Session Item

For radiation treatment of some head-and-neck cancer patients bite blocks are necessary. The function of the bite block is to stabilize the position of the jaw, tongue and/or lip. A suboptimal positioned bite block at the planning-CT scan degrades treatment quality and causes extra workload at the linac, because the bite block position cannot accurately be reproduced. The purpose of this study was to estimate the reproducibility of the bite block in an early stage of the treatment using CBCT scans.

Before the planning-CT scan a short CT scan was performed. In sagittal and transverse views it was examined if the bite block was properly positioned. If the bite block was not positioned correctly, it was repositioned, and the short scan was performed again. When the RTTs were not satisfied, a radiation oncologist was consulted for adjustment of the bite block. 30 patients were included in this study. For the first 10 patients the short CT scan was assessed by 4 RTTs with different expertise (CT, mouldroom, IGRT). After this initial training phase the short CT scans of the remaining 20 patients were judged by 2 CT RTTs.
Prior to the daily treatment for each patient an online CBCT scan was made and registered to the planning-CT scan. At the end of treatment all CBCT scans were analyzed and checked whether the position of the bite block was equal to the planning-CT scan.

In 7 out of 30 planning-CT scans (23%) the bite block was repositioned due to the short CT scan. For these  planning-CT scans with problems , in 83 out of 161 CBCTs (51%), the bite block was incorrectly positioned. For the 23 planning-CT scans without problems  in only 7 out of 642 CBCTs (1%) the bite block was incorrectly positioned.
We conclude that, when the RTTs had to reposition the bite block at the planning-CT scan, the RTTs had to reposition the bite block at the linac many times, leading to additional CBCTs. A frequently found problem was that due to the lack of teeth, patients had no grip on the bite block and the bite block could be positioned in various ways. Also it was found that if the bite block was difficult to place at the planning-CT scan, it appeared that during treatment it was not possible anymore due to toxicity.

By introducing the short CT scan for head-and-neck cancer patients, the RTTs at the planning-CT became more aware and critical with regard to placing and checking the position of the bite block.Also adjustment of the bite block at the planning-CT gave a better reproducibility at the linac. This caused less differences between planning-CT and CBCT scans. The position of the bite block became more representative for the whole treatment.  In addition we have gained better insight and understanding when to expect difficulties with the bite block at the linac. These patients are closely monitored by the RTTs.