Total scatter factors of small beams: A multidetector and Monte Carlo study

Abstract

The scope of this study was to estimate total scatter factors ͑s c,p ͒ of the three smallest collimators of the Cyberknife radiosurgery system ͑5-10 mm in diameter͒, combining experimental measurements and Monte Carlo simulation. Two microchambers, a diode, and a diamond detector were used to collect experimental data. The treatment head and the detectors were simulated by means of a Monte Carlo code in order to calculate correction factors for the detectors and to estimate total scatter factors by means of a consistency check between measurement and simulation. Results for the three collimators were: s c,p ͑5 mm͒ = 0.677± 0.004, s c,p ͑7.5 mm͒ = 0.820± 0.008, s c,p ͑10 mm͒ = 0.871± 0.008, all relative to the 60 mm collimator at 80 cm source-to-detector distance. The method also allows the full width at half maximum of the electron beam to be estimated; estimations made with different collimators and different detectors were in excellent agreement and gave a value of 2.1 mm. Correction factors to be applied to the detectors for the measurement of s c,p were consistent with a prevalence of volume effect for the microchambers and the diamond and a prevalence of scattering from high-Z material for the diode detector. The proposed method is more sensitive to small variations of the electron beam diameter with respect to the conventional method used to commission Monte Carlo codes, i.e., by comparison with measured percentage depth doses ͑PDD͒ and beam profiles. This is especially important for small fields ͑less than 10 mm diameter͒, for which measurements of PDD and profiles are strongly affected by the type of detector used. Moreover, this method should allow s c,p of Cyberknife systems different from the unit under investigation to be estimated without the need for further Monte Carlo calculation, provided that one of the microchambers or the diode detector of the type used in this study are employed. The results for the diamond are applicable only to the specific detector that was investigated due to excessive variability in manufacturing.

Diamond detector in absorbed dose measurements in high‐energy linear accelerator photon and electron beams

“…Most recently, some authors (7) investigated the utilization of natural diamonds in small field dosimetry, more specifically for the measurement of total scatter factor for the CyberKnife system and for dose measurements of radiation beams in intensity‐modulated radiation therapy (IMRT), respectively. An analysis of total scatter factor obtained using diamond detector was made, along with data from PinPoint ion chambers and semiconductor diodes.…”

Small field dosimetry: What have we learnt?

“…Francescon et al (27,35,39) provided detailed information on kΩ factor variability for multiple linear accelerator and detector configurations down to a field size of 5 mm square. Similar data has been also been reported by other groups (16,19).…”

Dosimetric characterization of a synthetic single crystal diamond detector in clinical radiation therapy small photon beams

“…[1][2][3][4] The development of high-resolution small volume detectors, as microionization chambers and solid state detectors, was studied by several authors with the aim of determining new devices and methods for accurate dosimetry in high-energy narrow photon beams. [5][6][7][8] However, no device was found yet, matching all the features requested to an "ideal" detector for small field dosimetry. PTW PinPoint 0.015 cm 3 microionization chambers (PTW, Freiburg, Germany), as types 31 006 (no longer available) and 31 014, were extensively examined [8][9][10][11] and assessed as proper dosimeters in the field size range 2 × 2 cm 2 to 10 × 10 cm 2 .…”