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The periportal space is a potential space surrounding the portal vein and its intrahepatic branches. A variety of neoplasms can involve the periportal region, whether primary or secondary, owing to contiguous spread from surrounding hepatic parenchyma or from adjacent organs. CT plays an important role in not only diagnosing these lesions but also determining the extent of the disease. Most of the malignancies leading to the periportal spread manifest as periportal hypodensity either distinctly or in contiguity with the primary tumour. Even in known malignancies, periportal hypodensity commonly results from non-neoplastic causes like periportal oedema; hence, a knowledge of the imaging findings to ascertain its presence as well as to conclude the definite neoplastic spread is prudent. Periportal spread of neoplasm may suggest locally aggressive or disseminated disease (in extrahepatic malignancies), which may change management accordingly.
The periportal region is a potential space surrounding the portal vein and its intrahepatic branches. Its contents include the hepatic artery, biliary radicle, nerves and lymphatics. A variety of neoplasms can involve the periportal region, whether primary (via haematogenous, lymphatics and biliary route) or secondary, owing to contiguous spread from surrounding hepatic parenchyma or adjacent organs. Differential diagnosis of periportal neoplasms is limited, which may be suggested on imaging; hence, radiologists need to be familiar with their findings. Nonetheless, diagnostic confirmation is a multidisciplinary approach, additionally requiring clinical and pathological correlation.
In the process of becoming intrahepatic, the portal vein pierces the hepatic capsule (Glisson's capsule or fibrosa perivascularis). As a result, the intrahepatic portal vein and its branches carry with themselves a sleeve of loose connective tissue, resulting in the formation of a “periportal space”. The contents of this space include neurovascular structures (hepatic artery, lymphatics and nerves) and biliary radicles, which embrace the portal vein.
On contrast-enhanced CT (CECT) scan, periportal lesions are most conspicuous in the portovenous or equilibrium phases, corresponding to the increased conspicuity of their anatomical landmark—the “portal vein”. Owing to its contents and strategic location, seedling of periportal space can occur from a variety of routes like haematogenous, lymphatic, neural, peritoneal or direct spread from contiguous neoplasms. Amongst these, the commonest cause of periportal pathologies is lesions affecting the portal vein.
For the purpose of simplicity, we have classified periportal neoplasms (Table 1) on the basis of mode of spread into primary (neurovascular or biliary origin) or secondary (infiltration from hepatic neoplasm or adjacent organ tumours).1–3
Cholangiocarcinoma (CCA) is one of the commonest malignancies of the biliary tract, ranking next only to carcinoma gall bladder in incidence. The average age of presentation is 50 years, with a slight female preponderance. Of the three morphological subtypes of CCA—intrahepatic mass forming, intraductal growing and periductal infiltrating, the latter spreads exclusively in the periportal space, with only occasional infiltration by other subtypes. The biliary radicles are supplied by the peribiliary vascular plexus, lymphatics and nerve plexus, which serve as a conduit to the periportal spread.
On CECT, periductal CCA lacks a distinct mass and appears as infiltrating asymmetrical hypodense soft-tissue thickening, growing along an irregularly narrowed or dilated bile duct (Figure 1). It encases the biliary radicles and portal vein, causing their abrupt cut-off or significant luminal attenuation with peripheral dilatation, owing to its scirrhous nature. Similar to other periportal neoplasms, portovenous or equilibrium phase is best to characterize CCA, although the lesion may show enhancement in arterial phase. In CCA, periportal hypodensity not only indicates periductal spread but also portends a poor prognosis, making the lesion unresectable. Periductal infiltrating CCA needs to be differentiated from periportal lymphatic dissemination of extrahepatic malignancies. Periportal involvement in periductal CCA is usually focal or segmental and it results in dilatation of upstream biliary radicles. On the contrary, in periportal lymphatic spread of extrahepatic malignancy, involvement is usually diffuse, and biliary radicles are usually not dilated.4–6
Variously referred as “myeloid or granulocytic sarcoma”, chloroma denotes localized extramedullary proliferation of the primitive malignant myeloid cells. Of all the myeloid neoplasms, chloroma is more preponderant in acute (seen in 3–5% of cases) than chronic myeloid leukaemia or other myeloproliferative disorders. It is usually seen in the relapse or remission phase of the disease and portends a poor prognosis. Common sites of chloroma include the bones, lymph nodes, soft tissues, skin and breast. It has a wide age of presentation but is commonly seen in children (<15 years) with no gender predilection.7,8
Leukaemic involvement of the liver is extremely rare and usually occurs secondary to extensive marrow infiltration. Rare case reports in literature describe an entity called primary biliary chloroma, with isolated involvement of the peribiliary space in the absence of bone marrow involvement. However, recent times have seen a surge in the cases of hepatobiliary chloroma, as more patients of myeloid leukaemia are being treated by intensive chemotherapy and bone marrow transplant.7,9
Myeloid cells can infiltrate either the hepatic sinusoids or periportal region. Corresponding to the pattern of spread, hepatic involvement may manifest as a nodular deposit (solitary or multiple) of varying sizes or the other rarer form, causing peribiliary infiltration. On CT (Figure 2), there may be hepatomegaly, with the presence of either well-circumscribed hypodense heterogeneously enhancing lesions or periportal hypodensities paralleling the biliary radicles (peribiliary chloroma).
Being non-specific, both these patterns of chloroma are great imaging mimics. Discrete lesions resemble lymphoma from which it is difficult to differentiate; however, certain morphological features may help. Lymphoma deposits are relatively well marginated, non-enhancing or mildly enhancing, and mass effect is considerably less for the extent of involvement. On the contrary, hepatic chloromas are usually infiltrating heterogeneously enhancing lesions with significant mass effect, resulting in biliary radicle dilatation. The presence of lesions at other sites may be a clue to lymphoma, especially in cases of secondary hepatic involvement. Peribiliary chloromas may remain undiagnosed until late, owing to their extremely rare occurrence as well as non-specific imaging findings, which are seen commonly in other conditions like cholangitis or CCA.9
Owing to its mass effect, hepatic chloroma can cause jaundice, the diagnosis of which may remain elusive until late, and radiologists are often the first ones to raise suspicion; nonetheless, prognosis remains poor. Early diagnosis of chloroma is prudent, as localized lesions in initial stages can be treated with curative intent by chemotherapy or focal ablative procedure like focused radiotherapy.8,10
Hepatic involvement in lymphoma can be either primary or secondary to the involvement at other sites, the latter being commonly seen in nearly half of the cases. The age of presentation is wide, ranging from childhood to eighth decade, males being affected twice as common as females. Owing to the paucity of lymphocytes, primary hepatic lymphoma (PHL) is an extremely rare entity and is diagnosed by the absence of extrahepatic involvement for at least 6 months after the onset of hepatic disease.8
On imaging, there can be various patterns of involvement—diffusely infiltrating form resulting in hepatomegaly, discrete nodules or masses, combined nodular infiltrating and an extremely rare periportal infiltrating form (Figure 3). Diffusely infiltrating is the most common pattern in secondary hepatic lymphoma, whereas in the primary form, solitary nodules are more common (Table 4).
On CT, lymphoma is usually hypodense and shows no enhancement. Central necrosis, scar and/or calcification may be seen in the aggressive variety or post chemotherapy. As lymphomatous deposits are soft lesions lacking desmoplastic response, the mass effect is disproportionately less compared with their size. In peribiliary lymphoma, by virtue of its distribution, biliary dilatation is relatively commoner and occurs early in the course of disease.
Labelling hepatic deposits as lymphomatous involvement with the evidence of disease elsewhere as in secondary hepatic lymphoma is usually not a diagnostic concern. However, dilemma arises in cases of PHL, in which differentiating it from other focal hepatic neoplasms can be painstakingly difficult. PHL with nodular deposits needs to be differentiated from other common hepatic malignancies like hepatocellular carcinoma (HCC), mass-forming intrahepatic CCA and metastasis, mainly owing to different therapeutic approaches. MRI may remain inconclusive, as all the above-mentioned lesions can be of intermediate signal intensity on T2 weighted images and may show restricted diffusion. Hence, differentiating amongst these conditions is a holistic approach requiring correlation of the imaging findings with clinical, laboratory profile and histopathological analysis (Table 2).8,11
“Periportal halo” resulting from dilated obstructed lymphatics can be seen in any morphological form of lymphoma. It differs from periportal lymphoma in being a linear circumferential hypoattenuation along the portal triad as opposed to the latter, which presents as a circumscribed mass in peribiliary distribution. Persistence of periportal halo may suggest refractoriness to chemotherapy or disease exacerbation.1,12,13
Lymphoma as a part of the spectrum of post-transplant lymphoproliferative disorders (PTLD) seen in the transplant recipients are usually extranodal, the liver being the commonest solid visceral organ to be involved. Three imaging patterns in PTLD of the liver have been described—poorly enhancing circumscribed masses, diffusely infiltrating pattern and an uncommon periportal infiltrating lesion. The periportal infiltrating form encases the portal vein and may cause its attenuation or thrombosis; however, the incidence of venous involvement is significantly lower than in HCC. Post-transplant recipients are vulnerable to opportunistic infections and other malignancies, which may metastasize to the liver. It is prudent to differentiate these lesions, as PTLD usually responds favourably by mere reduction in the immunosuppressant doses.1
Langerhans cell histiocytosis (LCH) is the most common histiocytic proliferative disorder, and the majority of patients are younger than 2 years of age. Hepatobiliary involvement is usually a part of the disseminated disease, seen in approximately 40–60% of the cases. On the contrary, isolated hepatobiliary LCH is relatively rare, with an incidence of approximately 14–18%. Hepatobiliary involvement of LCH can be of two types—diffusely infiltrating resulting in hepatomegaly or periportal involvement, owing to the marked proclivity of histiocytes for biliary radicles.
The periportal form can be further categorized into four stages—proliferative, granulomatous, xanthomatous and fibrotic. Initial proliferative and granulomatous stages are essentially a periportal triaditis, resulting in oedema on histopathology. On ultrasound, it manifests as hyperechoic nodularity and band in the periportal region with corresponding hypodensity on CT (Figure 4), showing occasional enhancement. In the xanthomatous stage, there is clustering of lipid laden nodules in the hepatic parenchyma in the peribiliary location. On ultrasound, it appears as a circumscribed hyperechoic nodule, which is hypodense on CECT. Ultimately, if allowed to progress, periportal fibrosis ensues, resulting in secondary biliary cirrhosis.14,15
Neurofibroma, a benign nerve sheath tumour and a feature of neurofibromatosis-1 (NF-1), can be either peripheral or plexiform. Peripheral neurofibromas are usually supradiaphragmatic in location—the head, neck and thorax being the usual sites of involvement. Gastrointestinal tract involvement in NF is extremely rare and is usually restricted to the occasional presence of leiomyoma or neurofibroma of the hollow viscera. Plexiform neurofibromas, which are pathognomonic lesions of NF-1, rarely occur below the diaphragm. Only a handful of case reports in literature describe its occurrence in the retroperitoneum and paraspinal region, which may contiguously infiltrate the hepatic hilum.
Intrahepatic periportal plexiform NF results from the involvement of the hepatic nerve plexus constituted by autonomic nerve branches of splanchnic and phrenic nerves. Further periportal spread is explained by the distribution of the hepatic nerve plexus along vessels and biliary radicles. Hepatic involvement may be either diffuse or segmental in distribution. On CT, it appears as homogeneous low-attenuation non-enhancing/hypoenhancing masses (approximately 15–20HU) in the periportal distribution emanating from the hepatic hilum (Figure 5). Low attenuation is variably attributed to the presence of the myxoid matrix, entrapment of adipose tissue and cystic degeneration. Quite characteristic of these lesions is vascular and biliary radicle encasement, without causing luminal narrowing or thrombosis, hence preserving the native architecture. Contiguous extension of the lesion from the retroperitoneum and/or mesenteric root is another key distinguishing feature.1,3,16–19
Schwannoma, the other nerve sheath tumour seen in NF-1, can occasionally involve the liver. Akin to intrahepatic neurofibroma, it is also extremely rare. Its most common site of involvement is the hepatic hilum (porta hepatis), owing to abundance of nerve fibres in the hepatoduodenal ligament.20 As opposed to the unencapsulated intrahepatic neurofibroma, which spreads longitudinally along the biliary radicles, schwannomas are usually encapsulated heterogeneously enhancing cystic masses, and they may also show calcification.1
Metastatic spread to the liver can occur through a variety of routes. Haematogenously disseminated metastases derive their vascular supply through the portal vein. Correspondingly, in the initial stages, such lesions seed along the periportal and subscapular locations (Figures 6 and and77).
Malignant lymphadenopathy at the porta hepatis or lesser omentum can lead to periportal hypodensity by either impeding the lymphatic drainage resulting in periportal lymphoedema or there may be a retrograde dissemination of malignant cells from the hilum. Hence, periportal hypodensity should always be interpreted cautiously to rule out occult neoplasms in the porta hepatis or lesser sac.2,21–23
HCC is the most common primary malignant hepatic tumour. It may show periportal spread either by invasion of the portal vein or through lymphatic dissemination. On CECT, it appears as an arterially enhancing mass lesion with washout in the portovenous and/or delayed phases, usually in the background of cirrhosis. Well-differentiated lesions may be circumscribed with the presence of a capsule; however, poorly differentiated forms are usually infiltrating lesions (Figure 8). Venous invasion is a remarkable feature of HCC. Periportal spread, which may be diffuse or segmental in distribution, is indicated by the presence of hypodensity tracking along the invaded portal vein (tumour in vein) or biliary radicles.3
It is the most common primary malignant hepatic tumour in children of less than 5 years of age. On CT, it is usually a well-encapsulated heterogeneously enhancing mass lesion, with variable presence of calcification or cystic degeneration. In some cases, periportal hypodensity (Figure 9) is seen, which may be secondary to the involvement of periportal lymphatics.24
Gall bladder carcinoma is the most common malignancy of the biliary tract. As the gall bladder lacks submucosa, there is early rapid spread of malignancies to the adjacent organs and hepatic parenchyma. Hepatic involvement can be either due to direct infiltration or can be haematogeneous. Further, the disease may show periportal spread (Figure 10), which can be due to direct portal tract invasion, lymphatic dissemination or involvement of the hepatoduodenal ligament.25,26
Glisson's capsule, which surrounds the liver, forms a sleeve of connective tissue around the intrahepatic portal vein referred to as Glisson's sheath. Glisson's sheath further continues into the subperitoneal spaces of gastrohepatic and hepatoduodenal ligaments, hence serving as a conduit to the periportal spread of peritoneal malignancies.
Peritoneal neoplasms like peritoneal carcinomatosis, pseudomyxoma peritonei or peritoneal lymphomatosis usually stud the hepatic surface corresponding to the peritoneal fluid circulation. Involvement of the lesser omentum leads to further spread to the periportal region contiguously from the gastrohepatic and hepatoduodenal ligaments.
On CT, peritoneal implants appear as low-attenuation plaque-like or nodular masses involving the liver surface and may show periportal spread, predominantly at the porta hepatis and along the left branch of the portal vein (Figure 11).
This periportal spread needs to be differentiated from the hepatic parenchymal metastasis especially in ovarian malignancies, as the latter portends a poorer prognosis, making the lesion usually unresectable. Periportal spread is relatively ill defined and is not circumferentially harboured by hepatic parenchyma, as opposed to the intraparenchymal metastasis.
As periportal spread of all neoplastic peritoneal disease processes remain similar, these can be differentiated by the primary site elsewhere. However, scalloping of the liver surface is a distinguishing feature of pseudomyxoma peritonei (Figure 12).21,27,28
Apart from peritoneal neoplasm, malignancies of adjacent organs like the stomach, duodenum, colon and pancreas or even from the anterior abdominal wall can contiguously infiltrate the perihepatic ligaments with further periportal spread (Figures 13 and and14).14). In the absence of distant metastasis, such cases are usually resectable, although surgery is more extensive.1,21,29
Kaposi's sarcoma is a low-grade vascular tumour and is the commonest intrahepatic malignancy in patients with acquired immune deficiency syndrome. CT examination shows hepatomegaly with the presence of ill-defined hypodense periportal nodules, which may show conglomeration. After contrast administration, these nodules may show enhancement with gradual filling in the delayed phases, making them indistinguishable from haemangioma. Similar lesions may be present in the spleen coupled with hyperenhancing lymphadenopathy, serving as a clincher to its diagnosis.30
Periportal hypodensity is most commonly due to non-neoplastic aetiologies (periportal oedema) like fluid overload, trauma, hepatitis, liver transplantation etc. (Figure 15). It is imperative to differentiate these causes from neoplastic aetiologies, in which certain morphological features of periportal hypodensity may help. In periportal oedema due to non-neoplastic causes, hepatic involvement is diffuse and the periportal halo is thin and sharply marginated with regular or symmetrical contour. On the contrary, periportal hypodensity resulting from neoplasm is usually focal or segmental in distribution and asymmetrically thickened with infiltrating margins.
A vast majority of neoplastic as well as non-neoplastic conditions can affect the periportal space, resulting in periportal hypodensity. By itself, periportal hypodensity is a non-specific finding. However, it may herald the presence of a sinister aetiology either within the liver or at other extrahepatic sites like the lesser omentum, peritoneum or adjacent organs. Arriving at a definite diagnosis is nonetheless a multidisciplinary approach, additionally requiring clinical and histopathological correlation. Radiologists need to be familiar with these imaging findings to suggest as well as curtail the differential diagnosis.