What dose constraints would you use for head and neck SBRT in a previously untreated field?
The use of SBRT as a primary modality in patients who have not undergone previous irradiation and are not felt to be candidates for surgery or definitive chemoradiation varies institutionally, with some preferring to treat with hypofractionated regimens (particularly when they are candidates for chemo) and others using stereotactic RT. SBRT has the potential to provide durable control and is an option worth considering, particularly when the target is not in a region associated with higher rates of toxicity (e.g., posterior pharynx).
We published a series of 66 HNC patients treated with SBRT and provided a table of constraints. Here is the link: Gogineni et al., PMID 32691496.
There are many other single institution series reporting outcomes with the use of 'Primary SBRT'. I've listed a few below. Most do not provide an extensive list of constraints, outside of the report by Vargo et al., PMID 25157336, which recommends keeping the spinal cord below 8-10 Gy and keeping the rest of the OAR's ALARA.
Other studies for reference:
Kawaguchi et al., PMID 21764492
At my current institution, we set our constraints for SBRT in unirradiated H&N by using BED calculations with an alpha/beta of 3 for normal tissues. So the constraint will vary if you use 1 vs 3 vs 5 fractions. I think others follow a similar approach, or use some variation of the validated constraints for organs with similar alpha/beta ratios (eg. use spinal cord constraint that is used for SBRT of other sites; use a/b of peripheral nerves when setting brachial plexus constraints; use a/b of great vessels for carotid a., and so on).
There is a recently published meta-analysis (Malik et al., PMID 33665492) that reports the following from the literature: "Only 1 study reported OAR constraints, with goals of keeping thyroid mean dose <50%, max carotid dose <10%, spinal cord <10%, and skin <10% of prescribed doses, while another reported constraints were used as reported in the survey of current practices for HNC SBRT (Karam et al., PMID 27842456). None of the studies mentioned brachial plexus dose constraints."