Quality assurance of the jaws only-intensity modulated radiation therapy plans for head-and-neck cancer

Intensity-modulated radiation therapy (IMRT) is a treatment technique which has become routine in developed countries. In most centers this technique is delivered with multi-leaf collimators (MLCs). However, the use of MLCs is not mandatory. Several oncology centres in developing countries are still using linear accelerators (LINAC) without MLCs, and can potentially deliver IMRT plans with the use of collimator jaws. In this report, we present the results of quality assurance of this Jaws-Only-IMRT (JO-IMRT) technique in treating nasopharyngeal carcinoma (NPC) patients. Twenty-five plans of nasopharyngeal patients were randomly chosen. For each patient, a JO-IMRT plan was generated and a series of pre-treatment verification measurements was performed including (1) point dose measurement with an ionization chamber, (2) planar dose measurement with a 2D-array detector and (3) 3-dimensional dose measurement using a rotatable phantom with a 2D-array detector. The average differences between the measured and TPS-calculated point doses were found to be 1.26 ± 0.77%, which is within the institution's dose constraint limits. For the planar dose and 3D dose measurements, the average gamma index based on 3%/3 mm criteria were 96.77 ± 2.33% and 94.72 ± 2.67%, respectively. Our measurements showed that the JO-IMRT treatment plans applied to the H&N patients were accurate for the treatment delivery based on our established pass criteria.