Verification of photon beam field absorbed dose and two-dimensional dose distribution in intensity modulated radiotherapy in Sichuan province

Objective To measure absorbed dose and verify two-dimensional dose distribution from IMRT MLC using thermoluminescence dosimeters (TLDs) and films. Methods The teflon phantom was scanned by CT and its images were transmitted to TPS for planning. The 6 Gy-corresponding MUs were calculated at 90 cm SSD and 10 cm depth and on 5 cm×5 cm radiation field. All the plans were implemented at 7 medical linear accelerators, which were produced by Varian, Elekta and Siemens and selected from 7 third-grade first-class hospitals according to the technical conditions of various regions in Sichuan province. The homogeneous solid phantoms used in hospitals were covered by 30 cm×30 cm and 25 cm×25 cm films. Then, the films were covered by thicker-than-20 cm phantoms. Finally, the plans were implemented by aligning the center of beams to the films center. Results The relative deviations of the measured absorbed dose to TPS-planned dose were 1.4%, 3.7%, -2.5%, -0.3%, 4.9%, 4.9%, 5.0% for TLDs and 4.7%, 4.3%, 1.5%, 3.9%, -1.6%, 3.3%, -1.3% for films, respectively, all consistent with the limit of less than 5%. The passing rates of 2D dose distribution (3 mm/3%) were 99.9%, 98.5%, 98.5%, 97.9% and 70.0% for 5 accelerators, with only one not consistent with the requirements. Conclusions It is convenient to measure the absorbed dose to photon beam field and verify two-dimensional dose distribution using TLDs and films, which can provide quality assurance for radiation treatment plans. Key words: Thermoluminescent dosimeter; Film; Intensity modulated radiotherapy; Absorbed dose; Two-dimensional dose distribution