Pilot dose intercomparisons of 3D and 4D advanced lung radiotherapy

Abstract

s / Physica Medica 30 (2014) e16ee44 e26 Contemporary therapeutic radiation oncology treatments require the de- livery of highly localized doses of radiation to well defined target regions inside the patient. The efficacy of the radiation treatment, however, re- quires knowledge of the absorbed dose in the organ of interest to better than ±5% as there is a higher risk of local recurrence or complications with incorrect exposure. Furthermore, since it is inevitable that healthy organs and tissue will also be exposed during treatment, overexposure increases the risk of secondary cancers. International regulations [1, 2] have been introduced to adapt to the fast introduction of new radiotherapy technologies and demand an improve- ment of in vivo dosimetry. Ideally, a real-time in vivo dosimeter would measure absorbed doses during radiotherapy. Optical stimulated luminescent (OSL) dosimeters, using the high sensi- tive material Al2O3:C, have been successfully used for measuring whole body doses that result from exposure to high energetic photon and beta radiation. OSL dosimeters have also been introduced in medical dosim- etry of low and high LET beams [3, 4]. Al2O3:C detectors can also be used as real time dosemeters, because the emitted stimulated light can be guided via light fibers to a remotely placed photomultiplier tube. Furthermore, during exposure to ionizing radiation Al2O3:C emits radi- oluminescent light (RL), of which the intensity is proportional to the dose rate [5]. In this study, we investigated the dosimetric response of an optical fibre coupled Al203:C RL/OSL dosemeter prototype. In our prototype, a PMMA fibre is coupled to an Al2O3:C detector composed of microparticles of Al2O3:C with diameters ranging from 5 mm to 35 mm dissolved in a photo- curable polymer. The RL/OSL dosimeter prototype is a portable and robust instrument that has been developed for the routine assessment of patient exposure to ionizing radiation during radiotherapy treatments. The design principles of hardware and software are described elsewhere [6]. In this study, we present the results obtained using radioluminescence (RL) from Al2O3:C irradiated with a 6 MV linear accelerator (Compact, Elekta, Crawly) and preliminary results obtained using optically stimulated luminescence (OSL). The dose rate dependence was assessed by varying the photon flux (MU/ min) of the linac, effectively changing the pulse rate and keeping the dose per pulse fixed. It was found that the RL measured dose response demonstrated low dose-rate dependency, to within 1%. The dose response was found to be linear in a dose range from 0.1 up to a dose of 6 Gy, with reproducibility below 0.5%. The dosimeter is benchmarked by evaluating the ability to measure depth- dose distributions and lateral dose profiles accurately. RL derived dose profiles have been compared with dose profiles measured with a standard ion chamber (PTW). Depth-dose distributions in water were acquired for a 6 MV photon beam using a 10 10 cm2 field, set at 350 MU/min, corre- sponding with 3.5 Gy/min at 10 cm depth. All data have been normalized to the depth-dose maximum. The RL measured dose agreed with the ionization chamber measured dose to within 1% (1 SD) for depths from 0.5 to 20 cm. Lateral dose profile was set at 350 MU/min (3.5 Gy/min at depth 10 cm), using a 10x10 cm2 field size. Differences between measured RL and ion chamber are within 1.5% for de direct beam and 5% in the penumbra region. These results show that the RL/OSL detector system makes it suitable for measurements of depth and lateral dose distributions in clinical photon beams. Furthermore, basic characteristics of OSL in irradiated fibers with Al2O3:C for dosimetry in therapeutic 6 MV photon beam were investigated.