Dosimetry audit for a multi-centre IMRT head and neck trial

PARSPORT was a multi-centre randomised trial in the UK which compared Intensity-Modulated Radiotherapy (IMRT) and conventional radiotherapy (CRT) for patients with head and neck cancer. The dosimetry audit goals were to verify the plan delivery in participating centres, ascertain what tolerances were suitable for head and neck IMRT trials and develop an IMRT credentialing program.

Centres enrolling patients underwent rigorous quality assurance before joining the trial. Following this each centre was visited for a dosimetry audit, which consisted of treatment planning system tests, fluence verification films, combined field films and dose point measurements.

Mean dose point measurements were made at six centres. For the primary planning target volume (PTV) the differences with the planned values for the IMRT and CRT arms were -0.6% (1.8% to -2.4%) and 0.7% (2.0% to -0.9%), respectively. Ninety-four percent of the IMRT fluence films for individual fields passed gamma criterion of 3%/3mm and 75% of the films for combined fields passed gamma criterion 4%/3mm (no significant difference between dynamic delivery and step and shoot delivery).

This audit suggests that a 3% tolerance could be applied for PTV point doses. For dose distributions tolerances of 3%/3mm on individual fields and 4%/3mm for combined fields are proposed for multi-centre head and neck IMRT trials.