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A Phantom-Based Analysis of Small Field Monitor Unit Calculation Agreement Between the Eclipse Treatment Planning System and RadCalc Software

S Ellefson*, E Clouser, Mayo Clinic Arizona, Phoenix, AZ


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To verify Varian Eclipse™ monitor unit calculations for our Varian linacs, a secondary calculation using RadCalc® software is used for every plan at our facility. Due to RadCalc’s assumption of electronic equilibrium, there is significant disagreement between Eclipse and RadCalc for certain plan types utilizing small fields. As our small field Eclipse calculations have been verified with measurements, we can conclude the RadCalc calculations are erroneous. The purpose of this work is to quantify the agreement between RadCalc and Eclipse for a variety of small field scenarios and investigate whether adjusting the RadCalc Sp curve can improve agreement between the systems.

Methods: Treatment plans were created in Eclipse on two virtual phantoms. The first phantom was a homogeneous 30x30x30cm³ water phantom. The second phantom was of similar design, but with layers of lung and bone tissue inserted on opposite sides of the central point, and the couch structure currently used clinically. Plans contained equivalent square field sizes ranging from 1x1cm² to 5x5cm², both square and rectangular. Plans utilized no MLC, static MLC, dynamic conformal arc, static gantry IMRT, and VMAT. Both perpendicular and oblique beam incidences were used. All beams were 6MV. Plans were also calculated in RadCalc, and results compared per-beam to Eclipse. The results were used to make adjustments to the RadCalc 6MV Sp curve, and subsequent results were compared to Eclipse.

Results: RadCalc showed significant disagreement with Eclipse for field sizes below 3x3cm² equivalent square for both phantoms. The largest discrepancies occurred with the smallest fields, with the maximum difference at 31.8% for the heterogeneous phantom. 6 MV Sp curve adjustments resulted in the average difference changing to approximately 0% and the maximum difference to 8%.

Conclusion: Adjustment of small field Sp values in RadCalc is a viable method to improve agreement with Eclipse.


Quality Assurance, Monitor Unit Calculations, Small Fields


TH- External beam- photons: General (most aspects)

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