An 85-year-old male presented with progressive fatigue and melena over several weeks. His medical history was notable for advanced age with overall frailty but no significant comorbidities. Physical examination revealed poor general condition with pallor and mild dehydration. No palpable lymphadenopathy, hepatomegaly, or splenomegaly was detected on initial examination.

Laboratory evaluation demonstrated severe anemia (hemoglobin 8.1 g/dL, hematocrit 27%) with significant leukocytosis (20.9 × 10⁹/L) and marked monocytosis (46%). Platelet count remained normal (181 × 10⁹/L). Additional findings included hypoalbuminemia (28.5 g/L), elevated LDH (218 U/L), and moderate renal impairment (creatinine 1.23 mg/dL, eGFR 53 mL/min).

Endoscopic evaluation revealed erosive pangastritis with antral and duodenal ulcers. Colonoscopy identified a 3.5-4 cm ulcerative, polypoid mass in the cecum with additional rectal involvement prompting biopsy.

Histopathological examination of gastrointestinal biopsies confirmed mantle cell lymphoma with characteristic immunophenotype: CD20(+), Cyclin D1(+), SOX11(+), BCL2(+), and CD43(+) with negative CD3, CD5, and CD23. The Ki-67 proliferation index was 20%, indicating moderate proliferative activity.

PET-CT staging revealed extensive disease with widespread lymphadenopathy involving cervical, axillary, mediastinal, retroperitoneal, and pelvic regions. Gastrointestinal involvement showed intense FDG uptake (SUVmax 12.1) in cecum and rectum. Diffuse hepatic and splenic involvement was present along with diffuse bone marrow uptake, establishing stage IV disease.

Given the patient's advanced age (85 years), frailty, history of gastrointestinal ulceration, and moderate renal impairment, intensive chemotherapy regimens were deemed inappropriate. Treatment was initiated with rituximab monotherapy (626 mg every 28 days) with antiemetic prophylaxis. BTK inhibitor therapy was considered but deferred due to high bleeding risk given active gastrointestinal ulceration.

Supportive care included proton pump inhibitor therapy and red blood cell transfusions as needed. The patient demonstrated good tolerance to rituximab therapy with early symptomatic improvement and stabilization of hematological parameters.

This case illustrates several important aspects of MCL management in elderly patients. The presentation with gastrointestinal bleeding and extensive disease is typical for MCL, which frequently involves the GI tract at diagnosis. The moderate Ki-67 proliferation index (20%) suggested less aggressive biology, supporting a less intensive treatment approach.

The decision to use rituximab monotherapy reflects the growing recognition that treatment intensity must be individualized based on patient fitness and comorbidities. While intensive regimens like hyperCVAD or Nordic protocols achieve superior outcomes in younger patients, they carry prohibitive toxicity in frail elderly populations.

Rituximab monotherapy has shown activity in MCL with response rates of 40-60% and manageable toxicity profiles, making it suitable for elderly, frail patients. The early tolerance and symptomatic improvement observed support this approach.

MCL management in elderly, frail patients requires individualized treatment decisions balancing disease control with quality of life. Rituximab monotherapy represents a reasonable option for patients unsuitable for intensive chemotherapy, providing disease control with acceptable toxicity. This case demonstrates the feasibility of this approach in carefully selected patients.