Don,

Oral-Maxillofacial Surgery report:
2009 xray suggests increase in periodontal ligament with potential furcation involvement. Early signs of apical pathology associated with #17 may be result of radiation induced ischemic changes. Patient is clinically negative. Suggest endodontic evaluation, observation, proceed when symptomatically mandatory. RO agrees. Endodontic eval on #17 was negative. Several other root canals performed.

Since 2009, teeth cleaned and checked every 4 months. Periodic bite wing and panorex xrays. No changes noted on #17.

9/2013, dull aching pain. Consultations with oral surgeon at Dana-Farber Cancer Center & local oral surgeon. Both agreed irreversible pulpitis & periapical periodontitis. I asked exactly what you are asking - why was this not picked up sooner if his teeth were being closely monitor? I was told that teeth sometimes have a smoldering infection that is not detectable in early stage. I don't know if that is really true.

I have read some studies that I will try to summarize and post tomorrow.