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Improved traceability chain for high dose rate (HDR) brachytherapy sources at PTB

15.08.2023

Brachytherapy is a specific tumor therapy using ionizing radiation where a radioactive source is positioned in the vicinity of, or in close contact with human tissue. It is often applied in the treatment of cervical, prostate, esophageal, lung and breast cancer. By irradiating the cancer cells directly, transmission through healthy tissue is not required, as in the case of external radiation beam therapy. However, due to the very short distances, the dosimetry in brachytherapy is quite challenging.

For almost three decades, PTB has been offering the calibration of 192Ir and 60Co high dose rate (HDR) brachytherapy sources in terms of the dose quantity reference air kerma rate (RAKR). The calibrated sources are used by manufacturers for their quality assurance of the source production. PTB also offers the calibration of well‑type chambers for both radionuclides. Well‑type chambers are used in clinics for quality assurance of the therapy.

The RAKR from a source is determined by measurements using a collimated radiation field achieved by positioning the source in the center of a lead chamber measuring 30 cm x 30 cm x 40 cm, with a 5 cm wall thickness and collimators made of DensiMet® (an alloy consisting of approximately 92 % tungsten, 4 % nickel and 4 % iron with a density of nearly 18 g/cm³) with a diameter of either 5.5 cm or 11 cm; a robot is used to position the standard in the central axis of the radiation field defined by the collimator (see Figure 1).

set-up for the calibration of HDR-brachytherapy sources

Figure 1: Set-up for the calibration of HDR-brachytherapy sources in a collimated field geometry: back: lead housing of the source with collimator; front left: custom made afterloading system; center: secondary standard ionization chamber LS‑01 (black) held by a robot (front right).

A secondary standard LS‑01 ionization chamber for air kerma free‑in‑air is used to determine the RAKR. To assess its position with respect to the center of the radiation source with an accuracy better than 0.1 mm, measurements at five different distances from 800 mm to 1600 mm in steps of 200 mm are made. After applying appropriate corrections for scattering and attenuation in air, the inverse square law is used to calculate the exact source position. The correction factors associated with the scattering and the attenuation in air were determined by measurements applying the shadow shield method, supported by Monte Carlo calculations (Selbach and Büermann 2004).

Until 2022, the LS‑01 was traceably calibrated to PTB’s air kerma primary standards for X‑rays, these being the free air chambers PK 100 and PK 400, as well as PTB’s air kerma primary standard for 137Cs and 60Co, the cavity chamber HRK‑3 (PTB 2023). In this previous calibration method, the calibration factor of the LS‑01 for 192Ir had been determined by calculation from the energy dependence of the chamber's calibration factor determined using several X‑ray qualities between 20 kV and 300 kV as well as 137Cs and 60Co radiation qualities. This energy dependent calibration factor was weighted with the line spectrum of the 192Ir source to obtain the corresponding calibration factor for the LS‑01 (Selbach and Büermann 2004). Regular repeat measurements of the energy dependent calibration factors of the LS‑01 against PTB’s corresponding primary free‑air and cavity chambers were then performed every two years. From these measurements, it was found that the chamber had been stable for more than 15 years, namely with ± 0.1 % for 137Cs and 60Co and within less than ± 1.0 % down to low energy X‑rays. Consequently, the LS‑01 calibration factor for 192Ir was not changed.

In 2016, the International Commission on Radiation Units and Measurements (ICRU) issued a report on updated Key data for ionizing‑radiation dosimetry: ICRU report 90 (ICRU 2016). To implement this report, at the beginning of 2018, the LS‑01 calibration factor for 192Ir and 60Co was lowered by 0.34 % and 0.84 %, respectively. The change for 192Ir was obtained by interpolating from the ICRU 90 change for N‑300 (a filtered X‑ray quality with 248 keV mean energy) and 137Cs (662 keV) to the mean energy of 192Ir (400 keV) (Büermann 2018). By this, the results stated in PTB’s calibration certificates before 2018 need to be multiplied by 0.9966 for 192Ir and by 0.9916 for 60Co to be compared with the results stated in calibration certificates issued as of 2018. Reference to ICRU 90 is given in PTB’s corresponding calibration certificates.

In 2023, PTB adopted new standards, namely, two PTW graphite‑wall, spherical, air‑filled cavity ionization chambers PS‑10 and PS‑50; fully characterized to be considered as primary cavity chamber standards for 60Co, 137Cs and 192Ir sources: volume determination and correction factors were determined experimentally and using Monte Carlo calculations (Pojtinger and Büermann 2021). The larger one of these standards, the PS‑50, was used to recalibrate the LS‑01 chamber at the beginning of 2023. With this chamber, it was possible for the first time, to perform the calibration of the LS‑01 against a primary cavity standard, the PS‑50, directly in an 192Ir radiation field of the same kind as used for PTB’s calibration services in HDR brachytherapy dosimetry, that is in a radiation field of an 192Ir brachytherapy source Microselectron V2 from the company Curium Netherlands B.V. Due to this optimization of the calibration method, the reference value was lowered by 0.83 % for 192Ir and enlarged by 0.47 % for 60Co, this means the results stated in former calibration certificates need to be multiplied by 0.9917 for 192Ir and by 1.0047 for 60Co to be compared with the values stated in PTB calibration certificates issued as of 2023. This further change is also explained in PTB’s corresponding calibration certificates.

In summary, the overall change in the LS‑01 secondary standard calibration factor from pre‑2018 to present is a reduction by 1.17 % for 192Ir and a reduction by 0.37 % for 60Co.

 

Literature

 

Contact

Opens local program for sending emailRolf Behrens, Department 6.3, Working Group 6.34

Opens local program for sending emailStefan Pojtinger, Department 6.2, Working Group 6.25

Contact

Head of Press and Information Office

Dr. Dr. Jens Simon

Phone: +49 531 592-3005
Email:
jens.simon(at)ptb.de

Address

Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig