Purpose: Helical tomotherapy is a modality of radiation treatment delivery which is equipped with an on-board imaging device (MVCT) allowing for daily patient set-up verification and correction in the medial-lateral (m-l), cranial-caudal (c-c), anterior-posterior (a-p) and transversal angular (roll) directions. In this study, we measured set-up deviations and evaluated different MVCT protocols for brain and head and neck (H&N) cancer patients.
Materials and methods: The daily set-up errors of 75 H&N cancer patients immobilized with 5-point fixation thermoplastic masks and 30 brain cancer patients immobilized with 3-point fixation thermoplastic masks were detected by matching the MVCT with the treatment planning CT images. This co-registration procedure was accomplished automatically by the system's software (automatic deviations), then corrected manually by the radiation therapists (total deviations). Systematic and random errors were analyzed on a patient and a population basis. Moreover, 2 MVCT protocols were retrospectively evaluated; MVCTs were either acquired during the first five fractions (FFFs) or on alternate week (ALT). Systematic deviations were calculated based upon prior "MVCT" fractions and applied during the "non-MVCT" fractions. The resulting residual deviations were then analyzed.
Results: The total systematic (and random) deviations reached 1.7mm (1.4mm), 1.6mm (1.5mm), 1.5mm (1.5mm) and 0.6 degrees (0.6 degrees ) for H&N cancer patients and reached 1.6mm (0.9mm), 1.7mm (1.1mm), 1.1mm (0.8mm) and 0.9 degrees (0.6 degrees ) for brain cancer patients in the m-l, c-c, a-p and roll directions, respectively. A t-test detected small but statistically significant differences between the automatic and total deviations. Both MVCT protocols gave rise to similar residual deviations. However, for H&N cancer patients the ALT protocol resulted in smaller residual deviations and CTV-PTV margins, particularly in the a-p direction.
Conclusion: The total systematic and random deviations were comparable to the previously published data. No clinical difference exists between the automatic and total deviations. Both MVCT protocols were similar. But, for H&N cancer patients, the ALT protocol gave rise to smaller residual deviations and therefore is the correct formula to adopt in order to reduce the frequency of pre-treatment MVCTs.