Lymphoma: This is a common cause of both lymph node enlargement and infiltration of the fat in the abdominal wall, particularly if the lymphoma is of a type that tends to involve the abdomen, such as diffuse large B-cell lymphoma or follicular lymphoma.

Metastatic Cancer: Cancers from various primary sites (e.g., breast, lung, melanoma) can metastasize to both the abdominal lymph nodes and the fat within the abdominal wall, leading to the observed findings.
Sarcoma: Although less common, sarcomas (e.g., liposarcoma) can arise within the fat of the abdominal wall and may also involve the regional lymph nodes.
Inflammatory or Infectious Processes: Conditions like tuberculosis or other granulomatous diseases can cause lymphadenopathy and may also involve the fat, although this is less common.

Desmoid Tumor (Aggressive Fibromatosis): Although rare, desmoid tumors can infiltrate the abdominal wall and may be associated with lymph node enlargement. They are important to diagnose because they can be locally aggressive and require specific management.
Retroperitoneal Fibrosis: This condition can cause infiltration of the retroperitoneal fat and may also lead to lymph node enlargement. It's crucial to diagnose due to its potential to cause significant morbidity by encasing and obstructing vital structures.

** Erdheim-Chester Disease**: A rare form of histiocytosis that can cause infiltration of various tissues, including the fat of the abdominal wall, and may also involve lymph nodes.
Lipoblastoma/Liposarcoma: Rare tumors of the fat tissue that can occur in the abdominal wall and potentially involve the lymph nodes.
Castleman Disease: A rare lymphoproliferative disorder that can cause lymph node enlargement and, in some cases, may involve the surrounding fat.