Dosimetry of mono-isocenter irradiation technique in radiotherapy for nasopharyngeal carcinoma

Objective: To study the dosimetry of mono-isocenter irradiation technique in radiotherapy for nasopharyngeal carcinoma. Methods: Conventional and mono-isocenter irradiation techniques were used to simulate irradiation of the phantom and nasopharyngeal carcinoma patients with Siemens primus linear accelerator equipped with asymmetric collimators. Dose uniformity isodose distribution and field overlap were measured at the plane of junction with TLD and film dosimeter on the phantom. The reproducibility of the set-up was tested on patients with verification films. Results: For an applied dose of 1 Gy, TLD dosimetry showed that the mean dose at the junction of fields were 1.01 Gy for the mono-isocenter technique as compared to 1.09-1.13 Gy for the conventional technique. An ideal isodose distribution at the junction with mono-isocenter technique was found by film dosimetry. The reproducibility of the set-up was obviously better for the mono-isocenter technique, with less field overlap (1 mm) and set-up deviation (0.5 mm) than those (6-14 mm and 3 mm) of the conventional technique. Conclusions: This investigation of junction dosimetry confirms the potential advantage of the mono-isocentric technique. Not only does it lead to superior dosimetry in the plane of junction but also associates with a better reproducibility of the set-up