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Advancing Alanine Dosimetry for Small Radiosurgery Fields

Summary:

Research is underway to expand the current range (20 Gy to 200 kGy) of alanine dosimetry to include therapy level dosimetry (as low as 1 Gy) for small-field radiation treatment applications (e.g., TomoTherapy or GammaKnife). The Hi-Art TomoTherapy System (TomoTherapy, Inc.), for example, uses a rotating linear accelerator to deliver a fan of small beamlets in a helical pattern.  This next generation of cancer therapies is growing rapidly to address demand for radiation treatment of the more than one million new cancers per year in the United States.

Description:

However, these improvements in radiotherapy pose interesting challenges for the dosimetry systems available for TomoTherapy certification. Typically, traceability to national standards is achieved by transfer dosimetry with ion chambers from a calibration laboratory.  However, ion chambers are not well-suited to this task due to the small size of each beamlet. 

Also, as the beam number increases in radiotherapy treatments, the dose delivered per beam decreases; this poses sensitivity/measurement challenges.  NIST is working with the dosimeter manufacturer (Gamma Service) and the measurement instrument manufacturer (Bruker BioSpin) to adapt the dosimeter and spectrometer to this new application.  Collaborations are underway with the Department of Radiation Oncology, University of Pittsburgh Cancer Institute, and the Radiology and Radiological Sciences Department of the Uniformed Services University of the Health Sciences.

Leksell Gamma Knife (LGK) International Calibration Survey 

In partnership with the Department of Radiation Oncology and Neurological Surgery, University of Pittsburgh Medical Center, NIST is providing measurement support for a survey of approximately 100 Leksell Gamma Knife (LGK) units worldwide to gather detailed information about calibration procedures, measure the output of the surveyed LGK units using alanine dosimeters, and compare these results with the user's calibration.  To date, 45 LGK units from 43 different centers in 12 different countries have participated in this project (23 from North America, 11 from Europe, and 11 from Asia). The deviations observed between the LGK user's calibration and the alanine dosimetry measurements were small with a mean value of 1.4 %. All 45 LGK units were within 5 % and 43 (96 %) LGK units were within a 3 % deviation.  Different calibration protocols are used worldwide (North America AAPM TG-21, Europe and Asia IAEA TRS398). A small (1.4 %) but systematic deviation is observed for LGK centers in Europe and Asia where the IAEA TRS398 protocol is used. This deviation can be explained by considering the ABS plastic calibration phantom to be water equivalent when performing calibrations in Europe and Asia. Very good overall agreement between a user's reported calibration and alanine dosimetry measurements was observed so far in this study.

Dosimetry System Advancements 

The Electron Paramagnetic Resonance (EPR) dosimetry facility and the gamma-ray irradiators that support the alanine dosimetry system are unique in design and capabilities.  Recently, these capabilities were further advanced through an MOU that was signed between NIST and the Uniformed Services University of the Health Sciences to transfer a state-of-the-art EPR spectrometer to NIST for collaborative research. In addition to the spectrometer upgrade, small-field dosimetry advancements for the alanine dosimetry system are being made by working with the dosimeter manufacturer (Gamma Service) to produce 2 mm and 3 mm dosimeters and the spectrometer application specialists (Bruker Biospin) to develop new measurement protocols for dosimetry below 10 Gy.  Samples of 2 mm and 3 mm alanine dosimeters were supplied to NIST for testing.  These were irradiated to doses from 0.5 Gy to 50 Gy and tested at Bruker Biospin with a new analysis protocol that was based on a newly designed in situ EPR-internal reference assembly with a proprietary EPR intensity reference material (a copy of this device was loaned to NIST for future testing) and a spectral analysis routine based on spectral least-squares fitting.

The first impressions of these advancements were that the 3 mm dosimeters may be useful in the 2 Gy to 4 Gy range at a minimum; measurements at 1 Gy could be possible.  The 2 mm dosimeter may be limited to doses above 10 Gy.