Tropic of Cancer question

Miller gave the following explanation of why the book's title was Tropic of Cancer: "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."

Cancer focuses on his Paris years, whereas in Capricorn he reminisces about his American past. The physical distance is also accompanied by a contrast in tone. But, primarily, I understand the title choice geographically, without reading too much into the metaphor.