accuracy of dose delivery in radiotherapy is stringent requirement for the expected outcome in patients . the dose delivery methods became very precise with digital linear accelerators , their monitoring electronics and automation in terms of use of record / verify systems . the measured dose in water at reference point in phantom is an absolute parameter for planning the treatment monitor units ( mu ) , both in conventional and intensity modulated / image guided treatments . therefore , traceability of dose accuracy depend mainly on the calibration factor of the ion chamber / dosimeter provided by the accredited secondary standard dosimetry laboratories ( ssdls ) , coming under the network of the international atomic energy agency ( iaea ) calibration laboratories . with the experience gained in maintaining an accredited laboratory elsewhere in the past , an attempt has been made to scrutinize the database in terms of surveillance of dosimetry standards at our radiotherapy clinic in the present center . objective of our present study , is referred to our earlier report on status of beam level dose delivery , and correlate to realization of true absorbed dose in field conditions , in the user institution . as there is no telecobalt machine available for surveillance of dosimetry methods , an attempt was earlier made to study the efficacy of an extrapolation chamber as a the iaea dosimetry protocol ( trs 398 ) highlights the uncertainties involved in various steps of dissemination of measurement parameters . the present report evaluates our reference dosimeters in terms of reproducibility in absorbed dose standards . the department has three reference class dose-1 electrometers ( iba , gmbh , scanditronix wellhofer , germany ) used for absorbed dose calibration in linear accelerator beams and one unidos electrometer ( ptw , germany ) for high dose rate brachytherapy measurements with suitable chambers . all electrometers have calibration factor 1.00 as per certificates provided from accredited ssdls , ( iba , scanditronix wellhofer , nurenberg , germany ; ptw , freiburg , germany ) . calibration factor for absorbed dose estimation in terms of gray / coulomb in water were obtained during 2003 , 2009 , and 2013 from the same ssdl ( iba chambers from iba , gmbh calibration laboratory ; ptw chamber calibrated by ptw , freiburg ) . a strontium-90 check source ( qsa global gmbh , germany ) of nominal activity 30 mbq ( 0.8 mci ) is used to periodically check the sensitivity variations of the 0.6 cc farmer chambers ( fc 65 ) routinely . this provides a dose rate of about 1.25 mgy / s at the position of the chamber ; requiring about 400 s for an accumulated reading of 500 mgy . postal dose tl intercomparison program of iaea , vienna , started in our hospital in 2008 , after ( sultanate of ) oman became a member state . and started participating in postal dose tl intercomparison program since 2008 . so far we participated in three intercomparisons , for three high energy photon beams 6 mv ( two ) , 15 mv ( one ) . the procedure followed here was to measure the absorbed doses in water ( 10 cm depth ) on the day of tl irradiations , and use the dose / mu for calculating the mu required for 2 gy dose . the stated dose by the user institution and iaea dose estimates in their dosimetry laboratory at seibersdorf , austria , validate existing dosimetry at the hospital . the method of maintaining a constant cgy / mu in our linac photon and electron beams was explained in a previous report . when the daily check of output / mu showed drift beyond 2% limits retuning is carried out by the local service personnel . the agreement of measured absorbed dose using three chambers / electrometer combinations was checked using a 5 cm depth perspex , isocenter test tool ( sun nuclear , usa ) in 6 mv x - ray beam from clinac 600 cd linear accelerator ( varian , usa ) . 100 mu with 10  10 cm field size at 100 cm fsd were delivered . absorbed dose / mu was calculated from the charge measured , multiplied by nd , w calibration factor , corrected for temperature pressure . the doses were not converted into dose to water , and compared as it is . three fc 65 chambers are connected one by one to different electrometers , their response and measured values were compared . the extent of agreement between dose estimates at the same reference point under constant geometry by two different ion chambers calibrated at different laboratories ( a and b ) is also checked . table 1 lists the nd , w values of four dosimeters during first and subsequent calibrations . slight changes in the nd , w factors in gray / coulomb are observed , effective on different dates of calibration . over a period of about 9 years , a mean variation of about 0.2% increase was observed for the iba chambers ( one to three ) , but the ptw chamber showed a nearly consistent response . table 2 highlights the results of chamber response to a strontium-90 beta check source , indicating the long - term stability of the ion chambers to the constant geometry beta source . the mean variation of the chamber response with corresponding electrometer , remained well within 1% for all three chambers used for beam level measurements . calibration factors nd , w for thimble ( fc 65 ) chambers from different laboratories strontium-90 check source , chamber response for reading 500 mgy it could be observed from table 3 , that the results of iaea postal tld intercomparison for three photon beams agreed well with our reported doses , over the period 2008 - 2012 representing the status of beam level dosimetry in this institution . tables 4 and 5 show the measured absorbed doses in phantom at constant geometry , using chambers and electrometers which obtained calibration factors from same accredited laboratory . it is observed clearly that all the chamber / electrometer calibrations had an agreement of 0.13% standard deviations in realized absorbed doses . from table 6 it could be appreciated that nd , w factors obtained from two different calibration laboratories ( in contrast to table 5 inference ) do not realize the absorbed dose exactly , but agree within 0.6% ( in perspex cylindrical phantom ) and with deviations as high as 1.5 - 2.0% in solid water , water equivalent phantom . absorbed dose estimates by iaea postal tld intercomparison dose estimate in cylindrical perspex phantom with nd , w corrected dose value from electrometers on gy display for different ion chambers estimate of absorbed doses with dosimeters traceable to different laboratories the above report enumerated calibration factor , nd , w , variability over a 9 year period , as is typical at the user level , and the extent of agreement / disagreement in application of nd , w factor in realizing dose were described . comparison of calibration factors for all dosimeters showed an upward trend in the nd , w factors [ table 1 ] which indirectly imply that they show slight under - estimation of dose , as the time progresses . though it is negligible percentage per year , the slight change in effective volume of ionization is not ruled out . the international dosimetry protocol(trs 398 , iaea ) indicates relative standard uncertainty in dose estimates using nd , w for cobalt reference , a value of 0.9% for telecobalt beams , and 1.5% for high energy linear accelerator photon beam , calibrated at an user institution . another report indicate uncertainties 0.8% for standards laboratory , total user beam level 2% with an overall uncertainty of 2.2% for the application of nd , w calibration factor . the status of beam level dosimetry is highlighted in table 3 for period 2008 - 2012 indicate a mean of 0.03% , maximum of + 1.4% , minimum of 1.4% for three photon beams for nine irradiations . however , beam output and energy stability have been evaluated since patient treatment with linacs began in 2005 , and have been documented in a previous report . table 1 has significance in a user clinic of megavoltage beams for treatment , as the nd , w factors form the basis for status of calibration over a long period of time . the drift in the calibration factor with a long time ( 8 - 10 years ) observed in table 1 is in agreement with the values indicated by tomas kron viz . an experience in a previous institution cited elsewhere showed that the departmental secondary standards showing about 1 - 1.5% variations in 10 years period . it was indicated by us earlier that a constant value of linear accelerator beam output cgy / mu is used in the department , and continuous monitoring of the beams are carried out to quantify routine variations . if changes beyond acceptable limits are observed , the original cgy / mu is restored such that dose delivered to patients are within 2% limits from the mu point of view . all the three dosimeters ( iba , germany ) in table 1 are reference class dosimeters , and therefore the changes in nd , w factor should be realized as genuine changes representing some response change in chambers , as electrometer calibration remained same . the realization of dose in terms of cgy / mu is within 0.13% standard deviation for different chamber / electrometer combinations by taking gray / coulomb calibration , and within same 0.13% standard deviation by taking gray / reading calibrations [ tables 4 and 5 ] . a careful scrutiny of strontium-90 check source response for these three chambers could not exactly correlate to the actual drift observed in nd , w factors over many years . it was highlighted that nd , w factors obtained from different laboratories ( three chambers calibrated by iba compared with ptw calibrated chamber ) [ table 6 ] did not estimate absorbed doses with good agreement , exactly in solid water phantoms ( in contrast to table 5 inference ) with deviations as high as 1.5 - 2.0% . but agreement was seen ( within 0.6% ) in perspex cylindrical phantom and in water phantom ( last four entries ) [ table 6 ] . in oman , we do not have an ssdl at present , though there are efforts to establish one in the university hospital , with iaea assistance . even then , our center will continue to be the only institution where high energy photons / electron sources are available . this overview has highlighted the point that about 2% uncertainty limit exist in maintaining patient dosimetry from medical physics point of view ; which is the achievable limit as per international standards .
the measured dose in water at reference point in phantom is a primary parameter for planning the treatment monitor units ( mu ) ; both in conventional and intensity modulated / image guided treatments . 
 traceability of dose accuracy therefore still depends mainly on the calibration factor of the ion chamber / dosimeter provided by the accredited secondary standard dosimetry laboratories ( ssdls ) , under international atomic energy agency ( iaea ) network of laboratories . 
 the data related to nd , water calibrations , thermoluminescent dosimetry ( tld ) postal dose validation , inter - comparison of different dosimeter / electrometers , and validity of nd , water calibrations obtained from different calibration laboratories were analyzed to find out the extent of accuracy achievable . 
 nd , w factors in gray / coulomb calibrated at iba , gmbh , germany showed a mean variation of about 0.2% increase per year in three farmer chambers , in three subsequent calibrations . another ion chamber calibrated in different accredited laboratory ( ptw , germany ) showed consistent nd , w for 9 years period . 
 the strontium-90 beta check source response indicated long - term stability of the ion chambers within 1% for three chambers . 
 results of iaea postal tl  dose intercomparison  for three photon beams , 6 mv ( two ) and 15 mv ( one ) , agreed well within our reported doses , with mean deviation of 0.03% ( sd 0.87% ) ( n = 9 ) . 
 all the chamber / electrometer calibrated by a single ssdl realized absorbed doses in water within 0.13% standard deviations . however , about 1 - 2% differences in absorbed dose estimates observed when dosimeters calibrated from different calibration laboratories are compared in solid phantoms . 
 our data therefore imply that the dosimetry level maintained for clinical use of linear accelerator photon beams are within recommended levels of accuracy , and uncertainties are within reported values .