ABSTRACT
Objective To establish a new method to analyze the position accuracy of multileaf collimator (MLC) in the dynamic mode.Methods The MLC test sequence was created in a field,where intentional leaf positional errors ranging from 0.1 to 1 mm per centimeter were introduced.In order to establish the relationship between the ion chamber readings and leaf position,whose slope indicated the leaf position error per centimeter,a two-dimensional ion chamber array was used to measure absorbed dose while leaves were moving at dose rates of 100,300 and 600 MU/min,respectively.For routine test,leaf position error was easily found via dose profile in y direction of the field created by dynamic leaves,where the position error could be quantitatively calculated as the slope of absorbed dose line of x direction of the same field.Results The error of 0.2 mm or more per centimeter was obviously shown through y dose profile.The calibration curve was linear at different dose rates.At 600 MU/min,a 0.1 mm leaf position error corresponded to a slope variation of 0.74％,and the differences between the tested errors and the introduced errors were within 0.1 mm.Conclusions The simple and reliable method is helpful to establish the intensity modulated radiation therapy (IMRT) quality control (QC) system.
ABSTRACT
Objective The aim of this work is to compare the performances of EDR2 film dosimetry with two-dimensional ion chamber array (2DICA) in quality assurance (QA) procedures and to investigate the origin of possible discrepancies between the two methods.Methods A 2DICA, I′mRT MatriXX and MULTICube equivalent solid water phantom from IBA Company were used to verify the dose distribution of 15 tomotherapy plan cases.The combined phantom which includes EDR2 film on the array was set up to measure the dose distribution from coronal and sagittal orientations.After the irradiation, the dose distributions of 2DICA and film were compared with those calculated in the planning system for verification.The results and efficiency were evaluated independently in the two methods.Results The mean number of points satifying γ parameter ≤1 in the coronal and sagittal planes was 97.00%±1.56%& 95.98%±2.52%(t=-2.22,P=0.043) and 98.28%±1.55%& 95.42%±1.99%(t=0.75,P=0.464) of the 15 cases respectively for 2DICA and EDR2 film.The ratio of more than 90% and 95% were 93.3% and 66.7%.The results we presented show a very good agreement between the two methods when used to assess the dose distribution between calculated and measured doses,and a certain degree of correlation (r=0.14,P=0.001).Conclusions The 2DICA may effectively replace both film and ion chamber dosimetry in routine IMRT QA.The good agreement between 2DICA and EDR2 film may give a possible check regularly just as a gold standard.
ABSTRACT
Objective To evaluate the dose distributions with different dose rates, and approach a reference to the dose rate for radiation. Methods Three classic static intensity modulated radiotherapy (IMRT) plans of prostate cancer, lymphoma and nasopharyngeal carcinoma were chosen for the study. For each plan, the dose verification of three different dose rates (100, 300 and 600 MU/min) was performed with the Varian 600CD linear accelarator by using the 2-DICA of I'mRT Matrixx. With the Pinncale planning system, each segment was used as a beam to form another IMRT plan. The OmniPro-I'mRT V1.6 was applied to compare the segments in the two IMRT plans, and then the actual weights were obtained. The simulated plans at different dose rate were designed when setting the weights back into the planning system. Results With the increase of dose rate, the passing ratio of the verification decreased and the D_(max), D_(min), D_(mean) and D_(95) of the planning tumor volume increased. The high dose area expanded significantly in target regions, and the 95% isodose line extended. At the dose rate of 600 MU/min, The D_(95) of GTV_(nd) in nasopharyngeal carcinoma increased by 5.33% than the original plan with the V_(110) up to 19. 38%. The irradiation dose of the organs at risk (OARs) increased. For the case of lymphoma, the V_(20) of the lungs in the original plan and the three simulated plans were 31.77%, 32. 11%, 32.60% and 33.26%, respectively. For the case of nasopharyngeal carcinoma, the V_(30) of the right parotid were 48.75%, 49. 56%, 51.65% and 53.91%, respectively. Conclusions With the increase of dose rate in static IMRT , the actual dose distribution deviates the original plan , and the high dose area and the OARs dose increases. The higher dose rate is suboptimal when the dose of the OARs is proximate to the tolerance limit.
ABSTRACT
Objective To evaluate the desimetrie performances of Seven29TM two-dimensional ion chamber array(2D-array).Methods Seven29TM 2D-array was calibrated by using solid water,IBA Dose 1 dosimeter and RIT 113 film dosimetry system.Tests and verifications were performed to evaluate the preci sion of absolute and relative doses of the 2D-array in solid water.Meanwhile ,the dose linear and energy re sponse of the 2D-array were studied.Results Solid water was a good equivalent phantom of water.For 2D-array,the calibration factor in solid water was 1.007 ,the absolute dose error was less than 2.1% ,the rel ative dose error was less than 3.8% and the errors of dose linear and energy response were hess than 0.2% and 1.2% ,respectively.Conclusions The Seven29TM 2D-array,possessing excellent desimetry perform ances,is a powerful tool for intensity modulated radiotherapy.
ABSTRACT
Objective To investigate the feasibility of dose verification of intensity modulated (IM) planning of helical tomotherapy (HT) using two-dimensional ion chamber array (2DICA),and develop an efficient way to validate the dose delivered under the parameters mirroring those during the treatment. Meth-ods A 2DICA,I'mRT MatriXX and MULTICube equivalent solid water phantom from IBA company were used to verify the dose distribution of 10 IM planning. The combined phantom was set up to measure the dose distributions on coronal and sagittal surface. The precise setup of phantom was guided by HTMVCT images. After the irradiation, the measured dose distributions on the coronal and sngittal plane were compared with those calculated by the IM planning system for verification. The results were evaluated and the feasibility of the different measuring methods was studied. Results The dose distribution measured by the MatriXX 2DICA was well consistent with that calculated by the treatment planning system. The errors between the measured dose and predicted dose in the selected points were within ±3%. In the comparison of the pixel-segmented ionization chamber versus treatment planning system using the 3 mm/3% γ criteria, the passing ratio of pixels with γ parameter ≤1 was 97.76% and 96.83%, respectively. Conclusions MatriXX is a-ble to measure the absolute and relative dose distributions simultaneously,which can be used for dose verifi-cation of IM planning.