Room: Exhibit Hall
Purpose: Some Treatment Planning Systems offer the possibility of couch structure inclusion to account for attenuation and bolus effect. This study focus on the so-called Varian Exact IGRT Couch, presenting a simple and practical way of evaluating the best HU value to be used when including couch structures on Varian Eclipse Treatment Planning System. This way, Medical Physicists can perform their own Exact IGRT Couch inclusion commissioning using their everyday tools at hand, which can be extended to other Treatment Planning Systems avaiable on the market.
Methods: A 4 centimeters thick slab water equivalent phantom was positioned on the couch and irradiated by an anterior and a posterior 6MV, 10x10 symmetric field. A farmer ionization chamber was placed inside this slab phantom on a central position. Absorbed dose was measured and compared with Treatment Planning System calculated dose obtained on the same conditions. By changing couch surface HU in Eclipse, it was possible to identify the best HU value which gives the best agreement between measured dose and computational calculated dose by AAA.
Results: Measured and TPS calculated mean dose for anterior field were first evaluated, showing a good accordance (0,2% deviation), so no corrections have to be done to account for couch effect for a posterior field. Measured mean dose was compatible to the value obtained for TPS calculated mean dose considering HU = -300 for couch surface, and HU = -1000 for couch interior. The percentage deviation for TPS calculated mean dose with and without couch inclusion was 0,6%.
Conclusion: Exact IGRT Couch inclusion in our Varian Eclipse TPS dose calculation had a slightly effect up to 0,6% deviation in dose for a single direct field. The TPS manufacturer recommended HU values for couch surface and couch interior could be validated using this simple and easy-to-perform metodology.
Acceptance Testing, Dosimetry, Treatment Verification
TH- External beam- photons: Standard field experimental dosimetry