瘤

Outer sickness radical - a person leaning against a bed, the standard marker for any disease character. Anchors 瘤 in the medical family with 病 illness, 痛 pain, 疫 epidemic, 痒 itch. Wraps around the right and bottom of the phonetic, like a sickbed enclosing a patient - the visual frame for any condition affecting the body.

Inner 留 supplies the sound liú with no drift, identical pinyin. 留 itself means 'to stay, remain,' which carries a strong semantic bonus: a tumor is a growth that stays, a lump of cells refusing to leave. Sound and meaning lock together cleanly - an abnormal mass that lingers.