An EGSnrc Monte Carlo study of the microionization chamber for reference dosimetry of narrow irregular IMRT beamlets

Abstract

Intensity modulated radiation therapy (IMRT) has evolved toward the use of many small radiation fields, or "beamlets," to increase the resolution of the intensity map. The size of smaller beamlets can be typically about 1-5 cm2. Therefore small ionization chambers (IC) with sensitive volumes < or = 0.1 cm3 are generally used for dose verification of IMRT treatment. The dosimetry of these narrow photon beams pertains to the so-called nonreference conditions for beam calibration. The use of ion chambers for such narrow beams remains questionable due to the lack of electron equilibrium in most of the field. The present contribution aims to estimate, by the Monte Carlo (MC) method, the total correction needed to convert the IBA-Wellhöfer NAC007 micro IC measured charge in such radiation field to the absolute dose to water. Detailed geometrical simulation of the microionization chamber was performed. The ion chamber was always positioned at a 10 cm depth in water, parallel to the beam axis. The delivered doses to air and water cavity were calculated using the CAVRZ EGSnrc user code. The 6 MV phase-spaces for Primus Clinac (Siemens) used as an input to the CAVRZnrc code were derived by BEAM/EGS4 modeling of the treatment head of the machine along with the multileaf collimator [Sánchez-Doblado et al., Phys. Med. Biol. 48, 2081-2099 (2003)] and contrasted with experimental measurements. Dose calculations were carried out for two irradiation geometries, namely, the reference 10x10 cm2 field and an irregular (approximately 2x2 cm2) IMRT beamlet. The dose measured by the ion chamber is estimated by MC simulation as a dose averaged over the air cavity inside the ion-chamber (Dair). The absorbed dose to water is derived as the dose deposited inside the same volume, in the same geometrical position, filled and surrounded by water (Dwater) in the absence of the ionization chamber. Therefore, the Dwater/Dair dose ratio is a MC direct estimation of the total correction factor needed to convert the absorbed dose in air to absorbed dose to water. The dose ratio was calculated for several chamber positions, starting from the penumbra region around the beamlet along the two diagonals crossing the radiation field. For this quantity from 0 up to a 3% difference is observed between the dose ratio values obtained within the small irregular IMRT beamlet in comparison with the dose ratio derived for the reference 10x10 cm2 field. Greater differences from the reference value up to 9% were obtained in the penumbra region of the small IMRT beamlet.

Dosimetric characterization and behaviour in small X-ray fields of a microchamber and a plastic scintillator detector

“…Some ionization chambers were recently especially designed for small-field photon dosimetry and showed a high sensitivity, stability and very good linearity. 3,6,11,12 Silicon detectors have a very high resolution; typically, unshielded p-type silicon diodes with very small active volume are used to perform small-field dosimetric characterization and output factor (OF) measurements. 13 However, the mass density of the sensitive volume and surrounding medium strongly influences the small-field measurements with ionization microchambers and diodes.…”

D-IMRT verification with a 2D pixel ionization chamber: dosimetric and clinical results in head and neck cancer

“…Each one of the 1024 individual chambers in the PXC has a diameter of 0.4 cm and this gives a distorted measurement of large gradients due to a geometric averaging effect, as reported by several authors (Bucciolini et al 2003, Capote et al 2004, Leybovich et al 2003, Martens et al 2000, 2001, Rustgi and Frye 1995, Stasi et al 2003. As expected, this distortion is even worse in the measurements obtained with IC15, which has dimensions larger than PXC (0.6 cm versus 0.4 cm inner diameter).…”

“…For a 2 × 2 cm 2 field, the PXC shows a difference of 3.1% when compared to the diamond detector. The OF was measured using detectors of sizes appropriate for the field dimensions, as it is well known that small fields require a detector with effective dimensions that insure lateral electronic equilibrium (Bucciolini et al 2003, Capote et al 2004, Leybovich et al 2003, Martens et al 2000, 2001, Rustgi and Frye 1995, Stasi et al 2003. Output factor measured with detectors with the smaller volume are close to the measurements with the diamond detector.…”

Development and Evaluation of a Phantom for Multi-Purpose Dosimetry in Intensity-Modulated Radiation Therapy

“…The SFD diode detector is very robust so as to be independent of bias, pressure, and moisture, and it requires no "warmup" time. The Markus and Roos chambers are planeparallel type chambers with vented sensitive volumes of 0.055 cm 3 and 0.35 cm 3 , respectively. Finally, due to its small size, a MOSFET has properties of a 360 degree isotropic response and little beam attenuation.…”

“…Because the dose is aimed precisely at the tumor volume, the dose can be increased without increasing the risk to nearby critical organs. In order to achieve such high conformity, radiation fluence often has complex shapes, and dose distributions exhibit sharp gradients at their boundaries [1][2][3][4]. Thus, in order to successfully carry out this state-of-the-art treatment, an intensive dosimetry is necessary during the implementation of the technique and treatmentverification processes [5].…”