key: cord-0907219-7ez6ywpm authors: Semwal, Manoj Kumar; Gupta, Harishchandra; Singhal, Sakshi; Kumar, Ashok; Kaushik, Aruna title: Variation in Dosimeter Calibration Factor (N(DW)) Over a Period of 20 Years date: 2021-08-07 journal: J Med Phys DOI: 10.4103/jmp.jmp_42_21 sha: 2d3d83dd1957c9cc21a950c577b0e16b8a7591f2 doc_id: 907219 cord_uid: 7ez6ywpm 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 (N(DW)) over a period of time. In this study, the N(DW)factor 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 N(DW)factor over this period was < ±3%. We, therefore, believe that a dosimeter with no history of repairs showed reasonable stability in the N(DW)factor over a long period. The absorbed dose to water calibration factor, N DW, is a crucial link for the accurate determination of dose delivered to patients during radiotherapy treatment. [1] [2] [3] Unknown variation in N DW 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 (N DW ) 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 N DW 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. 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. 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 (N DW ) over a period of time. In this study, the N DW factor 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 N DW factor over this period was < ±3%. We, therefore, believe that a dosimeter with no history of repairs showed reasonable stability in the N DW factor over a long period. 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 N DW (absorbed dose to water) calibration factors for both the chambers. Reference temperature and pressure conditions for the determination of N DW 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 Table 1 provides the N DW values for each calibration cycle for the Farmer and the Markus chambers along with other relevant details. The slight reduction in N DW values was observed with a reduction in polarizing voltage for the Farmer chamber. Table 2 displays the percentage deviation between consecutive N DW 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 N DW 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 N DW 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 N DW 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 N DW values was found to be 0.59% with a deviation of 1% at the upper 95% confidence level. Table 3 displays variation in N DW values with respect to a fixed initial N DW value. The maximum deviation noticed was 3.74% for the Farmer chamber at 20 years. Interestingly, if the initial reference N DW 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. 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 N DW factor between two consecutive cycles (3-year) is the most relevant deviation to be looked for. The maximum deviation in consecutive N DW 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 N DW 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 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. Nil Estimation of absorbed dose in clinical radiotherapy linear accelerator beams: Effect of ion chamber calibration and long-term stability Absorbed dose to water calibration of ionization chambers in a 60 Co Gamma-Ray Beam Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water Role of standards in radiation protection Ion chamber absorbed dose calibration coefficients, NDW, measured at ADCLs: Distribution analysis and stability Concerning radiotherapy standard dose meter calibration There are no conflicts of interest.