At imaging, the thymus appears in a variety of shapes and sizes, even in the same individual. It gradually involutes with age and may acutely shrink during periods of bodily stress. During the recovery period, it grows back to its original size or even larger, a phenomenon known as thymic rebound hyperplasia. These anatomic variations and dynamic changes appear to be the main source of confusion with pathologic conditions. In turn, these misinterpretations may lead to prolongation or alteration of the chemotherapy regimen or to unnecessary radiation therapy, biopsy, or thymectomy. Familiarity with the embryology, anatomy, and dynamic physiology of the thymus is essential to avoid unnecessary imaging or invasive procedures. Radiologists play a major role in differentiating normal thymic variants, ectopic thymic tissue, and nonneoplastic thymic conditions such as rebound hyperplasia from neoplastic conditions. Knowledge of the imaging findings of thymic tumors and their mimics may help radiologists arrive at the correct diagnosis.

Hepatoblastoma is the most common primary liver tumor in children, accounting for just over 1% of pediatric cancers. The etiology is unknown, but it has been associated with Beckwith-Weidemann syndrome, familial adenomatosis polypi, and low birth weight. The primary treatment is surgical resection, however, chemotherapy plays an important role by increasing the number of tumors that are resectable. The prognosis for patients with resectable tumors is fairly good, however, the outcome for those with nonresectable or recurrent disease is poor. Texas-Houston Medical School, 1515 Holcombe Blvd., Houston, Texas 77030, USA. Telephone: 713-745-0157; Fax 713-792-0608; e-mail: cherzog@ mdanderson.org Received June 21, 2000; accepted for publication September 6, 2000. ©AlphaMed Press 1083-7159/2000 INTRODUCTIONHepatoblastoma is the most common malignant tumor of the liver in children. Surgery remains the primary means of curative therapy, but the role of chemotherapy in both the adjuvant and neoadjuvant setting has become increasingly important over the past three decades. New insight has also been gained into the molecular biology of hepatoblastoma.

Twenty patients with osteosarcoma and pathologic fractures were treated with a chemotherapeutic regimen consisting of cis-diamminedichloroplatinum-II (CDP), Adriamycin (ADR) (doxorubicin) and high-dose methotrexate with citrovorum factor "rescue" (MTX-CF). Before the introduction of the regimen, the primary tumor in two patients was treated by immediate amputation and in 13 with preoperative intra-arterial CDP. Among these 13 patients, responses (healing) were observed in 11 (one required the addition of radiation therapy). In three patients, the responses were so dramatic that, at their request, surgery was deferred and treatment exclusively with chemotherapy was instituted. Based on this experience, treatment exclusively with chemotherapy was also administered to an additional five patients who were admitted without pathologic fractures. In the course of such treatment, pathologic fractures also developed; notwithstanding, chemotherapy was maintained and healing also occurred. One of the 20 patients had pulmonary metastases at diagnosis; these were resected after treatment and pathologic examination revealed no evidence of viable tumor. The remaining 19 patients were free of pulmonary metastases but these later developed in seven patients. These data were compared to a historical control series in which 16 of 21 patients with pathologic fractures developed pulmonary metastases. Three of the chemotherapy treated patients died of nonosteosarcoma related causes (leukemia, generalized varicella, and a metabolic complication). Overall, survival was improved in the chemotherapy treated patients as compared to the historical control series: 10 of 20 versus 6 of 21, respectively. Pathologic fractures in osteosarcoma may heal under treatment with chemotherapy, which also has a favorable impact on the eradication of pulmonary metastases and survival.

Mesenchymal hamartoma of chest-wall is a rare benign lesion that has varied histological characteristics and usually occurs during early infancy. We report the histological characteristics of mesenchymal hamartoma found in five patients aged respectively 25 days, 5 months, 8 months, and 4 and 8 years at presentation. Two patients presented with respiratory distress, two with an asymptomatic chest-wall mass, and one with a deformity of the left chest wall. Surgical resection was performed on four patients and a biopsy only in one patient. Overall, the tumors were well delineated, lobulated, tan to reddish in color, and on section showed blood-filled cystic spaces with interspersed small islands of cartilage and fibrous tissue. Histological analyses demonstrated mixtures of bone trabeculae with spindle-cell stroma, chondroblast-like proliferation, mature and immature hyaline cartilage, and aneurysmal bone cyst formation. All patients are alive and well without evidence of disease. Despite the alarming clinical and histologic presentation, mesenchymal hamartoma of the chest wall is a non-neoplastic benign lesion.

The variation over time in the intensity of contrast enhancement in HCC and cholangiocarcinoma differs sufficiently to make this a useful diagnostic criterion. The delayed phase is particularly important because it amplifies this difference.