Radiologists are essential for diagnosis, staging, and assessment of treatmen response.

Ultrasound, CT, and MRI are all modalities of choice for diagnosis of this condition although usage of CT is limited in this population due to ionizing radiation.

PET CT is very important in staging and provides functional information about the metabolic buried of the disease.

Pediatric lymphomas are staged using the International Pediatric Non-Hodgkin Lymphoma Staging System (IPNHLSS) for NHL.

Knee lymphoma

Musculoskeletal involvement is present in 7% of all NHL patients, mainly as metastatic disease. Infrequently NHL primarily affects the bone marrow of soft tissues.  Rarely, NHL mostly involves the joint's space. In a case of joint space involvement, the knee is the most commonly affected  (fig.1)

Patients with NHL of a joint space usually present with symptoms suggestive of an inflammatory arthropathy or a low-grade infection. The synovium of the joint space is commonly affected through direct extension from the bone.

Radiography may show para articular osteolytic lesions, and in some cases, it is insensitive with soft tissue swelling as the only finding.

MRI allows a complete assessment of the bone marrow and soft tissue involvement. Joint space/knee lymphoma is having nonspecific MRI signal intensity (T1W hypointensity, T2W hyperintensity) (figs 2 and 3). Usually, these tumors are followed by lymphadenopathy, which differentiates them from soft tissue sarcomas (fig 5)

Ovarian lymphomas

Because of differences in treatment and prognosis, the distinction between primary and secondary lymphoma is very important.

Patients are usually present with abdominal or pelvic pain or a mass (fig 6)

The ovarian lymphoma's appearance can be a nodular or lobular ovarian surface mass with an intact capsule. Solid ovarian lymphoma can contain areas of cystic necrosis or hemorrhage (fig 7)

Imaging findings in ovarian NHL are nonspecific.

On CT the ovarian lymphomas are usually presented as a large, homogenous, hypovascular mass (fig 8). The ultrasound appearance is nonspecific, with homogenous, hypoechoic, and mild vascular mass. On MR the ovarian lymphomas are presented as a large mass, that is usually hypointense on T1WI, intermediate to high signal intensity on T2WI, with mild to moderate enhancement.

The diagnosis of ovarian lymphoma could be considered in a case of bilateral ovarian homogenous mass.

Small bowel lymphomas

 The gastrointestinal tract is involved in one-third of children with NHL. The small bowel, the cecum, and the appendix are the most commonly involved sites. Nodular lymphoid hyperplasia may result in ileocecal intussusception.

Infiltration of the submucosal layer by lymphoid infiltration results in the replacement of the muscular layer with lymphoma cells and subsequent „aneurysmal dilatation“ of the bowel. Multifocal involement is not uncommon.

On ultrasound, the small bowel hypoechoic wall thickening with loss of normal wall stratification is usually seen. The loss of bowel wall stratification is important for differentiation from similar benign conditions, such as Crohn's disease.

At CT, the affected bowel loop wall is thickened with minimal postcontrast enhancement (figs 8 and 9). Thickening of the bowel wall is followed by significant mesenteric lymph node enlargement (fig 10)

At MRI, small bowel NHL shows homogenous intermediate signal intensity on T1WI and high signal intensity on T2WI, with postcontrast enhancement and restriction at DWI/ADC.

Anterior mediastinum

It is the second most common location for primary NHL.  The thymus is usually the origin of the primary NHL of the mediastinum.

Patients are commonly clinically presented with dyspnea, cough, or with superior vena cava syndrome.

The radiograph shows mediastinal widening and compression on the trachea and loss of retrosternal clear space.

At CT, anterior mediastinal NHL appears heterogeneously, with necrotic and cystic areas (figs 15 and 16).

Brain lymphomas

In childhood, primary brain lymphomas are rare. They consist of 1% of all patients with primary brain lymphomas. Usually, they are presented in immunodeficient adolescents.

Diffuse large B cell lymphoma (DLBCL) is the most common type of primary brain lymphoma. It has a better prognosis in children in comparison to adults.

Primary CNS lymphoma presents with symptoms of increased intracranial pressure, and motor or sensory deficit.

Usually, focal solitary brain masses, that involve cerebral hemispheres are present in 50% of cases. The Corpus callosum, ventricular wall, and choroid plexus are frequently involved.

At CT, primary brain lymphomas are hyperdense in comparison to the cortex, with insignificant peritumoral edema, and restriction on DWI/ADC.

Primary cutaneous lymphomas

Primary cutaneous lymphomas are rare in children. Only sporadic cases have been published. Imaging is having a limited role. On CT and MRI there is a nodular thickening of the skin (figs 19 and 20), which is hypermetabolic on PET/CT.