annually , an estimated 63000 new cases of primary nonmalignant and malignant central nervous system ( cns ) tumors are diagnosed in the united states with an estimated 13000 deaths .
most of the primary cns tumors are located within the frontal , temporal , parietal and occipital lobes of the brain .
there appear to be some difference between the patterns of brain tumor epidemiology in iran and western countries .
in the first report from iran by ameli et al . , the prevalence of glioma was estimated to be about 45% of all brain tumors , somewhat low in comparison to the western reports , but almost the same as southeast asian countries .
in fact , in iranian reports of glial tumor subtypes , the majority of lesions are low grade astrocytoma and ependymomas [ 2 , 3 ] which is very different from western countries , such as the us and france .
radiotherapy plays an important role in the management of most malignant and many benign primary cns tumors . with the goal of achieving uncomplicated loco - regional tumor control , balancing between benefits and side effects
the most important challenge in radiotherapy is delivering prescribed dose to the tumor and minimize dose to the normal tissues .
emami et al . obtained tolerance radiation dose of 28 critical organs ; their findings showed the td5/5 for lens , optic nerve , retina , chiasma , parotid and thyroid as 10 , 50 , 45 , 50 , 32 and 45 gray ( gy ) , respectively . an absorbed dose of 4 gy to the lens of eye in three weeks to three months leads to cataract .
so in the frontal lobe irradiation , lenses will not save at all and cataract will be induced .
in other organs , although organ dose is tolerated , but in order to minimize complications it seems necessary to develop new technique .
two major complications might occur after frontal lobe irradiation ; necrosis and visual disturbances which might be due to high dose received by chiasma [ 13 - 15 ] . in this study , for frontal lobe brain tumors three routine conventional radiotherapy techniques
were compared in terms of dose distribution and absorbed dose to critical organs using treatment planning system ( tps ) calculation and direct measurement in an anthropomorphic phantom .
then a new treatment technique was designed with same criteria as a mentioned above . due to the high probability of visual disturbances in frontal lobe brain radiotherapy
, we tried to reduce absorbed dose to the chiasma significantly considering new fields and bringing out the chiasma from radiation field with overall lower dose to other considered critical organs .

 construction of phantom 
  to construct the phantom , natural bone with paraffin wax with sodium chloride ( nacl ) as impurity was used ; the effective atomic number and electron density of phantom soft tissue were 6.57 and 3.361023 ( electron g-1 ) , respectively .
a hollow cavity and two hollow tubes were considered as the mouse cavity , trachea and esophagus , respectively .
six cylinders were made from phantom material which had cavities to insert thermoluminescent dosimeters ( tlds ) at several depths , one for parotid ( a transverse one from the left to the right parotid ) , one for chiasma ( perpendicularly inserted from top of brain ) , two for eyes and two for thyroids   ( figure 1 ) . 

tld measurement technique 
  radiation dosimetry was done using cubic lithium fluoride tld chips ( 3mm3mm1 mm ) .
the tlds ( tld-100 , harshaw , usa ) were initially sorted into groups of equal sensitivity .
this was accomplished by delivering a known dose ( 100 cgy ) from 6 mv x - ray ( siemens primus linac ) and consequently measurement of the output light from each tld using a tld reader ( harshaw model 3500 tld reader , usa ) .
the annealing cycle of tlds consisted a heat cycle of 1 hour at 400c and then immediately 2 h at 100c .
calibration procedure was done by exposing tld to 6mv photons in the range of 0 - 250 centigray ( cgy ) . in order to measure dose values ,
a calibration curve was plotted in which the tld reading in c is related to absorbed dose in cgy . besides , the linear calibration equation which used for dose measurement was fitted on experimental data points ( figure 2 ) [ 16 - 19 ] . 

tps and direct measurement on the phantom 
  an imaginary 4 cm putative tumor with 2 cm margin was considered in the right frontal lobe of the brain and phantom was ct planned .
three routine conventional techniques associated with frontal lobe brain tumors were considered as two lateral opposite - fields ( technique 1 ) , one lateral and an anterior field ( technique 2 ) , two lateral opposite - fields and an anterior field ( technique 3 ) .
a new conventional technique with the benefits of conventional techniques and significantly reduced dose to chiasma was designed .
this technique consists two 6 mv and 18 mv angled ap / pa fields with an oblique field with 18 mv photon beam . in angled ap / pa fields ,
the chiasma was brought out from radiation field with a couch rotation of 90 and gantry rotation of 10 ; plan of the new technique and chiasma are brought in figure 3 .
the phantom was planned using above techniques ( prowess panther treatment planning software , california , usa ) with a total prescribed dose of 6000 cgy in 30 fractions .
after contouring ptv and considered critical organs , dose distribution was obtained and absorbed dose to critical organs was extracted from the dose volume histograms ( dvh ) . as the dose fall - off was severe in organs closer to ptv , the selected points on plan ( roi ) were about the same points where tlds were placed in the phantom .
tlds were inserted in considered places in the phantom ( retina , optic nerve , chiasma , lens , submandibular glands , parotids and thyroid ) and the phantom was irradiated with above techniques .
the results of total absorbed dose to selected organs in selected points estimated from tps and tld in 30 fractions is presented in figure 4 and table 1 . in the assessed techniques ,
absorbed dose to all organs except lenses were at their tolerance dose levels and in the new technique , absorbed dose to chiasma is significantly reduced . among all techniques , technique 2 ( one lateral and an anterior field ) was the best in terms of dose distribution , because of its localized irradiated volume at the side of brain .
the differences between calculation and measurement in selected techniques are presented in table 2 as percentage difference . as it is obvious ,
the differences were as high as 70% in organs far enough from considered ptv such as thyroid and submandibular glands , although absorbed doses were not comparable with their tolerance dose . also in the new technique , differences were in the range of three routine techniques .
in clinical radiation therapy , usually tps is used to estimate dose to different organs ; but the obtained values might be different from the actual values depending on the clinical circumstances .
our findings showed differences in the range of 1 - 5% in all techniques between tps and direct measurements for all organs except salivary glands and thyroid ; although their distance from primary radiation field causes tld reading being small , but differs with tps ( about 60% ) which showed very smaller doses ; this might be due to scattered radiation which is not well considered in the tps .
it has been showed that an increase in parotid irradiated volume from 0% -40% to 90% -100% ( carried out in patients who had received a dose of 35 - 45gy ) , results in decreased secretion from100% to 10% .
due to high probability of cataract induction in lens exposed to 400 cgy in three weeks to three months , and underestimation of doses obtained from plan and measurement at about 5% ( ~20 cgy ) , it is important during planning to consider these differences to reduce the incidence of cataract . as showed in table 1 , in three routine techniques , chiasma receives high doses ( 1800 -3000 cgy ) . as previously has reported [ 13 , 15 ] , gensheimer et al . in a review on the outcomes of lacrimal gland adenoid cystic carcinoma treated with neutrons observed severe visual impairment which might be due to high dose to chiasma .
in studying with sino - nasal undifferentiated carcinoma treated using three fields ( weighted anterior field and two wedged laterals ) found that mean dose to the optic chiasm was 54 gy which exceeded from tolerance dose level ; these high doses are in accordance with our study .
so , as showed in table 1 , in the new technique we could reduce dose to chiasma as low as 596 cgy , and also save other organs better .
it is notable that in presented technique , although sparing other organ at risks , the absorbed dose to chiasma was measured as 596 cgy which showed a considerable dose reduction .
the present study sought estimates of the radiation doses received by organs at risk in the frontal lobe radiotherapy with direct measurement on the anthropomorphic phantom using tld dosimetry .
in addition , a comparison between the estimated values from treatment planning system and direct measurement shows differences in the range of 1.1% to 70% between the estimated and measured values .
the dose values of some organs despite the large difference between the two methods , was within their tolerance dose and so there will be no concern about elevated risk of radiation induced adverse effects .
the present study sought estimates of the radiation doses received by organs at risk in the frontal lobe radiotherapy with direct measurement on the anthropomorphic phantom using tld dosimetry .
in addition , a comparison between the estimated values from treatment planning system and direct measurement shows differences in the range of 1.1% to 70% between the estimated and measured values .
the dose values of some organs despite the large difference between the two methods , was within their tolerance dose and so there will be no concern about elevated risk of radiation induced adverse effects .
 backgroundradiotherapy plays an important role in the management of most malignant and many benign primary central nervous system ( cns ) tumors . 
 radiotherapy affects both tumor cells and uninvolved normal cells ; so , it is important to estimate absorbed dose to organs at risk in this kind of treatment . 
 the aim of this study was to determine the absorbed dose to chiasma , lens , optic nerve , retina , parotid , thyroid and submandibular gland in frontal lobe brain tumors radiotherapy based on treatment planning system ( tps ) calculation and direct measurement on the phantom.methodsa head and neck phantom was constructed using natural human bone and combination of paraffin wax and sodium chloride ( nacl ) as tissue - equivalent material . 
 six cylinders were made of phantom material which had cavities to insert thermoluminescent dosimeters ( tlds ) at several depths in order to measure absorbed dose to chiasma , lens , optic nerve , retina , parotid , thyroid and submandibular gland . 
 three routine conventional plans associated with tumors of this region and a new purposed technique were performed on the phantom and dose distribution and absorbed dose to critical organs were compared using treatment planning system ( tps ) calculation and direct measurement on the phantom.resultsabsorbed doses were measured with calibrated tlds and are expressed in centigray ( cgy ) . in all techniques absorbed dose to all organs except the lenses were at their tolerance dose levels and in the new purposed technique , absorbed dose to chiasma was significantly reduced.conclusionour findings showed differences in the range of 1 - 5% in all techniques between tps calculation and direct measurements for all organs except submandibular glands and thyroid . because submandibular glands and thyroid are far from primary radiation field , tld reading in these regions although small but differs from tps calculation which shows very smaller doses . 
 this might be due to scattered radiation which is not well considered in the tps . in the new technique , because the chiasma is out of the radiation field , absorbed dose was reduced significantly . 