ESTRO 2024 - Abstract Book

S4289

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

Maskless surface-guided head and neck cancer radiotherapy: Setup accuracy and intra-fraction motion

Marion Essers 1 , Lennart Mesch 2 , Maaike Beugeling 2 , Janita Dekker 2 , Willy de Kruijf 1

1 Institute Verbeeten, Medical Physics and Instrumentation, Tilburg, Netherlands. 2 Institute Verbeeten, Radiotherapy, Tilburg, Netherlands

Purpose/Objective:

Surface-guided radiotherapy (SGRT) is applied to improve patient set-up and to monitor intra-fraction motion. Head and neck cancer (H&N) patients are usually fixated using 5-point thermoplastic masks, that are experienced as uncomfortable or even stressful, especially for claustrophobic patients. Therefore, the feasibility of irradiating H&N patients without a face mask by using SGRT was examined. The patient setup accuracy, intrafraction motion, the resulting CTV-PTV margin, as well as the patient friendliness of irradiation with and without a mask was investigated.

Material/Methods:

Nineteen H&N patients were included in a simulation study. Once a week, before the standard treatment using a 5 point mask, a maskless treatment on a standard head rest was simulated, using SGRT for setup and intrafraction motion monitoring. Initial patient setup accuracy was determined using CBCT images before the setup. Intrafraction motion was determined by comparing the CBCTs before and after the (simulation) treatment and by the SGRT intrafraction motion data. The CTV-PTV margin for intrafraction motion was calculated. Using patient questionnaires with questions on discomfort, anxiety, pain, medication, etc, the patient-friendliness H&N irradiation with and without mask was determined.

Results:

Maskless setup with SGRT and CBCT was as accurate as with a mask. For all treatment fractions, the 6D anatomy match resulted in coverage of the treatment volume by the 3 mm PTV margin, both for treatments with and without a mask. For 19.3% of maskless simulation treatments and 9.1% of actual treatments in a mask, the 6D intrafraction motion was larger than 2 mm translation, 3 mm vector, and/or 2˚ rotations, which in future treatments will be the threshold for intervention th 2 . For 20 of the 21 fractions, a motion > th 2 would also have been found with SGRT. In 11 of 21 fractions, the patient position drifted gradually during the fraction and was still within th 2 for at least half of the simulation fraction. For 4 fractions, the position changed in and out of th 2 , also within th 2 for more than half of the fraction. For 5 patients, the position was within th 2 during the simulation treatment and only > th 2 during the second CBCT. The CTV-PTV margin correcting for intrafraction motion is 2.0 mm for maskless treatment without interventions, and 1.6 mm if the threshold th 2 is used for intrafraction setup corrections [1]. The total CTV-PTV margin also corrects for other uncertainties, such as initial setup errors, swallowing and respiratory motion. These other errors are identical for maskless and mask treatments, so the currently applied CTV-PTV margin of s 3 mm can also be applied without a mask. Sixteen out of 19 patients preferred treatment without a mask, while the 3 patients who preferred a mask did so since they feared motion without a mask and felt they had to concentrate more in order to lie still.

Conclusion: