Anders Ravnsborg Beierholm

Detector to detector corrections: a comprehensive experimental study of detector specific correction factors for beam output measurements for small radiotherapy beams

PURPOSE The aim of the present study is to provide a comprehensive set of detector specific correction factors for beam output measurements for small beams, for a wide range of real time and passive detectors. The detector specific correction factors determined in this study may be potentially useful as a reference data set for small beam dosimetry measurements. METHODS Dose response of passive and real time detectors was investigated for small field sizes shaped with a micromultileaf collimator ranging from 0.6 × 0.6 cm(2) to 4.2 × 4.2 cm(2) and the measurements were extended to larger fields of up to 10 × 10 cm(2). Measurements were performed at 5 cm depth, in a 6 MV photon beam. Detectors used included alanine, thermoluminescent dosimeters (TLDs), stereotactic diode, electron diode, photon diode, radiophotoluminescent dosimeters (RPLDs), radioluminescence detector based on carbon-doped aluminium oxide (Al2O3:C), organic plastic scintillators, diamond detectors, liquid filled ion chamber, and a range of small volume air filled ionization chambers (volumes ranging from 0.002 cm(3) to 0.3 cm(3)). All detector measurements were corrected for volume averaging effect and compared with dose ratios determined from alanine to derive a detector correction factors that account for beam perturbation related to nonwater equivalence of the detector materials. RESULTS For the detectors used in this study, volume averaging corrections ranged from unity for the smallest detectors such as the diodes, 1.148 for the 0.14 cm(3) air filled ionization chamber and were as high as 1.924 for the 0.3 cm(3) ionization chamber. After applying volume averaging corrections, the detector readings were consistent among themselves and with alanine measurements for several small detectors but they differed for larger detectors, in particular for some small ionization chambers with volumes larger than 0.1 cm(3). CONCLUSIONS The results demonstrate how important it is for the appropriate corrections to be applied to give consistent and accurate measurements for a range of detectors in small beam geometry. The results further demonstrate that depending on the choice of detectors, there is a potential for large errors when effects such as volume averaging, perturbation and differences in material properties of detectors are not taken into account. As the commissioning of small fields for clinical treatment has to rely on accurate dose measurements, the authors recommend the use of detectors that require relatively little correction, such as unshielded diodes, diamond detectors or microchambers, and solid state detectors such as alanine, TLD, Al2O3:C, or scintillators.

Characterizing a pulse-resolved dosimetry system for complex radiotherapy beams using organic scintillators

A fast-readout dosimetry system based on fibre-coupled organic scintillators has been developed for the purpose of conducting point measurements of absorbed dose in radiotherapy beams involving high spatial and temporal dose gradients. The system measures the dose for each linac radiation pulse with millimetre spatial resolution. To demonstrate the applicability of the system in complex radiotherapy fields, output factors and per cent depth dose measurements were performed in solid water for a 6 MV photon beam and compared with Monte Carlo simulated doses for square fields down to 0.6 cm × 0.6 cm size. No significant differences between measurements and simulations were observed. The temporal resolution of the system was demonstrated by measuring dose per pulse, beam start-up transients and the quality factor for 6 MV. The precision of dose per pulse measurements was within 2.7% (1 SD) for a 10 cm × 10 cm field at 10 cm depth. The dose per pulse behaviour compared well with linac target current measurements and accumulated dose measurements, and the system was able to resolve transient dose delivery differences between two Varian linac builds. The system therefore shows promise for reference dosimetry and quality assurance of complex radiotherapy treatments.

Comment on The Radiological Physics Center’s standard dataset for small field size output factors (J Appl Clin Med Phys. 2012;13(5):282–89)

Dear Editor, A standard dataset for small field output factors was published by the Radiological Physics Center (RPC).(1) The data are to be used as part of the quality control in other radiotherapy departments. As part of commissioning a new linear accelerator, we performed a comparison of our data against the original RPC dataset and found relative large deviations. We, therefore, conducted a comparison of the RPC dataset with measurements performed at four oncology departments in Denmark in order to validate the RPC dataset. The measurements were performed on Varian TrueBeam accelerators (Varian Medical Systems, Palo Alto, CA) using three different detectors all suited for small field dosimetry: a pinpoint chamber (PTW 31014; PTW, Freiburg, Germany), a prototype scintillator system developed at Technical University of Denmark,(2) and a diamond detector (PTW 60003). In addition we included some data measured on a Varian Clinac 2100C/D accelerator. Furthermore, we included output factors calculated by the treatment planning systems of the participating centers. Comparing our data to the original RPC dataset revealed a significant difference for all energies and a difference in output factors of about 2.5%. Recently an error in the original dataset was discovered, which lead to a major change in the output factors (see Followill, et al.: Erratum; Vol.15 #1). In Figs. 1 to 4, a plot of all measured and calculated values is seen for 6, 10, 15, and 18 MV beam qualities, respectively. All RPC data are taken from the corrected dataset and the error

Fiber-coupled organic plastic scintillator for on-line dose rate monitoring in 6 MV X-ray beam for external radiotherapy

Fiber-coupled organic plastic scintillators enable on-line dose rate monitoring in conjunction with pulsed radiation sources like linear medical accelerators (linacs). The accelerator, however, generates a significant amount of stray ionizing radiation. This radiation excites the long optical fiber (15-20 m), connecting the scintillator, typically with a diameter of 1 mm and 5 mm in length, with the optical detector circuit, causing parasitic luminescence in the optical fiber. In this paper we propose a method for circumventing this problem. The method is based on the use of an organic scintillator, 2-Naphthoic acid, doped in an optical polymer. The organic scintillator possesses a long luminescent lifetime (room temperature phosphorescence). The scintillator is molded onto the distal end of a polymer optical fiber. The luminescent signal from the scintillator is detected by a PMT in photon-counting mode. The long lifetime of the scintillator signal facilitates a temporal gating of the dose rate signal with respect to the parasitic luminescence from the optical fiber. We will present data obtained using a solid water phantom irradiated with 6 MV Xrays from a medical linac at the Copenhagen University Hospital. Also issues pertaining to the selection of proper matrix as well as phosphorescent dye will be presented in this paper.