Localization of Retroperitoneal Tumors

Before examining the specific imaging features of retroperitoneal masses, the radiologist must first confirm whether the mass is arising from the retroperitoneal soft tissue as opposed to the peritoneum or an organ that resides in the retroperitoneum. The clinical importance of making this distinction is that frequently, fat containing tumors arising from the retroperitoneal soft tissue turn out to be malignant while similar tumors arising from retroperitoneal organs like kidneys often turn out to be benign [8].

1.  Tumor location: displacement of retroperitoneal organs or vascular structures often suggest that a mass is retroperitoneal in location.

Fig 8: Signs for diagnosing primary retroperitoneal masses

2. Exclude organ of origin:

                              The following signs help in localizing the lesion to the retroperitoneum:

• Beak or claw sign
• Embedded organ sign
• Phatom organ sign
• Prominent feeding artery

Fig 9: Beak sign

Fig 10: Embedded organ sign

Fig 11: Phantom organ sign

Fig 12: Prominent feeding artery sign

Differential diagnosis of primary retroperitoneal masses

SOLID RETROPERITONEAL MASSES

Solid primary retroperitoneal masses can be broadly divided into neoplastic and non neoplastic categories.

Fig 13: Differential diagnosis of solid primary retroperitoneal masses Reference: Scali et al.(2014). "Primary Retroperitoneal masses: what is the differential diagnosis?". Abdominal imaging; 1-17.

Solid neoplastic mass lesions:

1. LYMPHOMA

- Lymphoma is the most common retroperitoneal malignancy, accounting for ~ 33% of all primary retroperitoneal masses.

- They typically present as para aortic masses with homogeneous attenuation and show mild homogeneous post contrast enhancement [4].

- They often appear as mantle like masses between the aorta and IVC, encasing the structures without compression. The aorta and IVC can be anteriorly displaced by the mass producing the classic "Floating aorta sign" or the "CT angiogram sign".

Fig 14: Case 1: 44 year old lady who presented with abdominal discomfort and nausea since 5 days. A - USG reveals a heteroechoic mass encasing the abdominal aorta. B - Scout CT reveals a large midline radio opaque mass lesion displacing the bowel loops to the sides. C and D - MRI abdomen axial and coronal views reveal an ill defined T1 and T2 intermediate signal intensity, diffusion restricting, retroperitoneal prevertebral lesion encasing the abdominal aorta and IVC with anterior displacement. E- FDG avid retroperitoneal mass lesion and lymph nodes.

Fig 15: Case 1: Histopathological analysis - Retroperitoneal Non Hodgkin's lymphoma, immunoprofile consistent with Diffuse large B cell lymphoma, NOS. B - Ki 67 positive. C - Bcl2 - diffuse moderate strong staining in lymphoid cells.

Take home point:  Homogeneously enhancing retroperitoneal lesions with the "floating aorta" sign should raise the suspicion of retroperitoneal lymphoma.

2. SARCOMAS

Sarcomas are rare neoplasms of mesenchymal origin which account for about 15% of primary retroperitoneal tumors.

    RETROPERITONEAL LIPOSARCOMA

 - Liposarcoma is a malignant adipose tissue tumor which accounts for 15% of all soft tissue tumors. 10-20% of liposarcomas originate in the retroperitoneum [1].

-  Histologically divided into 4 subtypes - 

• Well differentiated - most common subtype with good prognosis, low metastatic potential, approximately 10% undergo dedifferentiation
• Myxoid -  predominantly non fatty with a cystic appearance due to extracellular myxoid matrix. The presence of small amount of fat in lacy septa or nodular components is pathognomonic.
• Dedifferentiated - second most common subtype. Has the worst prognosis. Heterogeneous appearance with mixed solid and fatty components and the presence of calcifications is pathognomonic.
• Pleomorphic - least common subtype. Aggressive lesions with little or no macroscopic fat.

Fig 16: Case 2: 47 year old female who presented with complains of amenorrhoea and abdominal distension since 1 month. Scout CT reveals a large radio opaque abdominal mass in the midline extending towards the right with displacement of bowel loops towards the left. CECT abdomen reveals a large well encapsulated heterogeneously enhancing lesion with internal fat density areas, multiple nodular enhancing soft tissue components centered on the right side of retroperitoneum encasing the right ureter causing hydroureteronephrosis.

Fig 17: Case 2: Histopathological analysis - Dedifferentiated retroperitoneal liposarcoma with myxoid and pleomorphic areas - FNLLCC grade 3.

Take home point:  Heterogeneously enhancing retroperitoneal lesion with mixed solid and fatty components and calcifications should raise the suspicion of a dedifferentiated liposarcoma.

       RETROPERITONEAL LEIOMYOSARCOMA

- Leiomyosarcoma is a malignant tumor of smooth muscle cells with a retroperitoneal location in 12-69% of cases.

- They manifest as large soft tissue masses with internal heterogeneity and heterogeneous enhancement usually secondary to necrosis and hemorrhage. Fat components and calcifications are usually absent.

-  MRI: They normally show low to intermediate signal intensity on T1W and intermediate to high signal intensity on T2W images. 

Fig 18: Case 3: 44 year old lady presented with complains of low grade fever, intermittent lower abdominal pain and weight loss since 2 months. CECT abdomen reveals a well defined heterogeneously enhancing soft tissue density lesion in the right prevertebral region with multiple non enhancing areas within. No evidence of calcifications within. FDG PET shows moderate uptake within the lesion.

Fig 19: Case 3: Histopathological analysis - A and B - low and high magnification microscopic images confirm the diagnosis of retroperitoneal leiomyosarcoma with areas of coagulation necrosis. C - IHC: tumor cells show strong Desmin positivity

Take home point: A large non fat containing retroperitoneal mass with involvement of a contiguous vessel and varying internal necrosis should raise the possibility of a leiomyosarcoma.

          RETROPERITONEAL GIST

- Retroperitoneum is an exceptionally rare location for Gastrointestinal stromal tumors and account for less than 1% of cases [7].

- They arise from isolated Cajal's cells in the retroperitoneum.

- On contrast enhanced CT, they appear as heterogeneously enhancing lesions with no fat components or calcifications. Abdominopelvic MRI also helps to elicit the lack of fat within the lesion and the relationship with adjacent structures.

- Histopathological analysis with immunohistochemistry is required to establish the diagnosis. These mesenchymal tumors are c-KIT, CD-117, CD-34 and DOG-1 positive.

Fig 20: Case 4: 74 year old male patient who is a known case of carcinoma rectum, status post surgery and chemoradiation now presented with vague abdominal pain and generalized tiredness since 1 month. CECT abdomen reveals a large retroperitoneal heterogeneously enhancing lobulated mass lesion involving the posterior wall of duodenum. Large central cavitary changes within the lesion communicating with the lumen of third part of duodenum anteriorly. FDG PET reveals moderate uptake within the lesion. Final diagnosis after IHC: Retroperitoneal GIST, CD 117 positive.

3. NEUROGENIC TUMORS

 - Retroperitoneal neurogenic tumors account for about 10-20% of primary retroperitoneal tumors in adults.

- They are classified based on the tissue of origin:

• Nerve sheath:  Schwannoma, neurofibroma, malignant peripheral nerve sheath tumor 
• Ganglion cells:  Ganglioneuroma, ganglioneuroblastoma, neuroblastoma
• Paraganglionic cells:  Paraganglioma

Fig 21: Case 5: 58 year old lady with no known comorbidities presented with recent onset hypertension and weight loss since 3 months. Elevated normetanephrine and chromogranin levels were noted. A- USG abdomen reveals a well defined, heteroechoic lesion with calcifications and increased internal color Doppler flow in the para aortic region. B and C- CECT abdomen reveals an intense arterial phase enhancing lesion centered around the origin of inferior mesenteric artery. D - DOTA PET CT shows intensely avid uptake within the lesion.

Fig 22: Case 5: Histopathological analysis reveals epitheloid cells with nuclear atypia arranged in clusters in the classic "Zellballen pattern" separated by prominent fibrovascular stroma. IHC reveals chromogranin positivity within the tumor cells. Final diagnosis: Extra adrenal retroperitoneal paraganglioma.

Fig 23: Case 6: 36 year old lady, known case of neurofibromatosis came for routine follow up. CECT abdomen reveals a large well defined retroperitoneal nodular hypoenhancing lesion in the left upper para aortic region causing erosion of L1 vertebral body and widening of the neural foramen. No calcifications / fat containing areas within. FDG PET reveals mild uptake within the lesion. Final diagnosis after histopathological analysis: Malignant peripheral nerve sheath tumor.

Fig 24: Case 7: 14 year old male patient presented with complains of left paraspinal swelling in the lumbar region with firm consistency. MRI abdomen reveals a large retroperitoneal T1 and T2 intermediate signal intensity lobulated mass lesion crossing the midline and encasing abdominal aorta and its branches. Histopathological analysis reveals undifferentiated cells with salt and pepper chromatin arranged as "Homer - Wright pseudo rosettes" around a delicate, eosinophilic neuropil. Final diagnosis: Retroperitoneal neuroblastoma.

Take home points: 

• Extra adrenal retroperitoneal paragangliomas are hypervascular intense arterial phase enhancing lesions most commonly originating from the organ of Zuckerkandl at the origin of inferior mesenteric artery. Strongly enhancing retroperitoneal masses in a hypertensive patient should raise the suspicion of a functioning paraganglioma.
• Malignant peripheral nerve sheath tumors account for 10% of all soft tissue sarcomas. Distinguishing features from neurofibroma include large tumor size, peripheral enhancement, perilesional edema, ill defined borders and increased FDG uptake.
• Retroperitoneal neuroblastomas arise from the paraspinal sympathetic chain in the retroperitoneum. They frequently cross the midline and appear as heterogeneously enhancing lesions which often show areas of necrosis and hemorrhage.

4. GERM CELL, SEX CORD AND STROMAL TUMORS

• Primary extra gonadal germ cell tumors are rare, accounting for 1 - 2.5% of germ cell tumors. A gonadal primary must be excluded first as majority of retroperitoneal germ cell tumors represent metastases.
• Teratomas account for approximately 11% of primary retroperitoneal tumors and is the third most common retroperitoneal tumor in children after neuroblastoma and Wilm's tumor. Teratomas are of two types - mature (dermoid cyst) and immature.

Solid non neoplastic mass lesions

RETROPERITONEAL FIBROSIS

- Retroperitoneal fibrosis is a rare collagen vascular disease that is most common in men of fourth to sixth decades.

- Retroperitoneal fibrosis can be idiopathic or secondary to other etiologies like malignancy, medications or infections.

- On unenhanced plain CT, retroperitoneal fibrosis appears as a soft tissue attenuation mass surrounding aorta, inferior vena cava and psoas major with obliteration of fat planes. Retroperitoneal fibrosis displaces the ureters medially.

- In the acute stage of the disease, intense post contrast enhancement can be observed in CT and MRI. During the later stages, the lesion shows mild or even no contrast enhancement.

Fig 25: Case 8: 64 year old male patient with no known comorbidities complained of vague abdominal pain and loss of appetite since 1 month. CT abdomen reveals retroperitoneal. soft tissue thickening with adjacent fat stranding encasing the abdominal aorta, iliac vessels and right ureter causing upstream right mild hydroureteronephrosis. MRI abdomen reveals an ill defined T1 isointense, T2 hypointense lesion with minimal peripheral post contrast enhancement. There is no anterior displacement of aorta. Final diagnosis: IgG4 related retroperitoneal fibrosis.

Fig 26: Case 9: 67 year old male patient who is a known case of carcinoma esophagus came for a routine follow up CT which revealed an enhancing soft tissue thickening around abdominal aorta and its branches and IVC with mild anterior displacement of aorta. There is no narrowing / filling defects in the vessels encased. Encasement of left ureter also noted causing upstream left mild hydroureteronephrosis. Final diagnosis: Retroperitoneal fibrosis secondary to carcinoma esophagus.

Take home point: Displacement of aorta, invasion of adjacent soft tissue structures and bone, peripheral lobulation and variable contrast enhancement can help distinguish malignant from benign retroperitoneal fibrosis.

CYSTIC RETROPERITONEAL  MASSES

Common cystic retroperitoneal masses, their classification, clinical and imaging features have been enlisted in the table below.

Fig 27: Differential diagnosis and imaging features of cystic retroperitoneal masses. Reference: Yang, D.M; Jung, D.H; Kim, H et al. (2004). Retroperitoneal Cystic Masses: CT, Clinical and Pathological Findings and Literature Review. Radiographics, 24(5), 1353-1365.

ALGORITHMIC APPROACH TO PRIMARY RETROPERITONEAL MASSES

Given below is a step by step algorithmic approach which could be used to confirm the retroperitoneal origin of a mass lesion and arrive at an appropriate diagnosis after excluding differentials.

STEP 1:  Confirm the retroperitoneal origin of tumor based on the imaging signs discussed above.

STEP 2:  Based on the imaging appearance, classify the lesion broadly as solid or cystic.

STEP 3:  Classify the lesion based on location, vascularity, composition and specific imaging features.

Fig 28: An algorithmic approach for diagnosing primary retroperitoneal masses

Fig 29: Differential diagnosis of primary retroperitoneal masses based on composition

Fig 30: Specific imaging signs of various retroperitoneal masses