Michael Tallhamer, MSC, DABR
We use a combination of methods.
Your tolerances should reflect your setups, treatment techniques, chosen immobilization techniques and practicality of the sites in question. That makes taking a set of tolerances from one site and slapping them in your system not a easy as one would like.
Your tolerances are configured on a per treatment site basis (per the software configuration setup). Ours default site specific tolerances are configured to what we have determined is the best statistical range for each site based on an iterative process. We started off with a base set of tolerances we determined were acceptable for each site within the software configuration. We felt the overall ranges and tolerance levels for many of the sites within the software as constituted when you install it (defaults) where not clinically acceptable so we start by setting up the system to what we felt would be clinically acceptable and within the abilities of the software.
We then periodically review the stats on a number of patients from each treatment site category and slowly fine tune the default tolerances for each site. This allows us to apply a best practice set of tolerances as defaults to each of our patients. This process for example has taken us from our original guess of 5mm / 3 degree tolerances for our DIBH left breast patients down to 3mm / 2 degree tolerance allowing us to maintain a set level of quality without putting the therapists under stress to hit an impractical goal during setup.
You can also set the tolerances for each patient (or each ROI in the newest version of the software) based on any special circumstances that might exist for that specific patient. We often do this for SRS/SBRT/SRT patients that have specific margin constraints based on anatomy or other confounding variables which might make the default tolerances less than ideal.
In our case the patient would originally be assigned the standard default set of tolerances based on site selection at the time of import (the tolerances that have been statistically define as above). Physics would then adjust these tolerances as part of their QA process in order to address any special concerns for the patient in question. We like to try and set the tolerance at approx. one half the margin for these special cases if possible as long as the site itself doesn’t preclude use of such tight tolerances (e.g. large breathing amplitudes or body morphology concerns). So for a lung SBRT with symmetric 5mm PTV margins around a 4DCT defined ITV we would like to see the tolerances in AlignRT to be 2-3mm if practical,
Hope that helps