Alveolar Soft Part Sarcoma: Unusual Etiology of Mediastinal Mass in an Adolescent
A previously healthy 13-year-old girl presented with a five-day history of headache that was unresponsive to non-steroidal anti-inflammatory drugs. Other than a new mediastinal bruit, her physical examination was unremarkable. Chest radiograph revealed a large left mediastinal mass.\nOn contrast computed tomography (CT) (), the 9.6x8.6x8.6 cm anterior mediastinal mass appeared hypervascular, with large vessels, dense peripheral enhancement, and central non-enhancement, consistent with necrosis. No fat or calcifications were seen in the mass, and no vascular invasion, osseous destruction, or adenopathy was evident. Numerous well-defined pulmonary nodules suggested metastatic disease.\nContrast-enhanced head CT also revealed an avidly and homogeneously enhancing left parietal mass, measuring 3.3x3.0x 3.7 cm, with large feeding arteries from the left middle cerebral artery. Circumferential vasogenic edema and mass effect on the left lateral ventricle were present. Brain magnetic resonance imaging (MRI) showed the well-circumscribed left parietal mass to be minimally T1 hyperintense, T2 hyperintense, and homogeneously enhancing, without necrosis, hemorrhage, or diffusion restriction. Large vessels were present both in the mass and radiating from its lateral margin. On MRI perfusion imaging, the mass was hypervascular with increased peripheral blood volume and blood flow. Single voxel MR spectroscopy revealed a highly elevated choline-to-creatine ratio, no lactate peak, and an undetectable N-acetyl aspartate peak, consistent with a non-neuronal, nonnecrotic neoplasm with high cell turnover.\nThe patient underwent wedge biopsies of middle and lower lobe pulmonary nodules. Histopathology was diagnostic of ASPS (), with a pseudoalveolar architectural pattern, Periodic acid-Schiff (PAS) positivity, and uniformly positive anti-transcription factor E3 (TFE3) immunohistochemical staining. Due to presence of metastases and the projected difficulty of complete resection, the mediastinal mass was not excised. However, palliative left parietal metastasectomy was performed, again showing histopathology consistent with ASPS.\nThe patient was referred to the National Cancer Institute for a phase 2 trial of Cediranib. During pre-study evaluation, right thigh MRI revealed an additional 6.7x4.5x2.8 cm soft tissue mass with T1 and T2 hyperintensity, internal flow voids, large peri-tumoral vessels, and avid enhancement (). It was centered in the vastus intermedius muscle, abutting right femur. Normal signal intensity was maintained in the right femur, and no right femoral abnormality was seen on technetium-99m bone scan. Bone scan did reveal abnormal radionuclide uptake in right ischium and proximal right tibial metaphysis, potentially representing bone metastases.\nTen months after presentation, the patient has progressive disease. Due to increasing pain, she has undergone palliative resection of her mediastinal and right lower extremity masses. In addition, a new right frontal lobe brain metastasis has been excised, and two smaller brain metastases have been treated with stereotactic radiosurgery.