Evaluation of MRI-based MAGIC polymer gel dosimeter in small photon fields

Background: Accurate small radiation field dosimetry is essential in modern radiotherapy techniques such as stereotactic radiosurgery [SRS] and intensity modulated radiotherapy [IMRT]. Precise measurement of dosimetric parameters such as beam profile, percentage depth doses and output factor of these beams are complicated due to the electron disequilibrium and the steep dose gradients. In the present work the MAGIC polymer gel was used for dosimetry of small circular photon beams. The results of MAGIC were compared with EBT2 measurements and Monte Carlo [MC] calculations. Materials and Methods: Experimental measurements were made by mentioned dosimeters in four small field sizes 5, 10, 20 and 30 mm. The BEAMnrc code based on EGSnrc was used for simulation to calculate dosimetric parameters at these small fields. The phantoms were irradiated in a 6 MV photon beam Varian 2100C linear accelerator at SSD=100 cm. gel readout performed by 3 Tesla MRI scanner. Results: The results showed that the Percent depth dose [PDD] values measured and calculated by EBT2 film and MC had maximum local differences 4% and 5% with PDD values measured by MAGIC for field size of 5mm respectively. These differences decreased for larger field sizes. The measurements of output factor and penumbra [80%-20%] and [90%-10%] showed good agreement between the measurements and MC calculation. Conclusion: This study showed that the MAGIC polymer gel based on high resolution MRI images is useful detector for small field dosimetry but its agreement with MC is less than agreement of EBT2 film with MC

novel phantom design for brachytherapy quality assurance

Background: One major challenge in brachytherapy is to verify the accuracy of dose distributions calculated by the treatment planning system. In this project, a new phantom design has been introduced for quality assurance of dose distributions in gynocological [GYN] brachytherapy implants using EBT GafChromic film. Materials and Methods: This phantom has been designed and fabricated from 90 slabs of 18×16×0.2 cm[3] Perspex to accommodate a tandem and ovoids assembly, which is normally used for GYN brachytherapy treatment. In addition, this phantom design is allowing the use EBT GafChromic films for dosimetric verification of GYN implants with Cs-137 Selectron LDR system. With this assembly, GafChromic films were exposed using a plan designed to deliver 2.5 Gy dose to point "A" in Manchester system for tandem and ovoids configurations and to deliver 1.5 Gy of dose to 0.5 cm distance from the lateral surface of ovoids for using ovoid-pair. The measured dose distributions with GafChromic films were compared with the TPS isodose lines both numerically and spatially. For a quantitative analysis of the results, the measured doses values at several points of interest were evaluated with the treatment planning data and values obtained following the TG-43 dose calculation formalism. Results: The results of these investigations have indicated that the new phantom design enables us to measure differences of greater than +/-6% for LDR brachytherapy GYN treatments. Conclusion: The new phantom design could be utilized for the QA procedure of the brachytherapy remote after loading systems to confirm the accuracy of dose distribution in GYN implants

Evaluating the effect of Zinc Oxide nanoparticles doped with Gadolinium on dose enhancement factor by PRESAGE dosimeter

Background: New treatment modalities are developed with the aim of escalating tumor absorbed dose and simultaneously sparing the normal structures. The use of nanotechnology in cancer treatment offers some possibilities including destroying cancer tumors with minimal damage to healthy tissues. Zinc Oxide nanoparticles [ZnO NPs] are wide band gap semiconductors and seem to have a good effect on increasing the absorbed dose of target volume especially when doped with a high Z element. The aim of this study was to evaluate the effect of ZnO NPs doped with Gadolinium [Gd] on dose enhancement factor by 6MV photon beam irradiation. Materials and Methods: Various concentrations of ZnO NPs doped with 5% Gd were incorporated into PRESAGE composition, the 3D chemical dosimeter. Then by using a UV-Vis spectrophotometer optical density changes and also dose enhancement factor [DEF] were determined. Results: The results of this study showed that by incorporating 500, 1000, 3000 and 4000 micro g/ml ZnO NPs doped with Gd into PRESAGE structure the dose enhancement factor of about 1.57, 1.69, 1.78 and 1.82 in a 15 ×15 cm[2] field size could be found, respectively. Conclusion: The results of this study showed that ZnO NPs doped with Gd could be considered as new compound for increasing the absorbed dose

Basic investigation on performance of low-density polymer gel dosimeter

Background: in this study a series of basic dosimetric properties of a low-density [LD] gel dosimeter are investigated. The dose response is studied regarding to linearity, sensitivity, dose-rate and energy dependence as well as lung tissue equivalence

Materials and Methods: the LD gel was made by mixing the polymer gel with expanded polystyrene spheres. Energy dependence was studied at two different energies: 1.25 MeV and 6 MV photon beams which were produced by [60]Co and Linac machines. Investigation of dose rate dependence was performed in the low, medium, and high absorbed dose regions. Also reproducibility of dose response was studied in three sets of LD gel with identical preparation, irradiation and imaging procedure at three different days. Moreover the linearity and sensitivity were investigated up to 30Gy

Results: the results showed that the dose response was reproducible. The gel response was found linear up to 22Gy with r[2]=0.981 and sensitivity of 0.814S[-1]Gy[-1]. In the measured ranges, the dose response of LD gel was independent of beam energy within less than +/-0.02 and dose rate had no effect on the gel response. LD gel was nearly lung tissue equivalent with mass density 0.37 to 0.4g/cm3 and relative electron density 0.41

Conclusion: MAGAT LD gel dosimeter appears to be a promising dosimeter in all aspects of dosimetric properties evaluated in this study. In addition, its high linearity together with no dose rate dependence in different level of absorbed doses makes it a suitable dosimeter to measure 3D-dose distributions inside a non-homogeneous media

Three-dimensional gel dosimetry for dose volume histogram verification in compensator-based IMRT

Some tissues in human body are radiobiologically different from water and these inhomogeneity must be considered in dose calculation in order to achieve an accurate dose delivery. Dose verification in complex radiation therapy techniques, such as intensity-modulated radiation therapy [IMRT] calls for volumetric, tissue equivalent and energy independent dosimeter. The purpose of this study is to verify a compensator-based IMRT plan in anthropomorphic inhomogeneous phantom by Dose Volume Histograms [DVH] using polymer gel dosimetry. An anthropomorphic pelvic phantom was constructed with places for gel inserts. Two attached cubic inserts for prostate and bladder and a cylindrical insert for rectum. A prostate treatment case was simulated in the phantom and the treatment was delivered by a five field compensator-based IMRT. Gel dosimeters were scanned by a 1.5 Tesla magnetic resonance imaging [MRI]. Results were analyzed by DVH and difference of differential DVH. Results showed for 3D compensator-based IMRT treatment plan for prostate cancer, there was overall good agreement between calculated dose distributions and the corresponding gel measured especially in planning target volume [PTV] region. Our measurements showed that the used treatment plan configuration has had clinically acceptable accuracy and gel dosimetry can be considered as a useful tool for measuring DVH. It may also be used for quality assurance and compensator-based IMRT treatment verification

Dose measurement outside of radiotherapy treatment field [Peripheral dose] using thermoluminesent dosimeters

In radiation therapy, peripheral dose or the dose outside the radiotherapy field, is important when anatomical structures with very low dose tolerances are involved. One of these critical organs in pelvic irradiation is testis. The amount of radiation delivered to the testes in radiotherapy must be kept as small as possible, consistent with the limit dose of this organ. The threshold doses for temporary and permanent sterility are 0.15 Gy and 6-8 Gy, respectively. Therefore, in pelvic irradiation, protecting testis from these doses and also scattered radiation which could escalate testis dose should be considered. In order to determine the possibility of using testicular shield for high and low energies, TLD dosimetry was done on phantom and 27 patients involved with bladder, rectum and prostate cancers. Dosimetric results showed that the ability of testicular shield in reducing testes absorbed dose is approximately the same [about 7 cGy] for low and high energy photon beams. a testicular shield with a fixed thickness of 1.27cm can be used as testis protection for either high and low energies

Radiation exposure of the surgeons in sentinel lymph node biopsy

Sentinel node biopsy [SLNB] is the standard of care for breast cancer treatment and it is getting wide acceptance in Iran. The radiation safety of the procedure has been investigated under controlled conditions, but the standard dose of radiotracer and techniques are not always observed in the community setting. The aim of this study was to assess the magnitude of the absorbed doses of radiation to the hands of operating surgeons. Twenty consecutive SLNB procedures were studied. Radiation dose to the hands of the surgeons was measured by placing lithium fluoride thermoluminescent dosimeters [TLDs] in the surgeons' glove. The radiation dose to the abdomen and thyroid area was measured by placing TLDs at these areas. The injected dose of radiotracer, the time interval to the surgery and the duration of the surgery were recorded. The injected dose of radiotracer ranged from 0.1 to 5 mCi. The highest absorbed dose was recorded by TLD, placed on the non-dominant hand third finger [189.1 microSv]. Mean recorded doses were higher for non-dominant hand second finger [53.49 +/- 24.60 microSv]. The measured absorbed doses for the abdominal and thyroid area were lower than those for the fingers. This study has confirmed the procedure safety, even with high dose of radiotracer. Nevertheless, it is advisable to use the lowest dose of the radiotracer to avoid the waste of resources

Comparison of conventional and 3D conformal treatments using linac energies for prostate cancer

To evaluate the dosimetric difference between conventional and three-dimensional conformal Radiotherapy [3D-CRT] using 6 and 18 MV X-ray photons. Computed tomography scans of 26 pelvic patients were acquired and transferred to the 3D treatment planning system. For each patient, 8 Conventional plans [3, 4, 5 and 6 Fields] and one 3D-CRT plan were prepared using 6 and 18 MV photon energies. The minimum dose [D[min]], maximum dose [D[max]] and mean dose [D[mean]] to target [PTV] and organs at risk [OAR], Integral dose, Homogeneity Index and Conformity Index were compared for each plan. Also, Experimental measurements were performed using farmer ionization chamber on a patient based pelvic phantom. On Average, six-field [6F1] plans, offer minimum dose to critical organs and sufficient dose to prostate. Increasing the beam energy lead to a decrease in D[mean] of the bladder and femoral heads, as well as D[max] of PTV. The CI and ID were decreased by 4% and 11% respectively with increasing the energy and the number of beams. Experimental measurements were also in good agreement with calculations. 3D-CRT reduced D[mean] of bladder, rectum and femoral heads and also CI and ID were significantly improved by 44.6% and 30.8%, respectively. Increasing the photon energy and number of beams, improve the treatment parameters of bladder, femoral heads and PTV, except the rectum. 3D-CRT offered the most conformity in the delivery doses to the prostate while sparing dose to OARs, uninvolved structures with lower integral dose