An EGSnrc Monte Carlo study of the microionization chamber for reference dosimetry of narrow irregular IMRT beamlets a… Roberto Capote, b) Francisco Sa ´ nchez-Doblado, Antonio Leal, Juan Ignacio Lagares, and Rafael Arra ´ ns Hospital Universitario Virgen Macarena, Radiofı ´sica, Sevilla, Spain and Departamento de Fisiologı ´a Me ´dica y Biofı ´sica, Facultad de Medicina, Universidad de Sevilla, Spain Gu ¨ nther H. Hartmann Deutsches Krebsforschungszentrum, Abt. Medizinische Physik, Heidelberg, Germany Received 11 December 2003; revised 12 May 2004; accepted for publication 13 May 2004; published 19 August 2004 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 cm 2 . Therefore small ionization chambers IC with sensitive volumes 0.1 cm 3 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-Wellho ¨ fer NAC007 micro IC mea- sured 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 calcu- lated 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 Sa ´ nchez-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 1010 cm 2 field and an irregular 22 cm 2  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 ( D air ). 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 ( D water ) in the absence of the ionization chamber. Therefore, the D water / D air 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 radia- tion 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 1010 cm 2 field. Greater differences from the reference value up to 9% were obtained in the penumbra region of the small IMRT beamlet. © 2004 American Association of Physicists in Medicine. DOI: 10.1118/1.1767691 Key words: IMRT dosimetry, Monte Carlo, small beam, reference dosimetry, absolute dosimetry I. INTRODUCTION Intensity modulated radiation therapy IMRT is a highly conformal radiotherapy that is used when conventional meth- ods cannot deliver a prescribed target volume dose without compromising other critical organ’s maximum dose. To achieve a conformal dose distribution, IMRT fields are com- posed of small subfields, or ‘‘beamlets’’ with a typical char- acteristic area well below 10 cm 2 . For example the step-and- shoot IMRT employed in the Virgen Macarena hospital uses beams composed of multiple segments of which some of them may be quite small. 1 Similar situations occur at other centers employing dynamic IMRT 2 as well as tomographic IMRT. 3,4 Due to the complexity of IMRT treatment plans and delivery, many techniques developed for the verification of two-dimensional 2D and 3D treatment plans 5 cannot be used for IMRT. Thus, its plans are usually verified by phan- tom measurements, 2,6–9 including absolute dose measure- ments at several points using an ionization chamber IC. Since IMRT uses small fields, there is a tendency to employ small chambers with active volumes of 0.1 cm 3 or less for IMRT verification. 3,10,11 Recently it was experimentally veri- fied that pinpoint ion chamber with an active volume equal to 0.009 cm 3 may be used for absolute dose verification, 12 provided the area of uniform target dose has dimensions 1 cm and leakage corrections are taken into account. Some studies have been conducted on the dosimetry of small pho- ton beams, 13–17 often in connection with dosimetry in radiosurgery. 18–21 However, the use of ion chambers for nar- row beam absolute dosimetry remains questionable due to the lack of electron equilibrium in most of the field area. In 2416 2416 Med. Phys. 31 „9…, September 2004 0094-2405Õ2004Õ31„9…Õ2416Õ7Õ$22.00 © 2004 Am. Assoc. Phys. Med.