Absolute dosimetry on a dynamically scanned sample for synchrotron radiotherapy using graphite calorimetry and ionization chambers

Abstract

The absolute dose delivered to a dynamically scanned sample in the Imaging and Medical Beamline (IMBL) on the Australian Synchrotron was measured with a graphite calorimeter anticipated to be established as a primary standard for synchrotron dosimetry. The calorimetry was compared to measurements using a free-air chamber (FAC), a PTW 31 014 Pinpoint ionization chamber, and a PTW 34 001 Roos ionization chamber. The IMBL beam height is limited to approximately 2 mm. To produce clinically useful beams of a few centimetres the beam must be scanned in the vertical direction. In practice it is the patient/detector that is scanned and the scanning velocity defines the dose that is delivered. The calorimeter, FAC, and Roos chamber measure the dose area product which is then converted to central axis dose with the scanned beam area derived from Monte Carlo (MC) simulations and film measurements. The Pinpoint chamber measures the central axis dose directly and does not require beam area measurements. The calorimeter and FAC measure dose from first principles. The calorimetry requires conversion of the measured absorbed dose to graphite to absorbed dose to water using MC calculations with the EGSnrc code. Air kerma measurements from the free air chamber were converted to absorbed dose to water using the AAPM TG-61 protocol. The two ionization chambers are secondary standards requiring calibration with kilovoltage x-ray tubes. The Roos and Pinpoint chambers were calibrated against the Australian primary standard for air kerma at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). Agreement of order 2% or better was obtained between the calorimetry and ionization chambers. The FAC measured a dose 3-5% higher than the calorimetry, within the stated uncertainties.

Comparative toxicity of synchrotron and conventional radiation therapy based on total and partial body irradiation in a murine model

“… TD 50 doses calculated using PROBIT analysis 26 . 95% confidence intervals are stated in parentheses.…”

“…A full description of our dosimetry protocol for all three irradiation modalities is provided in a Supplementary Methods section. In short, SBBR and CRT dosimetry is based on full-scatter reference conditions as previously described 26 – 28 with Monte Carlo simulations used to adjust for the loss of backscatter due to the plastic mouse holder. The theoretical phantom used to mimic the scatter conditions of a mouse in the plastic mouse holder is shown in Supplementary Fig.…”

Water equivalent PRESAGE® for synchrotron radiation therapy dosimetry

“…The intensity of the IMBL beam is at a maximum at the center of the beam and decreases monotonically in both the horizontal and vertical directions. The central part of the beam is uniform in the horizontal direction to within 5% across 90% of the FWHM for a field size of 20 × 1 mm . The decrease in intensity near the edge of the x‐ray field in the horizontal direction is referred to as the ‘roll off effect'.…”

“…Percentage depth dose (PDD) profiles were measured with detector–phantom combinations appropriate for the respective x‐ray energy: a plane‐parallel chamber (PTW 23342) in a full scattering kV equivalent solid water phantom at different depths for the SXRT (120 kVp) beam; a pinpoint chamber (PTW 31014) in a mini water tank for the synchrotron (95.3 kV average energy) beam; and a plane‐parallel Roos chamber (PTW 34001) in a water tank for the 6 and 18 MV beams. Recombination corrections were not applied to the PDDs as the recombination factor is less than 0.2% for plane‐parallel chambers at low dose rates typically used in SXRT and linac treatments, and only 0.8% for the pinpoint chamber in the IMBL synchrotron beam …”

Quantitative characterization of the X-ray beam at the Australian Synchrotron Imaging and Medical Beamline (IMBL)

“…In order to put such discrepancies between calculated and experimental dose rates into context, we consider some recent comparisons between experimentally determined (synchrotron) dose rates obtained using different dosimeters. Lye et al (2016) have used IMBL to make such a comparison between dose rates (surface absorbed-dose rates to water) measured with an ADC IC-105 IC (operating as a FAC), a PTW 31014 PinPoint chamber, a PTW 34001 Roos chamber, and a graphite calorimeter (Harty et al, 2014). The FAC results were between 3 and 5% higher than for the calorimeter, the Roos results $ 2% higher, and the PinPoint results up to $ 2% lower.…”