Evaluation of Varian Machine Performance Check (MPC) and Ptw Quick Check for Daily Output Checks
Abstract
Purpose
Aim of study is to compare MPC output measurements with daily output measured by PTW Quick-Check and Ion chamber Based measurements for Varian Ture-Beam linear accelerator (LINAC).
Methods
MPC measurements were carried out on Varian LINAC capable of delivering 6, 10, 15, 6FFF and 10FFF (flattening filter free) MV photons energies and equipped with the aS1200 portal imager, with Millennium 120 MLC. The MPC daily check requires the Varian IsoCal™ phantom to be attached to couch top at index position H2. MPC procedure acquires MV images and compare them with set baseline. Daily output measurements were also carried out with PTW Quick Check and 0.6cc Farmer Ion-chamber 10x10cm2 field size at 100.0cm source to detector-distance and source to surface-distance respectively. Percentage variations in LINAC output with various methods has been calculated and compared with ion-chamber. All three sets of measurements were repeated 20 times over a period of six months. Agreement among output measurements was assessed using Bland–Altman analysis.
Results
Results showed that percentage variation in output with MPC with respect to ion chamber is 1.30±1.66%, 1.11±1.30%, 1.16±1.59%, 1.53±1.87% & 1.21±1.38% for 6MV, 10MV, 15MV, 6FFF &10FFF energies respectively. Similarly, the percentage variation in output with PTW Quick Check with respect to ion-chamber is 1.10±1.41%, 1.06±1.42%, 1.03±1.55%, 1.51±1.81% & 1.26±1.53% for 6, 10, 15, 6FFF and 10FFF MV energies respectively. Bland–Altman plot analysis for mean percentage variation between MPC and ion-chamber showed mean bias -1.08% (with 95% Limits of Agreement; 2.14, -4.3) indicating systematically overestimation, variation between PTW Quick-Check and ion chamber is -0.08% (with 95% Limits of Agreement; 2.69, -2.85) indicating less systematic variation.
Conclusion
PTW Quick-Check results are in good agreement with Ion-chamber Based measurements with minimal systematic variation (Bias). MPC results are also in agreement with ion chamber within 95% confidence limits.