ESTRO 2024 - Abstract Book

S5493

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

• 7 patients, of which 4 larynx carcinoma, 1 oropharynx carcinoma, 1 parotid carcinoma and 1 hypopharynx carcinoma

The DSPS developed by MacroMedics BV (Moordrecht, The Netherlands) was used in this pilot. Daily CBCT scans were acquired and online corrections were performed (XVI, Elekta, Stockholm, Sweden). For the first three fractions and once a week, an additional CBCT was made immediately after the treatment delivery. These CBCTs were retrospectively registered with the planning CT with focus on the cervical vertebrae through a clip-box, using a clip box around the cervical vertebrae, disregarding the tumor position (e.g. larynx or oropharynx) although this is taken into account when performing the online registration. CBCTs acquired at the start of the fraction were used to determine the reproducibility of the patient positioning, i.e. the interfraction variation. As an indication of the stability of the patient during treatment, the intrafraction motion was determined as the difference between the CBCT scans acquired before and after the treatment delivery. For both inter- and intrafraction motion, mean and standard deviation of the displacements were calculated in RL, AP and CC direction for each patient. These were used to calculate systematic (Σ) and random (σ) displacements and rotations (Σ_trans, Σ_rot and σ_tra, σ_rot) for both closed and open face DSPS. The prior mask material, the Posicast 5-point mask with individual head-support, was determined to be both replicable and dependable in a study conducted by Houweling et al. in 2010. Our data was compared with the data from this study.

Results:

The interfraction setup variation measured as translations was overall larger for the open face than for the closed mask, while the rotations differences were small (figure 1). The largest difference is found in the AP translation. This resulted for closed mask and open face mask respectively in Σ_trans <1.9mm and <3.2 mm and Σ_rot was <1.9 degree and <1.2 degree for setup variation. The random translations and rotations were quite similar for closed and open face mask with σ_tra of <1.7mm and <2.4mm and σ_rot <1.1 versus <0.9 degree for setup variation.

The intrafraction translation and rotation differences were small (figure 2). The largest difference is found in the AP translation. This resulted for closed mask and open face mask respectively Σ_trans < 0.4mm and < 0.7mm, Σ_rot <0.3 and <0.6 degree. The random translations and rotations were similar for closed and open face mask with σ_tra <0.7mm versus < 1.0mm and σ_rot of < 0.6 and <0.7 degree for intrafraction motion for open and closed mask respectively.