Evaluation of MVCT protocols for brain and head and neck tumor patients treated with helical tomotherapy

PurposeHelical 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 methodsThe 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.ResultsThe total systematic (and random) deviations reached 1.7 mm (1.4 mm), 1.6 mm (1.5 mm), 1.5 mm (1.5 mm) and 0.6° (0.6°) for H&N cancer patients and reached 1.6 mm (0.9 mm), 1.7 mm (1.1 mm), 1.1 mm (0.8 mm) and 0.9° (0.6°) 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.ConclusionThe 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.