To spawn cancer get some Quus (a type of fish) and bring them to Kuftal Tunnel. Inside the entrance from Altepa-go downstairs and to the left where all the haunts are. There are two ???s there-you're lookin for the one up north on a mini rock platform.
This specific ??? pops either a Robber Crab or Cancer each time you trade a Quus to it (the robbers wont link but they do aggro to sound and Cancer is no different plus a lil higher lvl). After each trade it takes about 15 minutes for the ??? to come back.
Word has it that Cancer spawns about 10% of your quus trades, and drops the arondight 2-5% of the time under normal conditions...(almost makes ya wanna cry huh?)
you dont have to kill the crabs that pop from your trade. If you leave a robber crab alone after it's popped it will disappear in like aminute or two...not sure if cancer does the same-but my guess is yes...and that no one wants to chance it to find out, so be prepared to claim it before someoen else or beore it dissapears.