- Mon Sep 05, 2022 12:06 pm
I have collected some ideas here concerning 5- and 3 point open masks with AlignRT from RTTs point of view. With both mask types we use faceROI for monitoring. With 3 -point open masks we additionally setup the chest and shoulders with the aid of postural video feature. With both methods head positioning and thereby mandible location after setup seems accurate if mouth is closed similarly at the CT and daily at the linac. This is what literature shows in the SRS cases from skulls part. Now it seems that neck setup and intrafractional control with AlignRTs face ROI is what we want to do, if it is asked from RTTs here right now, and specifically with 5 -point (Orfit) open masks. Intrafractional control of the head and thereby mandible location control seems reliable with both mask types with AlignRT. Sometimes with AlignRT we see shift caused by the nodding effect during treatment; faceROI shows pitch and VRT displacement and chin goes dorsally due to relaxation. This has as well been noticed in the images taken in those cases right after deltas were exceeded. With some patients we have even relied on AlignRT only in the head posture correction strategy during treatment, after systematically seen the similarity between AlignRT data and the images taken during treatment. Then we have asked patient to lift a chin/head a little to reach original 0-1°/mm error in AlignRT face ROI deltas in pitch and in VRT. Some RTTs wonder is there as much intrafractional movement inside closed masks, where the nose has its own location molded with mask material, for example? The answer should be found from the literature. Good things with open 3 point masks are that patient could most possibly choose it, if they see 3- point open, 5- point open and 5 point closed masks side by side, due to smaller amount of mask material to tie or cover them on to fixation. The downside seems to be that we need to use more time to setup the shoulders and chest. Some RTTs may feel it unpleasant if we need to play with the shoulders or chest rotations at the setup for a long time. This is what sometimes happens. On the other hand, with 3 point open masks we have possibilities to setup the patient shoulders and chest based on new better reference surfaces taken at the linac, in the cases when patient has lost weight and body has changed due to that, or if for some other reason chest or shoulders have shown systematic displacements in the images. With 5 point masks we see only free space between body and mask material and to correct those kind of systematic errors is in theory more challenging. These kind of patients however often need re-planning. With 3 point masks intrafractional control of the shoulders is more reliable with postural video than with 5 point open masks, because mask material is not covering the chest and shoulders. On the other hand, at the setup, in normal cases chest rotation and roll correction with 5 point masks is easy to do/see, because shoulder mask should guide the upper body to near correct position and posture in the first place. With 3 point masks, digital mask is more challenging to setup from shoulders and chest part. It may be difficult to understand with postural video and with 3 cameras is there roll/rotation in the patient chest or only displacement on the shoulders? In our workflow with 3 point masks we have considered it as a good order to correct chest rotation before shoulder positioning, both based on postural video. DICOM surface seems not that informative from chest rotation part as surface taken at the linac, because it typically ends too cranially and often this SGRT reference surface is in use, taken after images including as small postural and position errors as possible. AlignRT shows us what to do, with additional chest ROI (not used in this hospital), deformation workspace, postural video etc, but do we have enough interest or time to begin to do it with minimal mask immobilization? Is it worth it? What does patient think of it if setup on some days takes much time? Is the advantage of minimal mask immobilization from patients perspective lost, if setup and thereby the whole fraction takes more time? These are the questions some may think. I think it is worth it and ok to patients if some days setup takes more time and we will finally learn how to do the setup fast and accurately, patient at a time. We should not have that busy schedule that we do not have on average 1-2 minutes more time to do the setup process with minimal mask immobilization. To eliminate systematic posture errors with new better reference surfaces for the setup, we typically need offline review evaluation and/or re- setup(s). Posture and position should be excellent in the images before acquiring those surfaces for the setup. We cannot think that we could roughly position the patient (with minimal mask immobilization) and think that we can then correct the intended setup posture errors with IGRT, because posture errors in neck RT cannot be totally corrected with IGRT, even though 6D couch is available. Patient posture can deform in several ways inside or near large neck PTV. One suggested question for the study to evaluate approximate intrafractional accuracy could be: To compare the difference in the patient posture in the CBCT images taken before and after treatment and to compare the errors in those images between the groups with different mask types with and without SGRT?
Best regards Marko