glioblastoma multiforme ( gbm ) is the most common malignant tumor of the subcortical white matter of the cerebral hemisphere in adults . the treatment of gbm involves surgical resection , which is the first therapeutic modality for gbm , followed by radiotherapy that may be accompanied by adjuvant chemotherapy 2 . in general , patients with gbm have poor prognosis with about 20% of patients surviving beyond 2 years 2 . these factors include younger age , gender , unilateral tumor , a high karnofsky score , size of the tumor , extent of disease , and adjuvant treatments with chemotherapy such as temozolomide ( tmz ) 3 . in recent years , the development of state - of - the - art radiation therapy and recent advances in chemotherapy have increased the chances for a good prognosis for gbm patients 4 . intensity modulated radiotherapy ( imrt ) allows for a high dose of radiation to be delivered to the tumor while permitting maximal sparing of normal tissue which reduces the radiation toxicity 5 - 9 . in the case of glioblastoma multiforme , imrt has shown the potential to deliver a highly conformal dose to the target while minimizing dose to the organs at risk ( oar ) such as the optic chiasm 10 . this can allow for dose escalation , while on the other hand , also increase local control 6 , 7,11 . treatment with imrt fields involves the complex movement of a multileaf collimator ( mlc ) which consists of many small and irregular multileaf fields or segments that can be delivered in two main modalities , namely segmental imrt step - and - shoot ( ss ) or dynamic imrt ( sliding window ) 12 . in the imrt step - and - shoot ( ss ) technique , the shape of the leaves stays constant while the radiation beam is on and changes when the radiation beam is off , while in the dynamic sliding window technique each leaf pair moves continuously in one direction with independent speeds while the radiation beam is on 13 . imrt dose distributions have the characteristics of complex 3-dimensional dose gradients and a time- dependent fluence delivery 14 . the goals of the pretreatment quality assurance are to assure the precision of the imrt treatment plan and the application of the prescribed dose from the plan 13 . as a consequence of the complexity of the imrt technique , additional dose checking methods are required to confirm the exact calculation of the dose for all patients treated with imrt 15 , 16 . the most common applied dose evaluation tools encompass a direct comparison of dose differences that have a comparison of distance - to - agreement ( dta ) between the measured dose and the calculated dose distributions from the planning system 16 , 17 . the checking procedure for imrt includes several steps which then lead to the quality assurance ( qa ) for the whole imrt treatment plan . these steps include the multileaf collimator ( mlc ) qa , the measurements of individual patient fluence maps , the calibration of the tools used , and the reproducibility of patient positioning 18 . the planned dose fluence is compared with deliverable dose fluence , usually by using a two - dimensional array with ionization chambers , electronic portal imaging devices ( epid ) , or radiochromic film named  gafchromic ebt film  19 , 20 . in this study we used a two - dimensional array with 729 ionization chambers , which is a portal dose device for imrt plan verification . our imrt pretreatment dose verification method consisted of the following two independent measurements : first , point dose measurements at the isocenter using a two - dimensional detector matrix with 729 ionization chambers ( 2d - array ) ( ptw , freiburg , germany ) ; and second , using radcalc ( radcalc , lifeline software , inc . , tyler , tx ) to check independent monitor units ( mu ) for each beam . for each of the ten pretreatment plans , verification imrt plans were created using a varian eclipse external beam treatment planning system ( eclipse tps ) ( 8.1.18 , varian medical systems inc . , palo alto , ca ) . the dose was calculated using the pencil beam convolution ( pbc ) algorithm built - in in the 3-dimensional treatment planning system . all treatment parameters , i.e. , monitor units , field sizes , gantry angles , and leaf motion instructions , are stored in the database of aria oncology ( varian medical systems inc . , palo alto , ca ) , which is an oncology - specific electronic medical record ( emr ) that manages clinical activities such as radiation treatment . the system is connected through a network to all of the treatment units . the two - dimensional array used in this investigation ( 2d - array ) each detector covers an area of 5 x 5 mm and the measuring depth is at 5 mm water . these ionization chambers are uniformly arranged in a 27  27 matrix with an active area of 27  27 cm and dimensional area of 22 mm x 300 mm x 420 mm , interface : 80 mm x 250 mm x 300 mm , allowing absolute dose and dose rate measurements of high - energy photon beams . the 2d - array chamber is calibrated using a setup of 10 cm x10 cm field size , 100 mu , 10 mv beams at a depth of 10 cm , and a dose rate of 300 cgy / mu . in favor of the verification plans , the 2d - array setup consists of three solid water slabs of polymethyl methacrylate ( pmma ) with deferent thicknesses of 3 cm , 4 cm and 1 cm . the 3 cm thickness slab was used as a backscatter phantom , where the other two slabs with a total thickness of 10 cm was used as a buildup phantom . the 2d planar dose distribution was calculated at a 10 cm depth in the phantom using 1 mm pixel - dose grid resolution , and the point dose was calculated at the isocenter ; whereas the reference point was 5 mm behind surface . the individual fields are radiated in gantry and collimator position of 0 on the array and source - to - surface distance ( ssd ) of 94.5 cm , using dynamic multileaf collimation on a varian linear accelerator clinac 2100ex equipped with the 120-leaf millennium mlc ( varian medical systems inc . , the mlc system has 60 pairs of leaves in each bank and mlc leaf width projected at isocenter is 1 cm . the 2d - array chamber is connected to a laptop outside the treatment room which runs software from ptw . the software is matrixscan ( ptw - verisoft 3.1 ) which records the measurements with the 2d - array . prior to the treatment the temperature , pressure , and a correction factor for the machine is entered into the matrixscan software . each beam of the treatment plan is delivered to the 2d - array chamber , thus the dose at some reference points can be calculated . the imrt treatment plans for each of the ten patients consisted of 5 to 11 beams using 10 mv beams with total dose of 60 gy and a dose of 2.0 gy . every field is irradiated in each plan one after another on the 2d - array without interruptions or entering the treatment room and the combined dose is measured , reflecting the contribution from all beams for every plan . the measured dose by 2d - array was compared with the planned dose using verification software based on the gamma index criterion 19,20 . comparisons between measured and calculated dose distributions are reported as dose difference ( dd ) ( pixels within 5% ) , distance to agreement ( dta ) ( 3 mm ) , as well as gamma values (  ) ( dose 3% , distance 3 mm ) . data from each sample were run in duplicate and expressed as means  sd ( cgy , n = 10 patients ) . statistical analysis was performed by means of a graphpad prism package for personal computers ( graphpad software , inc . , san diego , usa ) and figures were drawn using the grafit package for personal computers ( erithacus software limited , surrey , uk ) . an anova analysis using tukey 's test for multiple comparison tests was performed on the data . data from each sample were run in duplicate and expressed as means  sd ( cgy , n = 10 patients ) . statistical analysis was performed by means of a graphpad prism package for personal computers ( graphpad software , inc . , san diego , usa ) and figures were drawn using the grafit package for personal computers ( erithacus software limited , surrey , uk ) . an anova analysis using tukey 's test for multiple comparison tests was performed on the data . in this study we evaluated our qa system for imrt plans that are going to be used to treat patients with gbm brain tumors . presently , we perform routine qa measurements for each imrt patient either immediately prior to the treatment or shortly after the first treatment . table 1 shows the total number of imrt fields for the ten selected treatment plans measured , the fractional dose for each plan , and the fractional measured dose by 2d - array . table 1 also shows the percentage dose different between the tps and the verisoft software measured dose in addition to the percentage of pixels passing gamma criterion . the average dose difference between planned and measured dose was -0.28% with a standard deviation of 1.06 . considering that the passing criteria for imrt plans is based on the percentage of pixels passing gamma index > 95% within dose difference ( pixels is within 5% ) , and distance to agreement dose is 3 mm , all of our ten selected treatment plans passed the gamma analysis test with an average of 97% pixels with an sd of 0.015 . glioblastoma multiforme ( gbm ) is the most frequently encountered and most malignant form of brain tumor , with a poor prognosis and low life expectancy 21 intensity modulated radiation therapy ( imrt ) is a new development of conformal radiotherapy which shows a better outcome for treatment , with a better sparing of the normal brain tissue and other critical structures 19 . imrt treatment plans are complex radiotherapy treatment plans that require a comprehensive qa field - by - field in addition to complex analysis methods 20 , 22 . the need for the sophisticated treatment plans and measurements increases if the tumor is located in an area surrounded by healthy and critical tissues . for example , a tumor in brain is surrounded by many organs at risk ( oar ) such as the brain stem and the optic chiasm 10 . in our study we evaluated our qa system of imrt plans that we use to treat patients with gbm . presently , we perform routine qa measurements for each imrt patient either immediately prior to the treatment or shortly after the first treatment , which is the protocol we use to avoid any delay for the treatment . the ten selected treatment plans were evaluated using 2d - array in addition to independent monitor unit calculations using radcal ; however , the study focused only on the measured dose by the ion chamber 2d - array . figure 2 shows the plan dose calculated by tps and figure 3 shows the measured dose by the 2d-array.the results showed agreement between the measurement dose by the 2d - array and the calculated dose produced by the tps . figure 4 shows the overlap of the planned dose and the measured dose using the gamma index . every point measured in these plans agreed to within 3% acceptability criteria . the evaluation of pretreatment plans for imrt qa is based on many factors such as patient position and patient immobilization and reproducibility ; however , here we only evaluated the imrt qa using the 2d - array ion chamber . all of our ten selected treatments plans successfully passed the gamma analysis criterion with more than 97% pixels in every defined field size for each treatment plan . patient specific dosimetric qa for imrt plan is an important component of clinical usage of imrt . our result showed a very good agreement between measurements dose and calculated dose which demonstrated that our treatment planning using imrt is accurately done compared with the dose planned by the tps . the 2d - array ion chamber measurement agreed with the planned dose , all the plans passed with > 95% gamma criterion with pixels under 5% dose difference and 3 mm distance to agreement for imrt patient - specific quality assurance ( qa ) . a good consistency was observed across the treatments . we concluded that using 2d - array for imrt verification plans is a fast method and possesses all the advantages of ionization chamber dosimetry .