Differential Diagnosis

The patient presents with right lower abdominal pain, a history of melanoma, and imaging findings suggestive of a diffuse inflammatory process in the right pelvic sidewall. The following differential diagnoses are considered:

• Single most likely diagnosis
  • Metastatic melanoma: The patient's history of desmoplastic melanoma, a subtype known for its aggressive behavior, and the presence of a large abnormal mass in the right inguinal region, make metastatic melanoma a strong consideration. The imaging findings of a diffuse inflammatory process and enlarged lymph nodes also support this diagnosis.
• Other Likely diagnoses
  • Appendicitis: Although the appendix is not distended, the presence of air foci within it and the patient's symptoms of right lower abdominal pain could suggest appendicitis. However, the absence of other typical symptoms such as nausea, vomiting, and fever makes this diagnosis less likely.
  • Diverticulitis: The patient's age and symptoms could also suggest diverticulitis, but the imaging findings do not support this diagnosis, and the patient's stool tests were negative for common pathogens.
  • Lymphoma: The presence of enlarged lymph nodes and a large abnormal mass in the right inguinal region could suggest lymphoma, but the patient's history and imaging findings do not strongly support this diagnosis.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Retroperitoneal sarcoma: Although rare, retroperitoneal sarcomas can present with similar imaging findings and symptoms. Missing this diagnosis could have significant consequences, as sarcomas can be aggressive and require prompt treatment.
  • Intra-abdominal infection: The patient's symptoms and imaging findings could also suggest an intra-abdominal infection, such as a abscess or infected lymph nodes. Missing this diagnosis could lead to sepsis and other serious complications.
• Rare diagnoses
  • Castleman disease: This rare lymphoproliferative disorder can present with similar imaging findings and symptoms, but it is much less likely given the patient's history and presentation.
  • Retroperitoneal fibrosis: This rare condition can cause similar symptoms and imaging findings, but it is typically associated with other systemic symptoms and laboratory abnormalities, which are not present in this case.

Management Modalities

The management of this patient will depend on the final diagnosis. However, based on the current information, the following steps could be considered:

• Biopsy and histopathological examination: The biopsy of the right inguinal node has already been performed, and the results are pending. Further histopathological examination, including immunohistochemistry and molecular studies, may be necessary to confirm the diagnosis.
• Imaging studies: Further imaging studies, such as MRI or PET-CT, may be necessary to better characterize the extent of the disease and to guide treatment.
• Surgical consultation: Depending on the final diagnosis, surgical consultation may be necessary to discuss potential treatment options, such as resection of the tumor or affected lymph nodes.
• Medical oncology consultation: If the diagnosis is confirmed to be metastatic melanoma, medical oncology consultation will be necessary to discuss potential treatment options, such as immunotherapy or targeted therapy.