How do you approach external beam RT to the thyroid bed for resected recurrent thyroid cancer in a patient with previous history of 150mCi of radioactive iodine?
This is a terrific question and speaks to ALARA; I know of no data or dosimetric capacity to accurately estimate a biological effect (dose-wise) from an unsealed radiopharmaceutical on OAR. Keep in mind that dose limits for I-131 are due more on bone marrow toxicity than local effects.
Previously, we had treated a pregnant woman with high risk thyroid cancer (with I-131), and using computer modeling, we estimated the potential dose to her uterus. But again, it was all pretty theoretical.
Just like there are no published dose constraints on the carotid arteries (despite the argument made that carotid sparing RT for glottic primaries has to be better than opposed laterals), I would argue that one should ask their dosimetry team to minimize dose to cord, larynx, major blood vessels of the head and neck (don't forget the posterior circulation), and esophagus when offering additional dose.
My quick review of the literature offers no assistance on this issue so, if there are case reports of toxicities experienced in this thyroid cancer patient population after I-131 followed later by XRT, please don't be shy. It's an area needing a body of literature to guide all of us.
I think the first question is, would you sacrifice target coverage? Without knowing the specifics of the case...I highly doubt you would.
Working from there, you can't go wrong with minimizing the dose to cervical esophagus.
This is a different situation from a spinal cord that received EBRT and you are deciding on underdosing your target.
In conclusion, I think most people do not take prior RAI into account for current EBRT plans.
I have not considered prior RAI a contraindication or complicating factor when planning. Would use routine conventionally fractionated normal tissue constraints, with a close eye on optimization to make sure OARs with potential prior RAI dose are being kept ALARA (which is good practice anyway), but aside from that, I don't consider it a big issue. There are reports of the routine use of adjuvant EBRT after RAI that don't mention the theoretically increased risks/alteration in their RT plan to account for RAI, a few examples below.
The absorbed radiation dose with 150 mCi as suggested by this study, which used several techniques to model the absorbed dose, to surrounding structures of interest are probably on the order of 200 - 400 cGy (Shahbazi-Gahrouei and Ayat, PMID 25709539). Although measurements of absorbed dose have their limitations, this is still informative. An important point to think about is that the beta particles with I-131 travel about 3mm in tissue so most of the dose will be confined to the thyroid bed.
I agree with @Dr. First Last and @Dr. First Last that you would not sacrifice coverage in this setting and would just keep the ALARA principle in mind.
Ignore the RAI.