Detector density and small field dosimetry: Integral versus point dose measurement schemes

Abstract

Purpose:The Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)] formalism for small field dosimetry proposes a set of correction factors (k f clin ,f msr Q clin ,Q msr ) which account for differences between the detector response in nonstandard (clinical) and machine-specific-reference fields. In this study, the Monte Carlo method was used to investigate the viability of such small field correction factors for four different detectors irradiated under a variety of conditions. Because kvalues for single detector position measurements are influenced by several factors, a new theoretical formalism for integrateddetector-position [dose area product (DAP)] measurements is also presented and was tested using Monte Carlo simulations. Methods: A BEAMnrc linac model was built and validated for a Varian Clinac iX accelerator. Using the egs++ geometry package, detailed virtual models were built for four different detectors: a PTW 60012 unshielded diode, a PTW 60003 Diamond detector, a PTW 31006 PinPoint (ionization chamber), and a PTW 31018 MicroLion (liquid-filled ionization chamber). The egs_chamber code was used to investigate the variation of k f clin ,f msr Q clin ,Q msr with detector type, detector construction, field size, off-axis position, and the azimuthal angle between the detector and beam axis. Simulations were also used to consider the DAP obtained by each detector: virtual detectors and water voxels were scanned through high resolution grids of positions extending far beyond the boundaries of the fields under consideration. Results: For each detector, the correction factor (k f clin ,f msr Q clin ,Q msr ) was shown to depend strongly on detector off-axis position and detector azimuthal angle in addition to field size. In line with previous studies, substantial interdetector variation was also observed. However, it was demonstrated that by considering DAPs rather than single-detector-position dose measurements the high level of interdetector variation could be eliminated. Under small field conditions, mass density was found to be the principal determinant of water equivalence. Additionally, the mass densities of components outside the sensitive volumes were found to influence the detector response.values for existing detector designs depend on a host of variables and their calculation typically relies on the use of time-intensive Monte Carlo methods. Future moves toward density-compensated detector designs or DAP based protocols may simplify the methodology of small field dosimetry.

Determination of the kQclin,Qmsrfclin,fmsr correction factors for detectors used with an 800 MU/min CyberKnife®system equipped with fixed collimators and a study of detector response to small photon beams using a Monte Carlo method

“…It is interesting to see that the polystyrene coating (mass density of 1.06 g cm −3 ) has no influence (Y MC,coat is less than 0.3% for all field sizes) which is consistent with the tissue-equivalence property of polystyrene. This result is in agreement with the recent work of Underwood et al 38 Regarding PTW 60016 and PTW 60017 diodes, the volume averaging and material effects are the same since they have the same silicon chip size. There is no volume averaging effect except for the 5 mm field size (Y MC,vol = −1.7%).…”

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators

“…The overresponse in small radiation fields of silicon diodes, which incorporate silicon of density 2.3 g/cm 3 and possibly other high‐density material in their sensitive volumes and surroundings, has been well documented 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 38 , 39 , 40 , 41 . Diamond detectors, which have a sensitive volume of density 3.5 g/cm 3 , have been reported to overrespond in small fields 16 , 39 , 41 , 42 .…”

“…Diamond detectors, which have a sensitive volume of density 3.5 g/cm 3 , have been reported to overrespond in small fields 16 , 39 , 41 , 42 . Monte Carlo simulations have shown that the magnitude of overresponse is proportional to the mass density of the material in the volume of interest, with a larger overresponse determined for a voxel of the same size in a radiation beam for diamond than for silicon in a 5 mm field for 15 MV (38) .…”

Spectral distribution of particle fluence in small field detectors and its implication on small field dosimetry

“…The output correction factor takes into account all the small‐field effects that have an impact on the output factor, such as partial volume averaging, atomic number, density and detector shielding. The new formalism was followed by a series of scientific papers providing numerical values of the correction factors for several beam quality, field size and detector combinations . Correction factors for most available radiotherapy and radiosurgery techniques (e.g., linear accelerators, CyberKnifes and Gamma Knifes) and modern radiation detectors (e.g., ionization chambers and solid‐state detectors) can be found in the cited papers.…”