What dose-volume constraint should be used for the normal brain parenchyma in a patient receiving 5-fraction SBRT?

This is tough, because most of the literature is based upon single fraction SRS for 1-3 lesions. There are no one criteria for brain doses for multi-session treatments as stated above. Tumor coverage is most paramount, and I agree with the need to respect the adjacent structure doses and to optimize conformity of the treatment. So the reason for multisession must be taken into account.

If it is for a small lesion that is close to brainstem or to optic structures, then the V12 should be low and similar to what we may consider acceptable for single fraction, or even extrapolation to a V16/V20 or similar.

For larger lesions and with higher overall doses, this will break down. And the guidelines may not be achievable. Trying to get 30 Gy in 5 fra...

We do not have established thresholds for V12-V16 for fractionated SRS and adverse events especially limiting dose evaluation to the "normal" brain.

We all need to remember, that the optimal models for estimating toxicity post-SRS include the target volume. YOUR TARGET IS YOUR TOXICITY. So first clinically, you need to select cases appropriate for SRS. Estimates of adverse radiation effect based on V10-12 are available, such as Sneed et al which you radiobiologically can extrapolate to V14-16 in in the 5 fraction setting

https://www.ncbi.nlm.nih.gov/pubmed/25978710

The gradient is your surrogate for a quality plan and while there is some data on fractionation vs single fraction for adverse radiation effect and control, no studies really evalu...

Agree with @Dr. First Last. Prioritize target and aim for V12 < 10 cc if possible. If not, don’t sweat as the UK consortium and AAPM TG 101, though older, did not have a 5 fxn SRS constraint for normal brain. Should always try to aim for as low as possible though.

If you want a metric to make you feel better though, you can use V4 Gy <20-23 cc based on Phase 2 data from Germany: https://www.ncbi.nlm.nih.gov/m/pubmed/16978720/

I don't have a hard dose-volume constraint for normal brain parenchyma. Depending on the case (brain mets vs benign brain tumors) I would prioritize target coverage, selectivity and gradient/conformity index. The more likely I'm able to keep my isodoses tight the less spillage into normal brain parenchyma. I don't recall a case where normal brain parenchyma was part of my treatment planning algorithm but instead I took my indexes into account. There isn't a 5-fraction constraint for brain parenchyma from the UK Consensus guidelines (https://www.ncbi.nlm.nih.gov/pubmed/29033164) or for RTOG/NRG protocols I'm aware of. I would try to keep my focus on target volumes while keeping normal brain dose at a minimum.

After the publication of these two articles, are people using these for V25 and V30 for brain? What if unable to meet constraint?

BrainV25 > 16 cm³ or BrainV30 > 10 cm³

Andruska et al., PMID 33310010.

Faruqi et al., PMID 31928848.