(Monday, 7/30/2018) 3:45 PM - 4:15 PM
Room: Exhibit Hall | Forum 3
Purpose: To evaluate the accuracy of the Varian Electron Monte Carlo (eMC) algorithm in predicting dose output and distributions in heterogeneous media and small fields. Previous works have established the accuracy of eMC for use in fields as small as 3 cm in diameter, certain melanoma and keloid patients may call for field sizes as small as 1cm. In implementing eMC for clinical use, it became necessary to establish its viability in these clinical cases.
Methods: Gafchromic EBT3 films and a RANDO phantom were utilized for testing the eMC algorithm for use in heterogeneous media. Films were placed between slices of sternum and lung and processed using a truncated-normal, triple-channel algorithm. Gamma analysis was performed comparing eMC calculated dose against film measurements. Small field doses were assessed with EBT3 film, pinpoint ion chamber (Exradin A16), and 3 different diodes (Sun Nuclear Edge, PTW Proton, and IBA SFD). PDDs, profiles, and output factors were measured for 5 cutout fields with diameters between 1 and 3 cm. Output factors were measured at the depth of maximum dose for each field size.
Results: Gamma passing rates comparing the film and eMC doses were above 92% for a gamma criteria of 3%/3mm, and above 96% at 4%/4mm for all energies. Profiles and PDDs showed good agreement for all field sizes when measured with all detectors except the Edge diode, which has a brass shell. Output factors, however, showed an under-dosing using the eMC algorithm of up to 16% for the smallest field size.
Conclusion: We have shown that while eMC calculated isodose lines accurately in heterogeneities and with small fields, the MU predicted for small fields were only reasonable for field sizes at and above 2.5 cm. The differences were additionally more pronounced in measurements of 6 MeV and 9 MeV.
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