Purpose: The purpose of this work was to evaluate the daily alignment accuracy of pre-treatment cone-beam computed tomography (CBCT) scans for laryngeal cancer patients.
Methods: CBCTs from the 1st, 11th, and 21st treatment fractions were exported to the MIM software package for 20 laryngeal cancer patients treated with VMAT. Each CBCT was rigidly registered to the treatment planning CT separately with respect to bony and soft tissue alignment. The displacement of the larynx was defined as the difference between the bony and soft tissue alignments, recorded for each of three orthogonal directions (right/left, anterior/posterior, and superior/inferior). In addition, the magnitude of the soft tissue-based registration shift from MIM was compared to the clinically recorded shifts from the patient’s treatment to provide quality assurance (QA) of the therapist generated registration shifts.
Results: The average larynx displacements in the R/L, A/P, and S/I directions were 0.0+/-1.0, -0.4+/-1.4, and 0.6+/-2.5 mm, respectively, with a maximum of 2.6, 5.5, and 8.9 mm. For each direction, 95% of all displacements were within 2.3, 3.0, and 5.8 mm. 4 out of the 20 patients recurred locally; however, there is no appearance of correlation of local recurrence with large larynx displacements (greater than 5 mm). When aligned to the larynx via soft tissue-based registration, the average difference between the registration performed in this work and the on-treatment applied registration shift based on soft tissue alignment was 1.1+/-1.2 mm, indicating excellent agreement with shifts generated during the patient’s treatment.
Conclusion: Despite being advised not to swallow during CBCT acquisition and during treatment, large S/I larynx displacements were observed, indicating the potential for swallowing to occur. In addition, the magnitude of the observed larynx displacements should be considered in the planning margin to account for potential swallowing during treatment.