The verification of dose distributions in nasopharyngeal cancer patients who were treated by intensity modulated radiotherapy

2. SUMMARY:Purpose: In this study our aim is to find the verification of dose distribution in nasopharyngeal cancers treated with intensity modulated radiotherapy (IMRT) technique. Therefore the dosimetric results of the treatment planning system (TPS) were compared with the relative dosimetric measurements which were obtained by using the film dosimetry technique in Alderson Rando Phantom (ARP).Material and Methods: The computed tomography (CT) images of the scanned ARP, which were taken at a treatment position in a Siemens Somatom CT for treatment simulation, were transferred to TPS. The targeted volumes and organs at risk were contoured by the responsible physican according to ICRU 50-62 protocol for nasopharyngeal cancer and three dimensional images were formed. By using the necessary data in the TPS the IMRT treatment plan for nasopharyngeal cancer was performed. Oncentra treatment planning (OTP) system software was used for the treatment planning. A coordinate system was formed for the three reference slices chosen at the TPS and a sum of 5 points were determined, one for each reference slice, one at the centre of the system and the rest on the x axis. The doses at these points were recorded. The equivalent points and slices at the phantom were found. The results of the planning system were sent to the linear accelerator (Primus, Siemens) and the phantom with the necessary Kodak EDR2 films was than treated with 6 MV-X rays. The irradiated films were read at the high concentrated laser scanner (FIPS plus) and trasferred to Mephysto software. Tables were prepared for each referance slice and for the 5 different points on the referance x axis with the median data of the relative doses during 30 fractions. The dose difference graphics of the comparison between TPS data and the median measurement data of the slices were then made for each referance slice.Results: When the point data, obtained by film dosimetry, were compared with the TPS data, the difference at the dose level (?D) was observed between -3.0% and +2.5%. And when the daily difference between each referance point according to film dosimetry was evaluated, it was observed that the standart deviation was between 0.5 and 1.1. When median measurement values and the standart deviation at the referance points of each slice were taken into consideration, the maximum difference from the median was found as -3.2% for the relative dose values during 30 fractions.Conclusion: In this study when the dose distribution obtained at the TPS during the treatment of nasopharyngeal cancer with IMRT was compared with the relative measurement results obtained by the film dosimetry method, it was observed that the dose difference between the TPS data and the film dosimetry results were at a considerable interval. In this study the effect of tissue heterogenity was taken into consideration by using ARF. At the same time the applicability of the Kodak EDR2 films were shown. According to the results, it was also found that the difference values of the dose profiles obtained at the TPS and the film dosimetry were quite correlated either with each other or with the similar studies in the literature. There was not significant difference between the measured dose distrubition and planned dose distrubition.Key words: Nasopharyngeal cancer, IMRT, Alderson Rando phantom, film dosimetry