Svjetlana Sunjic1, Crister Ceberg2, Tomislav Bokulic3. 1. Radiation Safety and Medical Physics Department, University Clinical Centre Sarajevo, Bolnicka 25, 71000 Sarajevo, Bosnia and Herzegovina. Electronic address: svjetlana.sunjic@kcus.ba. 2. Division of Medical Radiation Physics, Department of Clinical Sciences Lund, Lund University, 221 85 Lund, Sweden. Electronic address: crister.ceberg@med.lu.se. 3. Dosimetry Laboratory, Dosimetry and Medical Radiation Physics Section, International Atomic Energy Agency, Friedenstrasse 1, Seibersdorf, Austria. Electronic address: t.bokulic@iaea.org.
Abstract
PURPOSE: To investigate the statistical distribution of the gamma value under error-free conditions, in order to study the relation between the gamma evaluation failure rate and statistically significant deviations in the general situation. METHODS: The 2D absorbed dose distribution for 30 clinical head-and-neck IMRT fields were calculated in a QC phantom. For the same fields, dose measurements were simulated by assuming that the calculated value represented the expectation value, and by adding a random spatial uncertainty of 1-9 mm (1SD) and a random dose uncertainty of 1%-3% (1SD). The simulated measurements were then compared to the calculated dose using the gamma evaluation, and the distribution of the failure rate (i.e. the probability of gamma values above unity) was analysed. RESULTS: For a wide range of the random measurement uncertainty, a distinct peak in the failure rate distribution was observed. The presence of higher failure rates was associated with large values of the second order derivative of the dose distribution. For spatial uncertainties larger than or equal to the resolution of the dose matrix, and for reasonable dose uncertainties, the median value of the failure rate distribution was fairly constant. CONCLUSIONS: Simulations showed, in the general case, that the probability of having a gamma value above unity under error-free conditions was not spatially uniform. We believe that this shortcoming may be partly responsible for the limited ability of the gamma evaluation method to detect errors in clinically relevant situations.
PURPOSE: To investigate the statistical distribution of the gamma value under error-free conditions, in order to study the relation between the gamma evaluation failure rate and statistically significant deviations in the general situation. METHODS: The 2D absorbed dose distribution for 30 clinical head-and-neck IMRT fields were calculated in a QC phantom. For the same fields, dose measurements were simulated by assuming that the calculated value represented the expectation value, and by adding a random spatial uncertainty of 1-9 mm (1SD) and a random dose uncertainty of 1%-3% (1SD). The simulated measurements were then compared to the calculated dose using the gamma evaluation, and the distribution of the failure rate (i.e. the probability of gamma values above unity) was analysed. RESULTS: For a wide range of the random measurement uncertainty, a distinct peak in the failure rate distribution was observed. The presence of higher failure rates was associated with large values of the second order derivative of the dose distribution. For spatial uncertainties larger than or equal to the resolution of the dose matrix, and for reasonable dose uncertainties, the median value of the failure rate distribution was fairly constant. CONCLUSIONS: Simulations showed, in the general case, that the probability of having a gamma value above unity under error-free conditions was not spatially uniform. We believe that this shortcoming may be partly responsible for the limited ability of the gamma evaluation method to detect errors in clinically relevant situations.