一本词典对cancer的解释是这样的:a serious disease in which harmful cells spread through the body.

就是定语从句,理论上,完全可以用where 代替.
Cancer is a serious disease【 in which harmful cells spread through the body.】
癌症是一种【有害细胞遍布全身的】严重的疾病.首先感谢你的回答。我觉得只能用in which而不能用where代替，如果先行词是地点名词是可以用where的。首先我不明白的是in which在从句中做什么成分？宾语？状语？其次in在从句中和什么做搭配？(比如This is the house which i was born in.=This is the house in which i was born.in和从句中的born搭配)in which在句子中作状语；in和什么词搭配，你得看先行词。你给的例句中，先行词是house，be born in the house. 虽然where的先行词通常是表示地点场合的名词，但疾病作先行词也不少见，比如 Rheumatoid arthritis is considered an autoimmune disease where the body attacks healthy cells.又： AIDS is a disease where the body is unable to fight off many infectious diseases as it normally could.其实关系副词（where， when, why）引导定语从句时，在意义上常常相当于一个“介词+ which”结构。where=in which 或at which; why= for which; when=at/on/during/in which 所以理论上是可以替代的。