Teaching Case

Recent onset of left knee pain and localized swelling over left lumbar paraspinal area.

The patient was seen with 2-month history of worsening left knee pain. This gradually deteriorated over a period of time and for the 2 weeks prior to admission he had had difficulty walking. This was associated with paraesthesia of the lower leg and itching of the knee. In addition he had lost one stone in weight but there were no other systemic symptoms. He had a past history of Kawasaki’s disease at the age of 5, which was treated with immunoglobulins. He was born following a full term normal delivery with subsequent normal growth and development. On examination an absence of knee and ankle reflexes were noted. There was a firm subcutaneous left paraspinal mass approximately covering L3-L5 vertebral bodies. Magnetic Resonance Imaging of the back revealed a mass in the left paraspinal region in the area of the spinal muscles adjacent to the L3-S1 vertebrae as shown in the figures. There was extension through the L4 foramen and extension into the spinal canal. CT scan confirmed this mass along with some bony erosion. Ultrasound revealed this to be a vascular tumour. Biopsy of the lesion concluded that is was a peripheral primitive neuro-ectodermal tumour (PNET).

The peripheral PNET or extra osseous Ewing’s sarcoma belongs to a group of very primitive neuroectodermal, extracranial and extraspinal small round cell malignant tumours. It arises in soft tissues although there may be secondary involvement of bone due to local invasion or metastatic spread. These tumours have obvious neural differentiation as opposed to Ewing’s sarcoma which are characteristically undifferentiated. Both peripheral PNETs and Ewing’s sarcoma share the same cytogenetic anomaly involving a translocation of chromosomes 11 and 22. Peripheral PNETs are more common in males and occur usually in 1st or 2nd decade. These tumours can occur anywhere in the body. The trunk is a well recognised site for these tumours, most notably the chest wall. Origin in the paraspinal muscles is, however, rare. The distinction between primary osseous tumour and primary soft tissue tumour is often difficult. These tumours tend to metastasise to regional lymph nodes, lung and bones. Clinical presentation includes pain, swelling and tenderness of the local area and constitutional symptoms. Symptomatology may be non-specific and may delay diagnosis and treatment. With regard to radiological features, plain films may show soft tissue swelling and also the secondary involvement of the bone as vague lytic areas. Computed tomography and magnetic resonance imaging may identify and delineate the extent of the primary tumour and show secondary involvement of adjacent lymph nodes and organs. Biopsy of the tumour confirms the diagnosis, particularly with respect to immunhistochemistry on which PNETs are positive for CD99and S100. The overall prognosis is poor with a survival rate of around 50% even with a multimodal approach. These tumours are generally treated with treatment appropriate for Ewing’s sarcoma, currently the EURO Ewing protocol. As well as chemotherapy, local therapy including surgery and radiotherapy to the primary tumour is a vital part of management.