Absorbed Dose Standards for Brachytherapy Sources
MPD Completed on March 19, 2014
Objectives for MPD A.7.3
- Develop NIST traceable absorbed dose standards for diverse brachytherapy sources:
- A.7.3a Low dose-rate photon sources
- A.7.3b High dose-rate sources, as iridium-192
- A.7.3c Neutron brachytherapy
- A.7.4d Electronic brachytherapy
Background: Brachytherapy sources are coming into wider use for such applications as prostate implants and breast treatments. Presently, NIST offers air-kerma calibrations for these sources. Conversion of the air-kerma strength to a three dimensional dose-distribution in a medium is a long process, involving Monte Carlo analysis and in-air measurements of anisotropy and spectra. Radiochromic film is a convenient tool for some of this work, but requires construction of precise phantoms for each source geometry. Direct measurement of the dose-rate by an ionization chamber in a medium is a more direct method and would serve to tie together the theoretical modeling and the in-air measurements. It will also enable a direct measurement of source anisotropy.
With the increasing acceptance of implants as a leading method of treating cancer as well as a number of common non-cancerous conditions, the brachytherapy source manufacturers are responding by creating new source designs to compete for a part of the large market. Direct measurement of new source designs offers the advantage of increased accuracy and shorter validation times for clinical applications.
An alternative device for brachytherapy applications is a miniature X-ray generator. This device allows radiation to be delivered to small volumes of tissue through a needle-like applicator that can be inserted into the target tissue. Procedures for calibration of this device must be developed and implemented before it can be used in the clinic.
To appropriately address these needs, NIST must have source capabilities equivalent to each of the brachytherapy modalities noted above: a) low dose-rate photon sources; b) high dose-rate sources, as iridium-192; c) neutron brachytherapy sources; and d) an electronic brachytherapy source.
1 – Using three different detector systems, continue to characterize their reliability in measure dose from different brachytherapy sources.
2 – Adapt detector housings and software to enhance absorbed dose measurements for each of the different brachytherapy sources.
3 – Sustain sufficient NIST and industry support to complete the objectives of this MPD.
1 – A minimum of 4 person-years per year over the next three year time period is required to sustain efforts in this area with personnel being provided by both NIST and its industry partner.
Figure A.7.3 – Well-ionization chamber for clinic use (courtesy of NIST Ionizing Radiation Division)