Ionization chamber dosimetry of small photon fields: a Monte Carlo study on stopping-power ratios for radiosurgery and IMRT beams

Abstract

Absolute dosimetry with ionization chambers of the narrow photon fields used in stereotactic techniques and IMRT beamlets is constrained by lack of electron equilibrium in the radiation field. It is questionable that stopping-power ratio in dosimetry protocols, obtained for broad photon beams and quasi-electron equilibrium conditions, can be used in the dosimetry of narrow fields while keeping the uncertainty at the same level as for the broad beams used in accelerator calibrations. Monte Carlo simulations have been performed for two 6 MV clinical accelerators (Elekta SL-18 and Siemens Mevatron Primus), equipped with radiosurgery applicators and MLC. Narrow circular and Z-shaped on-axis and off-axis fields, as well as broad IMRT configured beams, have been simulated together with reference 10 x 10 cm2 beams. Phase-space data have been used to generate 3D dose distributions which have been compared satisfactorily with experimental profiles (ion chamber, diodes and film). Photon and electron spectra at various depths in water have been calculated, followed by Spencer-Attix (delta = 10 keV) stopping-power ratio calculations which have been compared to those used in the IAEA TRS-398 code of practice. For water/air and PMMA/air stopping-power ratios, agreements within 0.1% have been obtained for the 10 x 10 cm2 fields. For radiosurgery applicators and narrow MLC beams, the calculated s(w,air) values agree with the reference within +/-0.3%, well within the estimated standard uncertainty of the reference stopping-power ratios (0.5%). Ionization chamber dosimetry of narrow beams at the photon qualities used in this work (6 MV) can therefore be based on stopping-power ratios data in dosimetry protocols. For a modulated 6 MV broad beam used in clinical IMRT, s(w,air) agrees within 0.1% with the value for 10 x 10 cm2, confirming that at low energies IMRT absolute dosimetry can also be based on data for open reference fields. At higher energies (24 MV) the difference in s(w,air) was up to 1.1%, indicating that the use of protocol data for narrow beams in such cases is less accurate than at low energies, and detailed calculations of the dosimetry parameters involved should be performed if similar accuracy to that of 6 MV is sought.

Ionization chamber gradient effects in nonstandard beam configurations

“…Since chamber components ͑stem, wall, and central electrode͒ were not modeled in detail in one of these studies ͑i.e., Ref. 19͒ and because stopping power ratios ͑SPRs͒ were known to be quasi-insensitive to modulation, 21 these corrections could be attributed to the chamber replacement effect P repl , as calculations were in agreement with measurements. The present paper extends the previous investigations by quantifying the individual ion chamber correction factors for IMRT beams for two different chambers, modeled in full detail, using the Monte Carlo method.…”

Dosimetry tools and techniques for IMRT

“…High-spatial resolution is important for IMRT measurements. 14,[19][20][21][22][23][24][25][26][27][28][29][30] Table I lists some of the commercially available "small" volume ionization chambers and their design characteristics according to the manufacturer's literature. All listed chambers are waterproof and designed with air-ventilation.…”

Dosimetric Verifications of the Output Factors in the Small Field Less Than 3 cm2Using the Gafchromic EBT2 Films and the Various Detectors

“…Nonetheless, there are the issues to solve the problems in radiotherapy, such as 4D calculations, 1) tumor tracking on a moving tumors, 2,3) image quality and absorbed dose during cone beam computed tomography (CBCT) acquisition to verify patient set-up, 4) calculation-accuracy of radiation treatment planning system (RTPs) in an inhomogeneous area on low density, 5,6) and dosimetry of a small field (＜3×3 cm 2 ). [7][8][9][10][11][12][13][14][15][16][17][18][19][20] These issues have been great challenges in modern radiotherapy. Especially, small field dosimetry is the most important in modern radiotherapy because it has been frequently used to remove the tumor with high dose hypo-fractionated stereotactic radiosurgery (SRS) or high dose single fraction SRS with small size target.…”