The listed xray dose for a dental x ray is with 0.04 mSV really minuscule. For example those who received IMRT got typically around 66 Gray of radiation. To get the equivalent dose (SV) this is multiplied by Q(=1) and N(tissue specific ca 0.05) which gives you 3300 mSv.
If anything I am worried about the long term effect of the IMRT (or even CT scans)... but not a dental xray.

M

Partial glossectomy (25%) anterior tongue. 4/6/07/. IMRT start @5/24/07 (3x) Erbitux start/end@ 5/24/07. IMRT wider field (30x) start 6/5/07. Weekly cisplatin (2x30mg/m2), then weekly carbo- (5x180mg/m2). End of Tx 19 July 07.