Please tell me which organ does not affect by cancer
Secondary metastasis to brain is most common after lungs, skeletal bones and liver.
I'd say Spleen. Splenic sequestration doesn't allow most cancer cell formation. Also due to splenic sinusoids the secondary metastatic cells are unable to get lodged inside the organ and proliferate. However most cancers do cause splenomegaly but there's never an isolated case of carcinoma of spleen or even secondary metastases to spleen.
I had heard small intestines but I agree, it may be rare but any cell could mutate and proliferate