Journal of Medical Physics
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 Table of Contents    
Year : 2021  |  Volume : 46  |  Issue : 2  |  Page : 125-127

Variation in dosimeter calibration factor (Ndw) over a period of 20 years

1 Department of Radiation Oncology, Army Hospital Research and Referral, Delhi, India
2 Division of Clinical PET,Institute of Nuclear Medicine and Allied Sciences, Delhi, India

Date of Submission19-Mar-2021
Date of Decision17-Apr-2021
Date of Acceptance23-Apr-2021
Date of Web Publication7-Aug-2021

Correspondence Address:
Dr. Manoj Kumar Semwal
Department of Radiation Oncology, Army Hospital Research and Referral, Subroto Park, Dhaula Kuan, Delhi-110010
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jmp.JMP_42_21

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Long delays in renewal of calibration of secondary standards radiation dosimeters in radiation oncology centers due to the COVID19 pandemic have aroused concerns regarding accuracy in dose delivery to patients. The concerns are mainly due to the uncertainty in the absorbed dose to water calibration factor (NDW) over a period of time. In this study, the NDWfactor for two ion chambers, thimble type (Farmer) and parallel plate type (Markus), used in most of the radiotherapy centers, were retrospectively reviewed for 20 years. The calibration on all occasions except once was carried out at the Secondary Standards Dosimetry Laboratory, Bhabha Atomic Research Centre, Mumbai. The change in the NDWfactor over this period was < ±3%. We, therefore, believe that a dosimeter with no history of repairs showed reasonable stability in the NDWfactor over a long period.

Keywords: Calibration factors, farmer chamber, Markus chamber, secondary standards dosimeter

How to cite this article:
Semwal MK, Gupta H, Singhal S, Kumar A, Kaushik A. Variation in dosimeter calibration factor (Ndw) over a period of 20 years. J Med Phys 2021;46:125-7

How to cite this URL:
Semwal MK, Gupta H, Singhal S, Kumar A, Kaushik A. Variation in dosimeter calibration factor (Ndw) over a period of 20 years. J Med Phys [serial online] 2021 [cited 2023 Feb 7];46:125-7. Available from:

   Introduction Top

The absorbed dose to water calibration factor, NDW, is a crucial link for the accurate determination of dose delivered to patients during radiotherapy treatment.[1],[2],[3] Unknown variation in NDW value with time may lead to unacceptable uncertainties in the delivered dose. Thus, renewal of calibration factor has to be done every 2 or 3 years as per the regulatory norms of a country.[4] As it is known to all of us working in radiotherapy clinics in India, the prevailing COVID 19 pandemic has resulted in long delays in the renewal of calibration of clinical dosimeters in many radiotherapy centers. This happened mainly because there is only one accredited calibration laboratory for clinical dosimeters in India namely the Secondary Standards Dosimetry Laboratory (SSDL), at Bhabha Atomic Research Centre (BARC), Mumbai.[5] The delays have created unprecedented challenges and concerns for the clinical medical physicists as well as for the regulators. The concerns primarily arise from the unknown variations that may occur in the calibration factors in the absence of timely renewal of calibration.

In this retrospective analysis, the values of absorbed dose to water calibration factors (NDW) for a secondary standard dosimeter (SSD) comprising two ion-chambers have been reviewed over a period of 20 years involving seven cycles of calibration from 2001 to 2021. The dosimeter has been extensively used in the clinic over these years. The intention of submitting this article for publication is to apprise other clinical medical physicists and physicians of the variations in NDW factors over a long period. This can help them in taking an informed decision whether to use or not to use the dosimeter, whose calibration has got delayed, for dose measurements in these unprecedented times.

   Materials and Methods Top

A cylindrical Farmer-type ion-chamber (model 30006), a parallel plate ion-chamber (model Markus 23343) and an electrometer (model UnidosE) - all three from Physikalisch-Technische Werkstätten (PTW), Freiburg, Germany- were used in this study on five of the seven calibration occasions. On the last two occasions, the electrometer was changed to model Max 4000 from Standard Imaging, Middleton (WI), USA. The calibration laboratory was SSDL-BARC, Mumbai on 06 out of 07 occasions and PTW, Freiburg, Germany's accredited laboratory on one occasion. The calibration of the SSD was carried out in a Co-60 beam in a water phantom and the calibration laboratories provided NDW (absorbed dose to water) calibration factors for both the chambers. Reference temperature and pressure conditions for the determination of NDW values were 20°C and 1013.2 mbar for BARC and 22°C and 1013.2 mbar for PTW laboratory, respectively.

It is to be noted that no major repairs were carried out either on the chambers or the electrometers in the study period. The signal cable, however, was changed once during the calibration process when the calibration laboratory had to use its own signal cable (2021).

   Results Top

[Table 1] provides the NDWvalues for each calibration cycle for the Farmer and the Markus chambers along with other relevant details. The slight reduction in NDW values was observed with a reduction in polarizing voltage for the Farmer chamber. [Table 2] displays the percentage deviation between consecutive NDW values (provided at an interval of 3 to 4 years) over a period of 20 years for both the chambers. The maximum deviation of 2.72% in NDW was obtained for the calibrations done in October 2001 and July 2004 in respect of the Farmer chamber at PTW Germany and SSDL, BARC, respectively. The average percentage deviation in consecutive NDW values was found to be 0.87% with a deviation of 1.73% at the upper 95% confidence level for the Farmer chamber. For calibrations done at SSDL, BARC, the average percentage deviations in the consecutive NDW values was 0.49% with a deviation of 0.91% at the upper 95% confidence level for the Farmer chamber. For the Markus chamber, the average percentage deviation between consecutive NDW values was found to be 0.59% with a deviation of 1% at the upper 95% confidence level.
Table 1: Calibration factors(NDW) for PTW Farmer chamber and PTW Markus chamber

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Table 2: Percentage deviations between consecutive NDW values obtained for 20years for the Farmer chamber and the Markus chamber

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[Table 3] displays variation in NDW values with respect to a fixed initial NDW value. The maximum deviation noticed was 3.74% for the Farmer chamber at 20 years. Interestingly, if the initial reference NDW value for calculation of subsequent variations is taken as the first SSDL-BARC value, the maximum deviations noticed were 1.05% and 0.91% for Farmer and Markus chambers, respectively.
Table 3: Percentage deviation of NDW values from the initial calibration values provided by the calibration laboratories

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   Discussion Top

As per International Commission on Radiation Unit and Measurements recommendations, the accuracy of the physical dosimetry has to be within ±3% to ensure overall ±5% accuracy in the dose delivery to a patient.[4] In the present pandemic scenario renewal of calibration of absolute dosimeters for many radiotherapy centers has been delayed by about a year beyond the stipulated 3-year period. Hence the change in NDW factor between two consecutive cycles (3-year) is the most relevant deviation to be looked for. The maximum deviation in consecutive NDW values is 1.1% for six cycles of calibration involving SSDL- BARC. Only in one instance when there was a change of calibration laboratory from PTW Germany (2001) to SSDL-BARC (2004) did we observe a deviation of 2.70%. Ideally, such a difference due to a change of laboratories should not exist. We have no firm explanation for this difference except for speculating that the reason could be related to differences in primary/reference standards to which these calibration laboratories trace their secondary standards.[6] In the case of SSDL-BARC, the reference standard was NPL, UK whereas in the case of PTW it was the German National Standards Laboratory (PTB, Braunschweig). From the data, it was evident that the Markus parallel plate chamber is more stable as compared to the thimble chamber. We also observed that with decrease in polarizing voltage from 400 V to 300 V for the Farmer chamber (March 2017 and February 2021) there was a slight decrease in NDW value for the Farmer chamber. This could be related to decrease in ion collection efficiency with polarizing voltage. Karzmark reported accuracy of ± 2% in ionization chamber measurements taken over a period of 14 years and even calling off the need of calibration every 2 years.[7]

   Conclusion Top

Overall, we observed that the performance of the ion-chamber-based dosimeter was quite stable over 20 years in a clinical environment. We believe that this stability should be applicable to all quality SSDs of different makes available in the market. Therefore, in the absence of timely renewal of calibration due to unforeseen and extraordinary situations such as the present pandemic, an informed decision can be taken by the treating physicians and medical physicists whether to continue with the existing calibration factors beyond the stipulated recalibration period of 3 years or suspend treatment till the availability of updated calibration factors. However, the long-term stability of the dosimeters depicted in this study does not imply the extension of the calibration period beyond 3 years under normal conditions and calibrations must be renewed within the prescribed duration.

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Conflict of interest

There are no conflicts of interest.

   References Top

Ravichandran R, Binukumar JP, Davis CA. Estimation of absorbed dose in clinical radiotherapy linear accelerator beams: Effect of ion chamber calibration and long-term stability. J Med Phys 2013;38:205-9.  Back to cited text no. 1
[PUBMED]  [Full text]  
Minniti R, Shobe J, Stephen M Seltzer, Chen-Mayer HH, Domen SR. Absorbed dose to water calibration of ionization chambers in a 60Co Gamma-Ray Beam. Special Publication (NIST SP) 2006;250:250-74.  Back to cited text no. 2
Castro P, García-Vicente F, Mínguez C, Floriano A, Sevillano D, Pérez L, et al. Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration. J Appl Clin Med Phys 2008;9:70-86.  Back to cited text no. 3
IAEA, International Atomic Energy Agency, Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water, Technical Reports Series No. 398. Vienna: IAEA; 2001.  Back to cited text no. 4
Kulkarni MS. Role of standards in radiation protection. Radiat Prot Environ 2018;41:107-9.  Back to cited text no. 5
  [Full text]  
Muir BR. Ion chamber absorbed dose calibration coefficients, NDW, measured at ADCLs: Distribution analysis and stability. Med Phys 2015;42:1546-54.  Back to cited text no. 6
Karzmark CJ. Concerning radiotherapy standard dose meter calibration. Med Phys 1980;7:574-5.  Back to cited text no. 7


  [Table 1], [Table 2], [Table 3]


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