Monte Carlo Calculated Output Corrections Factors K(Q,f-Clin) for Varian_Clinac-IX 6MV Small Fields
H Benmakhlouf1*, J Sempau2, P Andreo1, (1) Medical Radiation Physics, Karolinska Hospital and University of Stockholm, Stockholm, Sweden,(2) Institut de Tecniques Energetiques, Universitat Politecnica de Catalunya, Barcelona, SpainSU-E-T-29 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To determine detector reading corrections for small fields from a Varian_Clinac-iX 6 MV linac, according to the international IAEA-AAPM formalism (Alfonso et al 2008) for different detectors.
Methods: The correction factor k(Q,f-clin) was calculated as the ratio of two MC calculated absorbed dose ratios, water to detector, for small clinical fields relative to the standard reference field of 10cmx10cm (machine-specific-reference field). Absorbed doses to various detectors positioned at 10 cm depth in water were calculated using the MC system PENELOPE/penEasy. Phase-space files for field sizes between 0.5 cm and 10 cm from the IAEA phase-space database were used as radiation sources to simulate the response of the detectors PTW T60017 (unshielded diode), PTW T60016 (shielded diode), PTW T31018 (liquid ion chamber, LIC) and IBA SFD (unshielded diode).
Results: For the two unshielded diodes, the corrections are up to 2% at intermediate fields (>1 cm side), becoming down to -5% for fields smaller than 1 cm. The shielded diode corrections decrease with field size from 0 to -9%. The correction for the LIC is within +-1% for all fields.
Conclusion: Shielded diode corrections are smaller than one due to the unbalanced lateral charged particle equilibrium between silicon and water. To this effect, unshielded diode corrections add the well-known gradual over-response to low-energy scattered photons in large fields (maximum for 10cmx10cm). All diode corrections are shown to be significant for field sizes smaller than 2 cm, being appreciable (up to 2%) at intermediate fields for the unshielded diodes. Normalization at fields smaller than 10cmx10cm partially masks the over-response of unshielded diodes. Results are consistent within 0.5% with other (EGS-based) MC calculations, some made for different 6 MV linacs (Siemens, Elekta). Differences can be used to estimate a type-B uncertainty.
Funding Support, Disclosures, and Conflict of Interest: Financial support from Radiation Safety Authority (SSM) (SSM2012-2331) is acknowledged.