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SU‐FF‐T‐407: Full Monte Carlo Computation of K Correction Factors Calculated in Tomotherapy Static and Helical Deliveries for Future Ion Chamber Reference Dosimetry Protocols of Non Standard Beams
Author(s) -
Sterpin E,
Mackie T,
Lu W,
Olivera G,
Vynckier S
Publication year - 2009
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3181889
Subject(s) - tomotherapy , monte carlo method , ionization chamber , dosimetry , physics , imaging phantom , nuclear medicine , computational physics , ion , optics , mathematics , ionization , statistics , medicine , radiation therapy , quantum mechanics
Purpose: Calculate correction factors for ion chamber dosimetry using Monte Carlo (MC) simulations for future IAEA/AAPM protocols adapted to Tomotherapy treatments. Material and methods: The formalism in Alfonso et al (Med. Phys. 35 2008) introduced two corrections factors: k Q MSRf MSRdefined for a “machine‐specific‐reference” field f MSR (10×5 cm 2 field, source‐surface distance of 85 cm) and k Q PCSRf PCSRfor a “plan‐class‐specific‐reference” field, f PCSR , closer to actual clinical treatments. Here, f PCSR was a helical sequence delivering a centered cylindrical homogeneous dose over 10 cm in a homogeneous cylindrical phantom. The k factors were calculated using MC with accurate simulation of static and helical deliveries (using TomoPen, based on PENELOPE) and detailed modeling of the geometry of the ion chambers Exradin A1SL, PTW‐30013, Wellhöfer IC‐70 and NE‐2571. Calculations in the phantoms were performed by the “cavity” EGS++ user code, for static beams, and PENELOPE for both helical and static beams. Correction factors were calculated from the formulas k Q MSR, Q Cof MSR, f 0=( D m /D a¯ )Q MSRf MSR( D m /D a¯ )Q Cof Coand k Q PCSR, Q Cof PCSR, f 0=( D m /D a¯ )Q PCSRf PCSR( D m /D a¯ )Q Cof Cowhere D m is the dose at the reference point in medium m (water or Solid Water™) and aD a¯ is the dose averaged over the air cavity. Results: For the A1SL chamber, k Q MSR, Q Cof MSR, f 0equaled 0.997±0.001 (3σ), which is consistent with the values previously reported using existing protocols. k Q PCSR, Q Cof PCSR, f 0was estimated at 0.995±0.002 for the procedure chosen. The work is in progress for the ion chambers listed above and other Tomotherapy “ PCSR ” settings, like a typical bilateral head and neck treatment delivery planned on a phantom. Conclusions: A powerful MC method is proposed here to compute accurate correction factors for Tomotherapy treatments. The data obtained may be of primary importance during the final definition of a new dosimetry protocol, adapted to nonstandard treatments like Tomotherapy and may also improve global treatment accuracy, especially for helical deliveries. Research partially sponsored by Tomotherapy Incorporated