LAUSR

Purpose or Objective The demands on accuracy in radiotherapy have increased especially in stereotactic treatments using IMRT or VMAT techniques where margins to the PTV are as small as 1 or 2 mm, but also for non-stereotactic treatments. Commercial systems providing better geometrical accuracy in patient positioning than conventional standard three-point masks, entail high costs. Although translations can be corrected with a couch shift, rotations cannot be corrected with a standard treatment couch and need to be prevented as much as possible. Therefore we aim to investigate whether the use of a dental fixation created with an inexpensive and simple wooden spatula, will improve accuracy in patient positioning. Material and Methods In 40 patients receiving non-stereotactic cranial radiotherapy, 144 conebeam CTs (CBCT) were acquired prior to treatment. Twenty patients had a standard threepoint thermoplastic mask with a standard base (MacroMedics®, Waddinxveen, The Netherlands); next 20 patients had an identical mask and base, but with the addition of a dental fixation moulded by a wooden spatula, to create an extra point of fixation between the teeth rows. Patients were asked to bite gently on the wooden spatula during moulding of the mask to create an indentation in the mask for dental fixation. After cooling and hardening of the mask, the wooden spatula is removed. During the acquisition of the planning CT and all treatment fractions patients are instructed to bite gently on the indentation. All CBCTs were registered on bony anatomy of the skull. For patients with an online correction protocol, all data were included. For patients with extended NAL correction protocol, only the data of the first ‘NAL’-phase were included. Thereby, the position inaccuracy was calculated on position errors before a position correction was applied. Individual systematic (Ʃ) errors were calculated and analyzed with Levene test. Individual random errors (σ) were calculated and analyzed with the Mann-Whitney test. Results The table summarizes the group setup errors for both fixation systems. Most errors are smaller when using the three-point mask with dental fixation created with a simple wooden spatula compared to the three-point mask alone. Geometrical accuracy shows significant improvement in the systematic and random error for the rotation over the X axis and the random error for rotation over the Y axis. Conclusion Adding a dental fixation point to a standard three-point cranial mask by a simple wooden specula improves geometrical accuracy, particularly by reducing rotational errors. This may be of clinical importance, since rotational errors cannot be corrected by a standard treatment couch. Although the absolute errors are already small for the standard three-point mask, but given the small effort and the low additional costs of a simple wooden spatula, we decided to accept the mask with dental fixation as our standard for non-stereotactic brain tumor radiotherapy.