No, not every tumor is visible. The tumor could be seen under a tooth, in the tonsils or in the base of tongue areas. Also cancerous cells are not visible with the naked eye.

Here is more info from the main OCF site.

OCF main pages, Oral Cancer Facts

Diagnosis from the main OCF site


Christine
SCC 6/15/07 L chk & by L molar both Stag I, age44
2x cispltn-35 IMRT end 9/27/07
-65 lbs in 2 mo, no caregvr
Clear PET 1/08
4/4/08 recur L chk Stag I
surg 4/16/08 clr marg
215 HBO dives
3/09 teeth out, trismus
7/2/09 recur, Stg IV
8/24/09 trach, ND, mandiblctmy
3wks medicly inducd coma
2 mo xtended hospital stay, ICU & burn unit
PICC line IV antibx 8 mo
10/4/10, 2/14/11 reconst surg
OC 3x in 3 years
very happy to be alive smile