I don't think that's it. I live in Houston about 2 miles from I-10. On the west side of downtown I-10 is known as "The Katy freeway" (cuz it goes to Katy, duh) or just "The Katy". I'll call it "The Katy" or I-10 (or perhaps, I-10 west), but never "The I-10".
I-610 makes a complete loop around the core of Houston and is an informal demarcation of 'city' vs. 'suburb' in Houston (although the city limits stretch far beyond The Loop). People here will call it "The Loop" or 610, but never "The 610". We have The Hardy, The Beltway, The Sam Houston, The Pierce Elevated, The LaPorte, The Southwest, The Katy, but we do not have The 610, The 290, The 59, or The 10 -- we have just 10, 59, 45, 290, 288.
No, this is a SoCal thang (not just LA). I spend about a week per month working in Sandy Eggo and they call I-5 "The 5" and I-15 "The 15 and my family in Orange Co. do as well.
This still goes nowhere in answering my query about The Cancer :-).
-j