-Malignancy defined according to extent of invasion into adjacent mediastinal fat + fascia!
Frequency:in 30-35% of thymomas

--StageI:intact capsule
--StageII:pericapsular growth into mediastinal fat
--StageIII:invasion of surrounding organs such as lung, pericardium, SVC, aorta
--StageIVa:dissemination within thoracic cavity (metastases to pleura + lung in 6%)
--StageIVb:distant metastases (liver, bone, lymph nodes, kidneys, brain)

-heterogeneous attenuation
-spread by contiguity along pleural reflections, extension along aorta reaching posterior mediastinum / crus of diaphragm / retroperitoneum (transdiaphragmatic tumor extension)
-irregular interface with lung
-unilateral diffuse nodular pleural thickening / pleural masses encasing lung circumferentially
-vascular encroachment
-pleural effusion UNCOMMON

DDx:
malignant mesothelioma, lymphoma, thymic carcinoma / malignant germ cell tumor (older male, no diffuse pleural seeding), peripheral lung carcinoma (no dominant mediastinal mass), metastatic disease (not unilateral) Rx:radical excision ± adjuvant radiation therapy

Prognosis:
5-year survival of
93% for stage I,
86% for stage II,
70% for stage III,
50% for stage IV;
2-12% rate of recurrence for resected encapsulated thymomas