title: Impact of energy variation on Cone Ratio, PDD10, TMR20/10 and IMRT doses for flattening filter free (FFF) beam of TomoTherapy Hi-Art(TM)machines.
abstract: OBJECTIVE
  The beam energy (PDD10: Percent depth dose) of a Tomotherapy Hi-ArtTM machine was varied in a controlled experiment from -1.64 to +1.66%, while keeping the output at 100% and the effect of this on IMRT output, MU chamber ratio (MUR), cone ratio (CR) and Tissue Maximum Ratio (TMR20/10) was studied
  
  
  METHODS
  In this study, Injector Current Voltage (VIC) and Pulse Forming Network Voltage (VPFN) were changed in steps such that the PDD10 was varied from golden beam value incrementally between -1.64 to +1.66%. The effect of this on other energy indicators was studied to verify the sensitivity of TMR20/10, MUR, and detector data-based-CR. To quantify the effect of energy variation on Intensity Modulated Radiation Therapy (IMRT) dose, multiple ion-chamber based dose measurements were recorded by irradiating a cylindrical phantom with standard IMRT plans. Dose variation across each commissioned Field width (FW) was tabulated against energy variation.
  
  
  RESULTS
  Good agreement between PDD10 and TMR20/10, MUR, CR was observed. CR was more sensitive to energy change than PDD10. More variation was observed across standard IMRT plan with increasing energy.
  
  
  CONCLUSIONS
  CR is more sensitive to energy changes compared to PDD10, and CR with MUR can definitely be used as surrogates for checks on a daily/weekly basis. Variation in output across the 6 standard IMRT plans can vary up to 2.8% for a 1.6% increase in energy. Hence, it is of utmost importance to manage the PDD10 tightly around +0.5% in order to regulate standard IMRT QA agreement to within 1% and patient IMRT QA within ±3%.
authors.affiliation: Department of Medical Physics, Hamilton Health Sciences, St. Catharines, Ontario, Canada.
authors.forename: Ranjini P
authors.initials: RP
authors.lastname: Tolakanahalli
authors.forename: Dinesh K
authors.initials: DK
authors.lastname: Tewatia
mesh_terms: D006801:Humans; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated