Extranodal lymphoma refers to non-Hodgkin or Hodgkin lymphoma involving organs other than lymph nodes,
thymus,
pharyngeal lymphatic ring,
tonsils and spleen.
Every organ can be affected by lymphoma.
The diagnosis of extranodal lymphoma is crucial as the treatment is mainly based on chemotherapy and radiotherapy,
meaning non-surgical.
The radiologist should keep in mind that lymphoma is a great disease imitator and be familiar with the imaging characteristics of lymphoma for an accurate diagnosis and an appropriate disease management.
In general,
lymphamatous involvement of solid organs appears as solitary or multifocal lesions or as a diffuse infiltration.
These lesions are homogeneous,
with ill-defined margins and,
after contrast enhancement,
there is usually mild homogeneous enhancement.
However various patterns of enhancement can be seen depending of the organ involved.
A heteregeneous enhancement may appear in the presence of necrosis.
Calcifications are rare before treatment.
On ultrasound,
the lesions are hypoechoic with no posterior acoustic enhancement,
which confirms their solid nature.
On CT images,
lymphoma is hypodense in comparison to normal parenchimal tissue and,
after contrast administration,
mild enhancement or wash out can be observed.
On MRI,
lymphoma is usually iso-hypointense on T1,
moderately hyperintense on T2 and shows restricted diffusion.
One characteristic of lymphoma is that it has a specific infiltrative pattern,
meaning that,
on one hand,
it does not respect anatomic boundaries; and,
on the other hand,
it can infiltrate the vascular structures without invading them (Fig.1).
Another key feature is that lymphoma has minimal or mild mass effect for its size.
CT is the preferred imaging tool of diagnosing and staging lymphoma because it permits assessment of size,
shape,
extension and relation with surrounding structures.
However,
CT is not as sensitive as MRI imaging in detecting meningeal,
spinal cord or brain lymphoma.
Ultrasound is useful for detecting focal lesions of solid organs but it is not used as a routine tool in the diagnosis of lymphoma or staging of lymphoma.
FDG PET/CT is a hybrid imaging tool useful in assessing most lymphoma subtypes. National Comprehensive Cancer Network (NCCN) guidelines recommend FDG PET/CT in primary staging,
early or final evaluation response in lymphoma patients.
However,
it should be noted that not all lymphoma subtypes are FDG avid,
as in MALT lymphoma.
One recent study indicates that FDG avidity is correlated with Ki-67 index; according to this study,
only patients with Ki-67 > 15% have FDG-avid lesions [1].
While it is true that definitive diagnosis requires biopsy,
imaging plays an important role in suggesting the diagnosis of lymphoma in order to avoid unnecessary surgical treatment.