Primary Retroperitoneal Tumors

Introduction

Primary retroperitoneal tumors are exceedingly rare. Primary masses in the retroperitoneum can be categorized as one of three entities: lymphomas, extragonadal germ cell tumors, and sarcomas. Although gastrointestinal stromal tumors (GISTs) arise in the intraperitoneal compartment, they can also mimic these retroperitoneal masses because of their large size. This chapter reviews primary retroperitoneal sarcomas (RPSs), which form about one third of all primary retroperitoneal tumors. RPSs lack specific clinical symptoms beyond the effect manifested from their impact on adjacent structures. As a result, they often progress to a large size before initial presentation. Successful management of RPSs requires the collaborative efforts of the radiologist, pathologist, radiation oncologist, medical oncologist, surgical oncologist, and other specialists. Imaging plays a key role in the initial detection, therapeutic planning, and follow-up of these patients.

Epidemiology and Risk Factors

Soft tissue sarcomas (STSs) form 1% of all newly diagnosed malignancies in the United States, and 15% of these arise in the retroperitoneum. STSs in children account for up to 15% of all pediatric malignancies. The mean annual incidence of RPS is 2.7 per million persons, and no significant change has been observed over time.

RPSs arise from the embryonic mesoderm. The vast majority do not have any identifiable predisposing cause. A small proportion of STSs may arise as a result of prior radiation exposure or in association with a genetic syndrome.

Exposure to ionizing radiation increases the incidence of STSs, with a median interval of 10 years (range 2–50 years) after exposure. These occur in the irradiated field, commonly in patients who receive radiation therapy for breast cancer, cervical cancer, lymphoma, or testicular tumors or for benign conditions. Patients treated with prior radiation exposure can develop either STSs or bone sarcomas.

Several genetic syndromes are associated with the development of STS. Neurofibromatosis type 1, also known as von Recklinghausen disease, is an autosomal dominant condition that arises because of a mutation in the NF1 gene. These patients have a high incidence of neurofibromas, as well as approximately a 10% increased risk of developing malignant peripheral nerve sheath tumors during their lifetime. Gardner syndrome, another autosomal dominant disease, is caused by mutation of the APC gene and is associated with multiple colonic polyps, colon cancer, and desmoid tumors. STS has also been reported in patients with Li–Fraumeni syndrome, which is caused by a germline mutation in the P53 tumor suppressor gene. Children with hereditary retinoblastoma owing to a germline mutation in the RB1 tumor suppressor gene face a higher risk of STS and osteosarcoma. The risk is further increased because these patients receive radiotherapy for the initial treatment of retinoblastoma.

Anatomy and Pathology

Anatomy

Macroscopically, parts of the genitourinary tract, gastrointestinal (GI) tract, aorta and its branches, inferior vena cava and its tributaries, retroperitoneal fat, and lymphatic and nervous systems form important components of the retroperitoneum ( Fig. 24.1 ). The pancreas, ascending colon, descending colon, and duodenum are located anteriorly within the anterior pararenal space, and the aorta, inferior vena cava, and lymph nodes are in the midline. Laterally, the kidneys and adrenals are surrounded by renal fascia within the perirenal space. The psoas, quadratus lumborum, paraverterbral muscles, and bony skeleton form the posterior boundary of the retroperitoneum. Retroperitoneal fat, vessels, lymphatics, and nerves continue from the retroperitoneum into the small bowel mesentery, providing anatomic continuity between these compartments ( Fig. 24.2 ). The displacement or contiguous involvement of these major organs, and in particular of the major vascular branches, is of critical importance when planning surgical resection.

Key Points

Anatomy

• Duodenum, ascending and descending colon, and pancreas lie anteriorly in the anterior pararenal space.

• Kidneys with perinephric fat and adrenals surrounded by renal fascia laterally lie within the perirenal space.

• Muscles and bones form the posterior boundary of the retroperitoneum.

• Arterial encasement, venous involvement, and the displacement or invasion of adjacent organs are important considerations in surgical planning.

Clinical Presentation

Primary retroperitoneal tumors are rare. In essence, primary masses in the retroperitoneum can be categ­orized as one of three entities: extragonadal germ cell tumors, lymphoma (particularly non-Hodgkin lymphoma), and RPSs.

RPSs lack specific symptoms or laboratory findings. RPSs usually progress to a very large size before prompting clinical suspicion. Examples of potential presenting signs and symptoms include painless abdominal mass, abdominal distention, back pain or pain referred to the hip, urine retention, hematuria, early satiety, or weight loss, occurring singly or in combination. Neurologic deficits may be a presenting symptom when there is spinal cord, nerve root, or sciatic plexus involvement. Venous compromise can result in lower extremity edema. Although there is a broad age range, the sixth decade is a common time for presentation in adults. There is slight male predominance. Imaging, typically computed tomography (CT), leads to discovery of the tumor. Some 11% of patients with RPS have metastases to the liver or lungs at presentation. Two-thirds of the tumors are high-grade at the time of diagnosis. Therefore, at initial presentation, RPSs are at a higher stage and, consequently, their prognosis is worse than for extremity sarcomas.

The differential diagnosis of retroperitoneal tumors can frequently be narrowed based on initial history, physical examination, and laboratory testing. For example, testicular examination and ultrasonography coupled with standard germ cell serum markers will strongly suggest the possibility of germ cell tumor, whereas elucidating a history of B symptomatology such as night sweats, fever, and weight loss suggests the possibility of a lymphoma. However, image-guided tissue biopsy provides the definitive diagnosis of RPS from these entities and is crucial for treatment planning before surgical resection.

Staging Evaluation

The eighth edition American Joint Committee on Cancer (AJCC) staging system changed significantly for STS and now has a separate staging system for RPS and is based on a combination of anatomic as well as pathologic data ( Fig. 24.3 ). Previous versions of the AJCC staging system included tumor depth, but this is not included in the current edition. The tumor-node-metastasis (TNM) characteristics of the tumor are provided by imaging and may be modified after surgery. The histologic subtype and grade of the primary tumor are determined after biopsy or surgical excision to yield tumor stage as defined by AJCC criteria.