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Chibani O , Ma CMC
On the discrepancies between Monte Carlo dose calculations and measurements for the 18 MV Varian photon beam
Medical Physics. 2007 Apr;34(4) :1206-1216
PMID: ISI:000245842500009   
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Abstract
Significant discrepancies between Monte Carlo dose calculations and measurements for the Varian 18 MV photon beam with a large field size (40 X 40 cm(2)) were reported by different investigators. In this work, we investigated these discrepancies based on a new geometry model ("New Model") of the Varian 21EX linac using the GEPTS Monte Carlo code. Some geometric parameters used in previous investigations (Old Model) were inaccurate, as suggested by Chibani in his AAPM presentation (2004) and later confirmed by the manufacturer. The entrance and exit radii of the primary collimator of the New Model are 2 mm larger than previously thought. In addition to the corrected dimensions of the primary collimator, the New Model includes approximate models for the lead shield and the mirror frame between the monitor chamber and the Y jaws. A detailed analysis of the phase space data shows the effects of these corrections on the beam characteristics. The individual contributions from the linac component to the photon and electron fluences are calculated. The main source of discrepancy between measurements and calculations based on the Old Model is the underestimated electron contamination. The photon and electron fluences at the isocenter are 5.3% and 36% larger in the New Model in comparison with the Old Model. The flattening filter and the lead shield (plus the mirror frame) contribute 48.7% and 13% of the total electron contamination at the isocenter, respectively. For both open and filtered (2 mm Pb) fields, the calculated (New Model) and measured dose distributions are within 1% for depths larger than I cm. To solve the residual problem of large differences at shallow depths (8% at 0.25 cm depth), the detailed geometry of an IC-10 ionization chamber was simulated and the dose in the air cavity was calculated for different positions on the central axis including at the surface, where half of the chamber is outside the phantom. The calculated and measured chamber responses are within 3% even at the zero depth. (c) 2007 American Association of Physicists in Medicine.
Notes
ISI Document Delivery No.: 159CH Times Cited: 0 Cited Reference Count: 38 Cited References: 1983, MED PHYS, V10, P741 *ICRU, 1976, 24 ICRU *ICRU, 1984, 37 ICRU ABDELRAHMAN W, 2005, MED PHYS, V32, P286 ALMOND PR, 1999, MED PHYS, V26, P1847 ANDREO P, 2000, TECHNICAL REPORT SER, V398 BERGER MJ, 1988, MONTE CARLO TRANSPOR, P21 BERGER MJ, 1996, DLC174 OAK RIDG NAT CHIBANI O, 1994, NUCL INSTRUM METH B, V94, P1 CHIBANI O, 1994, THESIS P SABATIER U CHIBANI O, 1995, NUCL INSTRUM METH B, V101, P357 CHIBANI O, 2002, MED PHYS, V29, P835 CHIBANI O, 2004, AAPM M PITTSB CULLEN DE, 1997, DLC179 OAK RIDG NAT DING GX, 2002, MED PHYS, V29, P2459 DING GX, 2002, PHYS MED BIOL, V47, P3251 FIPPEL M, 1999, MED PHYS, V26, P1466 FUJISAKI T, 2000, P 2 INT WORKSH EGS T, P200 GOUDSMIT S, 1940, PHYS REV, V57, P24 GOUDSMIT S, 1940, PHYS REV, V58, P36 HARTMANNSIANTAR CL, 1996, MED PHYS, V23, P1128 HARTMANNSIANTAR CL, 2001, MED PHYS, V28, P1322 KAWRAKOW I, 2000, PHYS MED BIOL, V45, P2163 KAWRAKOW I, 2001, ADV MONTE CARLO RAD, P229 KAWRAKOW I, 2006, MED PHYS, V33, P1829 KOCH HW, 1959, REV MOD PHYS, V31, P920 MA CM, 1995, PIRS509B NRCC MA CM, 2000, USE COMPUTERS RADIOT, P123 PETTI PL, 1983, MED PHYS, V10, P18 RILEY ME, 1975, ATOM DATA NUCL DATA, V15, P443 ROGERS DWO, 1995, MED PHYS, V22, P503 ROGERS DWO, 2001, PIRS702 NAT RES COUN ROGERS DWO, 2002, PIRS0509 NAT RES COU SEMPAU J, 2000, PHYS MED BIOL, V45, P2263 SHEIKHBAGHERI D, 2002, MED PHYS, V29, P379 SHEIKHBAGHERI D, 2002, MED PHYS, V29, P391 VELKLEY DE, 1975, MED PHYS, V2, P14 ZHU TC, 1998, MED PHYS, V25, P12