Patient-related QA for helical TomoTherapy with Delta4: analysis of the results

AbstractThe biplanar diode arrays Delta4PT and Delta4+ has been used in our hospital since the introduction of the TomoTherapy in 2013 to ensure a good agreement between the calculated and the measured dose distributions in patient-related QA with helical TomoTherapy. The aim of this presentation is to evaluate the quality of the measurement procedure with the Delta4 phantoms Delta4PT and (since January 2016) Delta4+. This includes the influence of a cross calibration with a treatment plan with low modulation.Two analyses were performed: (i) All treatment plans in a period of three months (n=86) were not only calculated and measured with Delta4PT or Delta4+ but also with an ionization chamber (Exradin A1SL) in the homogeneous “cheese phantom”. (ii) All data measured from January 2016 to April 2017 (Delta4+, n=132) were analyzed regarding median dose deviation, Gamma analysis and others.The comparison with chamber measurements shows that all measurements with Delta4 and almost all with the ionization chambers (79 of 86) yield a deviation of measured vs. planned dose in the PTV of less than 2.5%, but with a lower variation of the Delta4 measurements. However, a strong correlation between both was not observed.The separate analysis of the measurements with the newer Delta4+ (since January 2016) shows a mean dose deviation in the PTVs of only 0.14% with a standard deviation (S.D.) of 0.69%. Before every measurement a cross calibration has been performed. Without this cross calibration, the deviation would be 0.96% with an increased standard deviation of 0.93%.It is concluded that the Delta4 systems are well suited for patient-related QA for helical TomoTherapy treatment plans. The comparison with chamber measurements shows a plausible accordance between both systems whereas the variation of single measurements is quite different.With the help of a daily cross calibration the variability of the Delta4 results is further decreased and the results show higher accuracy and reliability. According to our experience, a daily cross calibration is mandatory for a reliable patient-related QA.