Minimally Invasive Microsurgery of the Skull Base
Rhabdomyosarcomas (RMSs) and other benign and cancerous tumors of the skull base require special expertise combining the talents of skull base surgeons and head and neck surgeons. Newer approaches including endoscopic techniques to access these tumors through small and precise exposures, have made a dramatic impact on the overall outcomes by reducing mortality and morbidity, reducing hospital stays and improving overall prognosis.
Overview
Rhabdomyosarcoma (from Greek, rhabdo, meaning rod shape, and myo, meaning muscle) is the most common soft tissue sarcoma in children. Sarcomas are cancers that develop from connective tissues in the body, such as muscles, fat, membranes that line the joints, or blood vessels; and RMS is a cancer made up of primitive muscle cells, called "rhabdomyoblasts", that develop in the skeletal muscles of the body. The tumor shows differentiation resembling that of skeletal muscles.
Soft tissue sarcomas account for about 7-8% of childhood cancers and RMS is the most common soft tissue sarcoma in children. Since skeletal muscle is located throughout the body, RMS can appear at numerous locations in the body. However, the most common site in which it occurs is the head and neck (approximately 35-40%), particularly around the eyes, the genitourinary tract (20%), the extremities (15-20%), and the trunk (chest and lungs) (10-15%).
RMSs account for 5-8% of childhood cancers and 70% of all rhabdomyosarcoma cases are diagnosed in the first ten years of life. It usually affects children between the ages of 2 to 6 and 15 to 19. The peak incidence is in the 1-5 age group and approximately 87% of patients are younger than 15 years, and 13% of patients are aged 15-21 years. In the US, the incidence of RMS is 6 cases per 1,000,000 per year in children younger than 15 years. RMS rarely affects adults, no obvious race predilection exists and overall, RMS occurs slightly more frequently in males than females, the male-to-female ratio is 1.2-1.4:1. Differences exist according to the site of primary disease.
Depending on the "histology" of the cells, there are two main types of RMS that occur in children:
Embryonal type, this is the most common type and it tends to occur in the head and neck area, bladder, vagina, and in or around the prostate and testes. It is called embryonal type because the cells resemble the developing muscle cells. Two variants of the embryonal type, botryoid and spindle cell RMSs, tend to have a better prognosis than the more common forms.
Alveolar type, this type occurs more often in large muscles of the trunk, arms, and legs and typically affects older children or teenagers. This type is called alveolar because the malignant cells form little hollow spaces, or alveoli. This type arises more commonly in the extremities.
Metastases can occur in RMS and are found predominantly in the lungs, bone marrow, bones, lymph nodes, breasts, and brain.
Causes
The cause of RMS is unknown; however, several genetic syndromes and environmental factors are associated with an increased prevalence of RMS. The genetic syndromes include Neurofibromatosis, Li-Fraumeni syndrome, Rubinstein-Taybi syndrome, Gorlin basal cell nevus syndrome, and Beckwith-Wiedemann syndrome. Environmental factors that appear to have an influence on the development of RMS include parental use of marijuana and cocaine, intrauterine radiograph exposure, and prior exposure to alkylating agents.
Symptoms
RMSs usually manifest as an expanding mass; symptoms depend on the tumor's size and location. Pain may be present and if metastatic disease exists, symptoms of bone pain, respiratory difficulty, anemia, thrombocytopenia, and neutropenia may be present. If disseminated rhabdomyoblasts are present in the bone marrow, the disease may mimic leukemia, both in its symptoms and light microscopy findings.
Some typical presentations of RMS by location are: Orbital RMS, causing proptosis or dysconjugate gaze; or parameningeal RMS (ear, mastoid, nasal cavity, paranasal sinuses, infratemporal fossa, pterygopalatine fossa), causing upper respiratory tract symptoms or pain. When RMS occurs in superficial locations, it may be palpable and detected relatively early, but those in deeper locations may grow large before causing symptoms.
Diagnosis
A biopsy best allows an adequate tissue sample for histopathologic diagnosis and further treatment of RMS. Plain radiograph films, computed tomography (CT) scanning and magnetic resonance imaging (MRI) are the different imaging modalities that are used to evaluate RMS. In general, MRI provides better definition of the mass and better evaluation of any invasion of adjacent structures such as the paranasal sinuses and the skull base, whereas CT is superior in showing the bony anatomy and evaluating any possible bony invasion by the tumor. Other methods used as part of a "metastatic work-up" to survey RMS patients include bone scanning to search for any bony metastases, ultrasound to obtain images of the liver in patients with abdominal complaints or to survey for any abdominal metastasis.
Treatment
RMS is treated by a combination of surgery, chemotherapy, and radiation. External beam radiation is usually used to deliver radiotherapy. Chemotherapeutic agents that are most commonly used are vincristine, cyclophosphamide, dactinomycin, adriamycin, and ifosfamide.
Prognosis
In patients with limited disease, overall survival rates have improved with the combined use of surgery, radiation therapy, and chemotherapy. In general, the embryonal type of RMS is considered the most treatable form of the disease. Survival rates depend on the initial tumor size, location, its appearance under the microscope, and whether the disease has spread to other parts of the body.
