No need to get your globe out: Tropic of Cancer isn't about the Northern tropic.
Instead, it's a little play on words. We can actually look to Miller himself for an explanation of the title: "It was because to me cancer symbolizes the disease of civilization, the endpoint of the wrong path, the necessity to change course radically, to start completely over from scratch" (source).
To Miller, Paris at the time was a place of great transition. Disease, death, and poverty were overwhelming culture, which is why you'll notice so many references to syphilis, excrement, bugs, and general disease. But is the message to use a condom and carry a nit comb with you? No. Embrace it, Miller would say—turds and all. Maybe the cancer will burn away all of the excess and greed and the world will have itself a rebirth.