Actual metrological conditions for ionizing chamber calibration in radiotherapy

Abstract

BACKGROUND: Since 1976 it has been recognized that an accuracy of ±5% in the delivery of an absorbed dose to a target volume is necessary for successful therapy treatment. Recent studies have concluded that combined standard uncertainty in dose delivery should be smaller than ±3.5 %. The basic radiotherapy requirements initiated some changes in calibration approach. New approach included beam as vital part of calibration chain and also insisted on realization of measurement quality assurance through legal metrology, international key and supplementary intercomparisons, national comparisons, and routine calibration. METHODS: In past twenty years there were three various protocols for absorbed dose determination in radiotherapy that had been based on various principles and various calibration concepts. As there were three conversions in air kerma concept the basic national protocol was changed. We gave up air kerma concept and developed absorbed dose primary standard by ionometric approach and assured appropriate transfer of calibration through four various laboratory levels. The primary standard was realized with combined uncertainty better than 0.3%, 1 s. Transfer of calibration was realized through calibration coefficient determination. RESULTS: Before Code of Practice IAEA 398 was adopted some steps were made in verification of absorbed dose to water primary standard. This standard was established after bilateral intercomparison with Hungarian National Office of Measure (OMH) in 1999 and also after international supplementary comparison organized by International Bureau of Weights and Measures (BIPM) in Sevres, in 2001. Results of the BIPM intercomparison were presented in this paper and they are recognized as national input true value of absorbed dose. Verification of national absorbed dose true value gave us the opportunity to establish new calibration protocol in our radiotherapy centers. We also introduced the new regulatory paper for determination of ionization chamber calibration coefficient. New metrological conditions and calibration manual for radiotherapy chamber were presented in this paper. CONCLUSION: As the method for in-water calibration for gamma and high-energy photons generated in accelerators has been established in our country it gives us possibility to join regional EUROMET program for high-energy photon beam calibration. The first step of calibration in gamma beam quality included also users of high-energy beam in order to fulfill the main metrology goal: calibration in conditions similar to those of users as much as it is possible