Forums SGRT – Best Practice Head and neck setup Reply To: Head and neck setup

Marko Laaksomaa

Hello Helga
Thank you very much of your feedback. Neck is a complex treatment site for radiation therapy in several ways. It is a site, which demands high accuracy to protect the organs at risk, such as parotid and submandible glands and medulla. As well, shrinking of the body outline due to weight loss and tumor shrinking may cause underdosage of the target. Volume changes and/or displacement of the named structures typically leads to re- planning to optimize local control, overall survival and quality of life.
In addition to changes in internal soft tissue anatomy, head and neck patients have daily variation in bony structures. Bony regions best demonstrate whether the posture of the patient is good or not. This is something that we can affect with fixation, positioning and with good workflows in making the masks. In the literature, this relationship between the bony sub-regions around and inside large neck PTV is firstly and best demonstrated in the study of van Kranen et al. For C1, C7/TH1, mandible and occiput bone we have 5 mm action level after daily image guidance in our unit. The reason that we do not accept larger than 5 mm residual errors in bony structures is from this part to save the organs at risk and to ensure correct hitting to the target.
If I consider the posture things between AlignRT and 3 point open masks and 5 point closed masks with laser setup then. Inside 5 point closed mask systematic shoulder position errors seems to be more difficult to evaluate and daily correct than with 3 point open masks with AlignRT and postural video. FaceROI and positioning the pitch delta to zero error with AlignRT, seems to lead to excellent mandible accuracy, if patient bites the teeth similarly at the CT and at the linac and if the systematic errors in the DICOM are corrected with taking the new reference surface for the setup, when needed. Good headrest is important in all the cases and using time to find a good headrest at the CT seems mandatory, since I do not for example know, is it possible with SGRT to detect small spine pitch errors at the positioning. Small spine pitch is typical, but not the main problem with these neck patients with 5 point closed masks. May be deformation workspace could be useful to detect these errors with colors. We have not had such large systematic spine pitch errors yet, so difficult to say. With AlignRT, we have used Orfit 3 point open face hybrid masks, which are rather robust. The good thing is that typically head is in good posture in the first place with those masks (rotation deltas below 1°) after automatic send to couch with Truebeam, but if it is not, the head posture seems difficult to correct inside the mask from rotations and rolls part. To remove the mask and position the head without mask may improve the positioning, but may as well force the head to the same incorrect rotations after installing the mask back onto patients face. AlignRT 6D correction is not the solution in neck cases, since faceROI is NOT demontrating the whole area of the large neck PTV. Possibly, for this head positioning problem and to make patient feeling more comfortable, to use even more minimal mask material only to support the chin (which is done in USA for examle), seems justifiable. One thing to mention is that 3 point open mask positioning is slightly more time consuming, since with 5 point closed masks with laser setup, we only need to check the straightness of the patient with lasers, check that the head/mandible and shoulders are in their correct location inside the mask, then use Truebeam automatic function to go to planned couch values. This is the moment where the laser setup with 5 point masks ends, but as well the moment where positioning with faceROI and with postural video with AlignRT and 3 point open masks only begins. However, I think that everything is worth it with AlignRT, due to found excellent accuracy on the mandible with faceROI, small systematic errors of the shoulders, improved patient comfort (publication) and possibility for intrafractional control. Evaluation is in progress, but this is how it seems now.

Best regards Marko