Benign retroperitoneal tumours and masses
The retroperitoneum — the space behind the abdominal cavity containing the kidneys, adrenal glands, major vessels, ureters, pancreas and lymph nodes — can harbour a wide range of masses. Many are found incidentally on scans performed for unrelated reasons, and their discovery is often more alarming than their nature warrants. A significant proportion are benign. The clinical work is characterising the mass accurately, which means resisting two opposite errors: assuming a mass is benign and removing it inadequately, and assuming it is malignant and subjecting a patient to an unnecessary major operation.
The governing rule is the same as for malignant retroperitoneal tumours: a retroperitoneal mass without an obvious explanation should be assessed in a specialist centre before biopsy or surgery.
The main benign masses, and how they form
Schwannoma
The commonest benign retroperitoneal neoplasm. It arises from Schwann cells of a peripheral nerve sheath — typically a lumbar or sacral nerve root, or the sympathetic chain. It grows eccentrically, displacing the nerve fibres to one side rather than passing through them, which is why it can often be enucleated without sacrificing the nerve.
On imaging, schwannomas are well-encapsulated, often with cystic degeneration, haemorrhage and calcification, giving a heterogeneous appearance that can look worrying. Histologically they show the characteristic alternating cellular Antoni A and loose Antoni B areas with Verocay bodies, and stain strongly for S100 protein. Malignant transformation is very rare. Multiple schwannomas suggest schwannomatosis or neurofibromatosis type 2.
Ganglioneuroma
A fully differentiated tumour of the sympathetic nervous system, composed of mature ganglion cells and Schwann cells. It represents the benign end of a spectrum whose malignant counterpart is neuroblastoma, and it may in fact result from spontaneous maturation of a neuroblastoma. It occurs in children and young adults, grows slowly, and characteristically envelops rather than displaces major vessels — wrapping around the aorta and its branches without narrowing them, which is a helpful radiological sign. A minority secrete catecholamines or vasoactive intestinal peptide, causing hypertension or diarrhoea, and biochemical screening is worthwhile.
Paraganglioma
Arises from extra-adrenal chromaffin tissue of the sympathetic chain, most often near the organ of Zuckerkandl at the aortic bifurcation. Although often described as benign, all paragangliomas have malignant potential, and the distinction is made on behaviour rather than histology. Many secrete catecholamines, producing paroxysmal hypertension, headache, sweating and palpitations.
This is the single most important lesion not to biopsy blindly, since biopsy or manipulation of an unsuspected functioning paraganglioma can precipitate a hypertensive crisis. Any retroperitoneal mass near the aorta should have plasma or urinary metanephrines measured before intervention. A high proportion carry germline mutations — most often in the SDHx genes — and genetic testing is recommended in all cases.
Lipoma
A benign tumour of mature fat. A true retroperitoneal lipoma is genuinely uncommon, and any fatty retroperitoneal mass must be regarded as a well-differentiated liposarcoma until MDM2 testing proves otherwise — the two are indistinguishable on routine histology and on imaging. This is the most important practical point in this whole group.
Cystic lesions
- Lymphangioma — a developmental malformation of lymphatic channels, thin-walled, multiloculated, often crossing tissue planes and insinuating between structures rather than displacing them.
- Bronchogenic and enteric duplication cysts — developmental remnants of foregut origin.
- Mullerian and epidermoid cysts, and pancreatic pseudocysts, which must be distinguished by their relationship to the pancreas and the clinical history.
- Lymphocele — a collection of lymph after retroperitoneal or pelvic surgery or transplantation, common and usually self-limiting.
Other benign lesions
- Extra-adrenal myelolipoma — a benign mixture of fat and bone marrow elements, with a characteristic appearance.
- Angiomyolipoma — usually arising from the kidney, containing fat, muscle and vessels; associated with tuberous sclerosis when multiple, and carrying a risk of haemorrhage when large.
- Castleman disease — a lymphoproliferative disorder producing a highly vascular nodal mass; the unicentric form is cured by excision, and its vascularity makes biopsy and surgery bloodier than expected.
- Retroperitoneal fibrosis — a fibro-inflammatory process encasing the aorta and ureters, causing obstructive renal failure. A proportion are IgG4-related disease and respond to corticosteroids and immunosuppression rather than surgery — a diagnosis well worth making, since treatment is entirely medical.
- Extramedullary haematopoiesis, in patients with chronic haematological disorders.
- Psoas abscess and haematoma — inflammatory rather than neoplastic, and readily mistaken for tumour.
The crucial differentials that are not benign
Several conditions present as retroperitoneal masses and are treated entirely without surgery. Missing them and proceeding to an operation is a serious error:
- Lymphoma — treated with chemotherapy; often suggested by multiple confluent nodal masses encasing vessels without occluding them.
- Germ cell tumour — in young men particularly; AFP and hCG must be measured, and the testes examined and scanned.
- Metastatic nodal disease — from testicular, renal, gastrointestinal or gynaecological primaries.
- Tuberculosis — producing necrotic nodal masses, particularly relevant in patients from endemic areas.
Symptoms
- frequently none — most are incidental findings
- vague abdominal or back discomfort
- a palpable mass when large
- leg pain, numbness or weakness from nerve involvement
- urinary symptoms or hydronephrosis from ureteric compression
- leg swelling from venous compression
- hypertension, headache, sweating and palpitations with a functioning paraganglioma
- diarrhoea or hypertension with a secreting ganglioneuroma
How the diagnosis is made
- CT with contrast — defining size, density, fat content, calcification, vascularity and the relationship to surrounding structures.
- MRI — superior for characterising tissue, defining the relationship to nerve roots and the spinal canal, and distinguishing cystic from solid lesions.
- Biochemistry before anything else — plasma or 24-hour urinary metanephrines in any mass that could be a paraganglioma; AFP, hCG and LDH in young patients where germ cell tumour is possible.
- Image-guided core biopsy — after excluding a functioning paraganglioma, and planned with the centre that would operate, using a retroperitoneal route. Biopsy is particularly valuable where lymphoma or germ cell tumour is in the differential, since these are treated medically.
- MDM2 testing on any fatty lesion.
- Functional imaging — MIBG or Ga-68 DOTATATE PET for suspected paraganglioma.
Treatment
- Observation — entirely appropriate for a confidently characterised, small, asymptomatic benign lesion such as a small schwannoma or a myelolipoma, with interval imaging.
- Surgical excision — for symptomatic lesions, growing lesions, lesions where malignancy cannot be excluded, and functioning tumours. Schwannomas can often be enucleated with nerve preservation, and the possibility of a neurological deficit should nonetheless be discussed beforehand.
- Paraganglioma — requires alpha-blockade for two to three weeks before surgery, with beta-blockade added subsequently where needed, to prevent an intraoperative hypertensive crisis. Operating on an unprepared functioning tumour is dangerous. Genetic testing follows in all cases.
- Retroperitoneal fibrosis — corticosteroids and immunosuppression, with ureteric stenting or nephrostomy to protect renal function; surgery (ureterolysis) for selected cases.
- Lymphoma and germ cell tumour — chemotherapy, with surgery only for residual masses in defined circumstances.
- Lymphocele — drainage or sclerotherapy if symptomatic; most resolve.
Minimally invasive and robotic approaches are increasingly used for well-characterised benign retroperitoneal lesions.
Recovery
Depends entirely on the lesion and the approach. Laparoscopic or robotic excision of a schwannoma typically involves two to four days in hospital and a recovery of three to four weeks. Open excision of a large mass adjacent to major vessels is a considerably larger undertaking. Numbness or weakness in the distribution of a sacrificed nerve may be permanent and should be anticipated in the consent discussion.
Follow-up
Interval imaging for observed lesions, lengthening as stability is established. Lifelong biochemical and imaging surveillance for paraganglioma, particularly where a germline SDHx mutation is found, and screening of family members.
When to seek an opinion
Any retroperitoneal mass without a clear explanation should be discussed with a specialist centre before biopsy or surgery. Measure metanephrines before touching a mass near the aorta. Regard any fatty retroperitoneal mass as a liposarcoma until proved otherwise.
Common questions
A scan done for something else found a mass. How worried should I be?
Incidental findings in this area are common and a good proportion turn out to be benign. What matters is characterising it properly — the right scans, the right blood tests, and in many cases a biopsy taken in the right way — rather than either rushing to surgery or dismissing it. That process usually takes a few weeks and is worth doing carefully.
Why are blood or urine tests needed before a biopsy?
Because one type of tumour in this region produces adrenaline-like hormones, and putting a needle into it, or handling it at surgery, can cause a dangerous surge in blood pressure. A simple blood or urine test excludes this beforehand, and if it is present the tumour is treated with blocking medication for a few weeks before any intervention.
I was told it is a lipoma. Is that reassuring?
Only once a specific genetic test has confirmed it. In this part of the body, a genuine lipoma is actually uncommon, and a well-differentiated liposarcoma looks almost identical both on scans and under the microscope. A test for MDM2 amplification distinguishes them, and it should be done before anyone calls a fatty mass in the retroperitoneum harmless.
Can it be left alone?
Sometimes, yes. A small, clearly characterised benign lesion causing no symptoms can reasonably be watched with scans at intervals. Removal is advised where it causes symptoms, where it is growing, where the diagnosis remains uncertain, or where it is producing hormones.
Will the operation damage a nerve?
It may, and this should be discussed with you beforehand. Many of these tumours grow from a nerve, and although the commonest type can often be shelled out with the nerve preserved, numbness or weakness in the area that nerve supplies is a real possibility. The likelihood depends on which nerve is involved and is assessed on the MRI.
Why might I need chemotherapy rather than surgery?
Because some masses in this region are lymphoma or, in younger men, a germ cell tumour, both of which are treated very effectively with drugs and are not helped by an operation. This is one of the main reasons a biopsy is often taken before surgery is planned — it prevents a major and unnecessary operation.
Why should this be dealt with in a specialist centre?
Because the range of possible diagnoses is wide, several require entirely non-surgical treatment, and the operations for those that do need surgery often involve major blood vessels, the kidney and nerve roots. Getting the assessment right at the start avoids both unnecessary surgery and inadequate surgery.
Is it inherited?
Most are not. The exception is paraganglioma, where a high proportion carry an inherited gene change, and genetic testing is recommended for everyone diagnosed with one, along with screening of relatives. Multiple nerve sheath tumours also suggest an inherited condition worth investigating.
Related conditions
Other conditions of the sarcoma & retroperitoneal covered on this site:
- Retroperitoneal liposarcoma
- Leiomyosarcoma
- Soft tissue sarcoma of the trunk and abdominal wall
- Desmoid tumour (aggressive fibromatosis)
This page provides general information and does not replace an individual medical consultation. A retroperitoneal mass should be assessed in a specialist centre before biopsy or surgery.