Use of a new type of radiochromic film, a new parallel-plate micro-chamber, MOSFETs, and TLD 800 microcubes in the dosimetry of small beams

Abstract

The dosimetry of the fields usually employed in radiosurgery requires the use of small detectors to measure Total Scatter Factor (Sc,p), Tissue Maximum Ratio (TMR), Percentage Depth Dose (PDD), and Off Axis Ratio (OAR). In this paper new dosimeters are investigated: a new type of radiochromic film, a micro parallel-plate chamber (filled with both air and tetramethylsilane, TMS), MOSFETs, and TLD-800 microcubes. Their behavior has been compared with the response of radiographic film and with the values obtained with BEAM Monte Carlo simulation. The experimental data confirm that dosimetry with radiochromic films and TLDs gives consistent results for all beam diameters. The parallel-plate micro chamber underestimates the Sc,p for the smallest field diameters (4.4 mm and 6.7 mm); MOSFETs show an over-estimation for the Sc,p of the 4.4 mm, 6.7 mm, and 10.5 mm field diameters. BEAM Monte Carlo simulation employing a parallel beam and a standard 6 MV x-ray spectrum has been used to obtain a correction factor as a function of the field size for both the parallel-plate micro chamber and MOSFETs. High accuracy measurements of PDD and TMR have been made in a water phantom both with radiochromic film and with the micro parallel-plate chamber and have been compared with the data computed by BEAM Monte Carlo simulation. The latter dosimeter is preferred because of the quicker and simpler use and because it gives immediate readout. Measurements of OAR made with radiochromic films and with radiographic films give differences in the 80%-20% penumbra width within 0.6 mm for field diameters ranging from 4.4 mm to 19 mm.

Measurements of the Relative Output Factors for Radiosurgical CyberKnife Collimators Using a Glass Rod Dosimeter

“…For each electron beam, the responses of the GRD for 6 and 20 MeV were lower by approximately 4.5% and 3.8%, respectively, than the responses for a 60 Co r-ray beam. These values are consistent with previously reported data obtained with the same GRD model 2) . It was therefore necessary that the correction factor should be determined and applied for the electron energy used during assessment of the absorbed dose.…”

“…The following dosimeters may be used for output factor measurements in small radiosurgical fields: diode, ionization chamber, diamond detector, thermoluminescent dosimeter (TLD), metal-oxide-semiconductor field effect transistor (MOSFET), radiographic and radiochromic film. Advantages and disadvantages of each dosimeter for small field dosimetry have been discussed in the literature [1][2][3] . A radiophotoluminescent (RPL) glass rod dosimeter (GRD) system has become commercially available and is being increasingly used for radiosurgery dosimetry.…”

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

“…Similar correction factors were defined in previous works for radiosurgery 11 and for IMRT. 31,40 However, since accuracy of Monte Carlo simulation depends on the choice of beam parameters such as energy, divergence, and radial distribution of the electron beam incident on the target, 41,42 a prerequisite of this study has been to obtain a means to infer source parameters and to choose the appropriate correction factor for a given detector and for a specific Cyberknife system, even if its beam characteristics do not strictly match those of the unit used for this study.…”

“…[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] It is known that the main problems with the detectors are retraceable to their finite size compared to the small size of the beams and to the nonwater equivalence of the materials. Moreover, the dosimetry of small beams is complicated by the lack of lateral electronic equilibrium.…”

In-vivo dosimetry for conformal arc therapy using several MOSFET in stereotactic radiosurgery computed by an inverse model

“…The sensitive surface of a MOSFET is so small that it can be considered as a point measure. Therefore, MOSFETs could be suitable for in-vivo assessment of radiosurgery microbeams [7,6,8]. In the literature their use has been described extensively for such in vivo procedures as: in vivo dosimetry for external radiotherapy [9,10,11], dose verification in intensity modulated radiotherapy (IMRT) [12,13,14], measurements in head and neck Tomotherapy [15], skin dose measurement [16], entry dosimetry [17], implantable detectors for in-situ testing during radiation therapy treatment [18], dose verification of permanent low-dose-rate implants [19] and as a dosimeter for imaging in radiological procedures [20].…”