An understanding of the possible mechanisms by which disease can spread within the abdominal cavity can greatly enhance the radiologist’s ability to identify the epicenter of a disease process and predict the likely path of disease progression. Pathways for disease transmission within the abdominal cavity include hematogenous, transperitoneal, and subperitoneal spread [3]. Transperitoneal dissemination is confined to the peritoneal cavity and includes ascites, abscesses, pneumoperitoneum, and peritoneal tumor deposits. Subperitoneal spread of disease refers to disease spread within the ligaments, mesentery, mesocolon, or under the peritoneal surface of the organs [4]. This interconnecting potential space represents a significant conduit for bidirectional spread of disease within and between the peritoneal and retroperitoneal compartments. Subperitoneal spread may also follow the lymphatic drainage of organs along the blood vessels in the mesentery, mesocolon, and ligaments.

Retroperitoneal and mesenteric lymphadenopathy can be an unexpected imaging finding and represent the first sign of disease. Given the increased frequency with which lymph nodes are identified on cross-sectional imaging and the wide differential diagnosis for lymphadenopathy in these regions, a careful evaluation of lymph node imaging characteristics (size, distribution, CT attenuation coefficient, signal changes on MRI, post-contrast enhancement) should be performed. Correlation with the patient’s clinical history should be made in an effort to rule out a neoplastic condition. Bulky retroperitoneal lymphadenopathy in association with mesenteric lymphadenopathy is suggestive of lymphoma. Abdominal lymphadenopathy, disproportionately involving the mesentery, is suggestive of tuberculosis [5].

The most common soft tissue masses to affect the retroperitoneum and peritoneal structures are metastases and lymphoma [6]. Metastases to the retroperitoneum most commonly originate in the pelvis (rectum, cervix, prostate) [7]. Peritoneal carcinomatosis and metastases to omentum and mesentery frequently arise from tumors of the ovary, colon, stomach, pancreas, uterus, and bladder [8–11]. Tuberculosis can closely mimic peritoneal carcinomatosis radiologically. Forty to eighty percentage of gastrointestinal carcinoid tumors metastasize directly or via lymphatic spread to the mesentery [12, 13]. The infiltrative pattern of mesenteric disease seen with carcinoid, desmoid tumors, retractile mesenteritis, and peritoneal mesothelioma can sometimes be difficult to differentiate on CT.

Primary neoplasms of the retroperitoneum are uncommon and are derived from mesenchyme, neurogenic tissue, or embryonic rests. Primary malignant tumors are about four times more common than benign neoplasms [14]. Liposarcoma and leiomyosarcoma account for over 90 % of malignant tumors. Approximately two-thirds of omental tumors are benign and include leiomyomas, lipomas, and neurofibromas. Malignant tumors include leiomyosarcoma, liposarcoma, fibrosarcoma, mesothelioma, and hemangiopericytoma [6, 15]. Primary peritoneal tumors are a rare group of tumors that arise from the mesothelial and submesothelial layers of the peritoneum [16]. They include malignant mesothelioma, multicystic mesothelioma, primary peritoneal serous carcinoma, leiomyomatosis peritonealis disseminata, and desmoplastic small round cell tumor. Many of these tumors have imaging findings similar to peritoneal carcinomatosis.

For many patients, the information provided by the clinical history and imaging studies is insufficient to exclude a malignancy. The imaging features of many primary neoplasms of the retroperitoneum and peritoneum are nonspecific. While the presence of calcifications, fat, necrosis, or cystic components are key to providing a differential diagnosis, there is often considerable overlap between neoplastic and nonneoplastic lesions and between benign and malignant tumors. Moreover, cytological analysis of ascitic fluid rarely results in site-specific tumor diagnoses in cases of peritoneal carcinomatosis [17]. Percutaneous needle biopsy of the retroperitoneum, mesentery, or omentum may be required to establish the benign or malignant nature of a lesion, to obtain material for microbiologic analysis in patients with known or suspected infections, and to stage patients with known or suspected malignancy when local spread or distant metastasis is suspected. A repeat biopsy may be required if there is a strong clinical suspicion of malignancy and the previous biopsy result was negative or inconclusive, or the result is discordant with the clinical and imaging findings.

At our institution, one of the most common indications for percutaneous biopsy of a retroperitoneal or mesenteric soft tissue mass is for the diagnosis, staging, and restaging of lymphoma. While the clinical practice guidelines issued by the European Society of Medical Oncology recommend excision biopsy for the diagnosis of newly diagnosed and relapsed lymphoma (except in emergency circumstances) [18], numerous studies support the use of core biopsy in providing a definitive histological diagnosis. Core biopsy diagnostic rates of 83–88 % for lymphoma have been quoted for nodes inside and outside of the retroperitoneum [19–22]. Controversy also surrounds the diagnosis of retroperitoneal soft tissue sarcomas. In the past, definitive diagnosis and treatment of these tumors was achieved with surgical resection. High local recurrence rates of 50–60 % at 5 years and 90 % at 10 years have led to efforts to downstage the disease with preoperative and intraoperative radiotherapy [23–26]. In this patient, population pretreatment percutaneous biopsy is appropriate. While surgical excision is the treatment of choice for many cystic masses in the retroperitoneum and mesentery, percutaneous needle aspiration can be useful to confirm the nature of a nonneoplastic cystic mass, e.g., urinoma, lymphocele, hematoma, or pseudocyst.

The radiologist’s pre-procedure evaluation should include a review of the patient’s clinical history and physical examination and correlation with available imaging studies. With a differential diagnosis in hand, the radiologist must determine if a diagnosis is possible with the evidence available, whether the diagnosis can be confirmed by less invasive means or that biopsy is indeed indicated. Not all lesions identified by imaging need to be biopsied, (e.g., FDG-avid mesenteric lymph nodes on PET-CT may be sufficient to confirm the presence of metastatic disease; an enhancing soft tissue mass involving bowel, with linear areas of soft tissue attenuation radiating outward in the mesenteric fat, is a CT finding strongly suggestive of carcinoid tumor). In order to avoid unnecessary and possibly hazardous interventions, the radiologist must be familiar with common “pseudotumors” and normal variants. Left-sided paraortic lymphadenopthy can mimic duplication of or left inferior vena cava (IVC). Retrocrural lymphadenopathy may mimic an enlarged azygos vein in the retrocrural space. Retroperitoneal lymphadenopathy can mimic circumaortic left renal vein. Retroperitoneal fibrosis is an inflammatory disorder that may be misinterpreted as a malignant process, as it envelops the aorta and IVC, often displacing and encasing the ureters. Other nonneoplastic entities that can have a mass-like appearance on CT include cystic masses (hematoma, urinoma, lymphocele, pseudocyst, duplication cyst), omental torsion, omental infarction, and inflammatory pseudotumor.

Before embarking upon biopsy of a deep retroperitoneal or mesenteric lesion, consideration should be given to biopsy of more accessible sites, e.g., the axillary or inguinal regions in the case of lymphadenopathy (Fig. 19.3). To minimize the risks associated with biopsy of deep lesions, the safest and shortest needle trajectory should be identified. A careful risk-benefit assessment should be performed prior to trans-intestinal biopsy of deep mesenteric lesions or trans-caval biopsy of retroperitoneal lesions. Biopsy may be contraindicated if the lesion is highly vascular. Markedly hypervascular tumors include paragangliomas and hemangiopericytomas. Moderately hypervascular tumors include some of the soft tissue sarcomas including leiomyosarcoma which can also be partly intravascular. Hypervascular retroperitoneal lymphadenopathy is a feature of Kaposi’s sarcoma and Castleman disease.

Many retroperitoneal and deep mesenteric biopsies are performed with the patient. Close proximity to viscera, e.g., kidneys or bowel, may require breath holding during the procedure. The patient’s ability to cooperate and lie supine or prone should be assessed. Patient comfort and cooperation may be optimized with the use of moderate sedation administered by dedicated nursing staff. At our institution, respiratory compromise, decompensated cardiac failure, morbid obesity and diagnosed sleep apnea, pregnancy, pediatric patient, and inability to lie in the required position for the procedure are among some of the reasons that referral to anesthesia may be required.

A history of anticoagulation therapy and antiplatelet agents should be obtained. The Society of Interventional Radiology provides guidelines for the cessation of these drugs in patients undergoing percutaneous biopsy [27]. At our institution, aspirin and clopidogrel are held 5 days and coumadin 3–5 days prior to percutaneous biopsy. Coagulation status is tested in those on coumadin therapy and those with known or suspected liver disease. If the risks associated with complete cessation of anticoagulation are high, the patient is converted to unfractionated heparin (biological half-life 1–2 h). Patients on low molecular weight heparin have one dose held prior to the procedure. In patients with malignancy, biopsy of mesenteric and retroperitoneal lesions may involve additional risks owing to treatment-related coagulation disorders or the cancer itself, e.g., chemotherapy-induced thrombocytopenia. Although numerous studies have demonstrated a relatively low predictive value of abnormal laboratory screening parameters in predicting bleeding, laboratory criteria for biopsy at our institution include a platelet count greater than 50 × 10⁹ and an international normalized ratio of less than 1.6 within 1 month before the procedure.

There are no absolute contraindications for percutaneous needle biopsy. Relative contraindications may include:

The choice of imaging modality is based on equipment availability, radiologist preference, the size and location of the target lesion, patient body habitus, potential access routes, the ability to visualize the lesion, and cost. Factors to be considered in a patient with a high body mass index include the maximum weight load of the CT/MRI table, the ability to visualize the lesion on ultrasonography, and the ability of the interventional radiologist to compress the subcutaneous tissue with the ultrasound probe while performing the biopsy. Although many interventional radiologists prefer to perform CT-guided biopsies of retroperitoneal and mesenteric lesions in obese patients, ultrasound-guided biopsy has been demonstrated to be very effective in this subset of patients [28].

Retroperitoneal and mesenteric lesions can be accessed from an anterior or posterior approach. When the posterior approach is performed, the needle usually passes alongside or through the quadratus lumborum and psoas muscles (Fig. 19.7). Although this is a safe trajectory, the operator should be aware that passing the needle through these muscles decreases the possibility of correct needle trajectory. Some proponents of a posterior paraspinal approach argue that in the event of a bleeding complication, the result would be a contained retroperitoneal hematoma rather than free intraperitoneal hemorrhage [49].