A round-robin gamma stereotactic radiosurgery dosimetry interinstitution comparison of calibration protocols.

PURPOSE: Absorbed dose calibration for gamma stereotactic radiosurgery is challenging due to the unique geometric conditions, dosimetry characteristics, and nonstandard field size of these devices. Members of the American Association of Physicists in Medicine (AAPM) Task Group 178 on Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance have participated in a round-robin exchange of calibrated measurement instrumentation and phantoms exploring two approved and two proposed calibration protocols or formalisms on ten gamma radiosurgery units. The objectives of this study were to benchmark and compare new formalisms to existing calibration methods, while maintaining traceability to U.S. primary dosimetry calibration laboratory standards. METHODS: Nine institutions made measurements using ten gamma stereotactic radiosurgery units in three different 160 mm diameter spherical phantoms [acrylonitrile butadiene styrene (ABS) plastic, Solid Water, and liquid water] and in air using a positioning jig. Two calibrated miniature ionization chambers and one calibrated electrometer were circulated for all measurements. Reference dose-rates at the phantom center were determined using the well-established AAPM TG-21 or TG-51 dose calibration protocols and using two proposed dose calibration protocols/formalisms: an in-air protocol and a formalism proposed by the International Atomic Energy Agency (IAEA) working group for small and nonstandard radiation fields. Each institution's results were normalized to the dose-rate determined at that institution using the TG-21 protocol in the ABS phantom. RESULTS: Percentages of dose-rates within 1.5% of the reference dose-rate (TG-21+ABS phantom) for the eight chamber-protocol-phantom combinations were the following: 88% for TG-21, 70% for TG-51, 93% for the new IAEA nonstandard-field formalism, and 65% for the new in-air protocol. Averages and standard deviations for dose-rates over all measurements relative to the TG-21+ABS dose-rate were 0.999±0.009 (TG-21), 0.991±0.013 (TG-51), 1.000±0.009 (IAEA), and 1.009±0.012 (in-air). There were no statistically significant differences (i.e., p>0.05) between the two ionization chambers for the TG-21 protocol applied to all dosimetry phantoms. The mean results using the TG-51 protocol were notably lower than those for the other dosimetry protocols, with a standard deviation 2-3 times larger. The in-air protocol was not statistically different from TG-21 for the A16 chamber in the liquid water or ABS phantoms (p=0.300 and p=0.135) but was statistically different from TG-21 for the PTW chamber in all phantoms (p=0.006 for Solid Water, 0.014 for liquid water, and 0.020 for ABS). Results of IAEA formalism were statistically different from TG-21 results only for the combination of the A16 chamber with the liquid water phantom (p=0.017). In the latter case, dose-rates measured with the two protocols differed by only 0.4%. For other phantom-ionization-chamber combinations, the new IAEA formalism was not statistically different from TG-21. CONCLUSIONS: Although further investigation is needed to validate the new protocols for other ionization chambers, these results can serve as a reference to quantitatively compare different calibration protocols and ionization chambers if a particular method is chosen by a professional society to serve as a standardized calibration protocol.

Comparison of myocardial tissue Doppler with transmitral flow Doppler in left ventricular hypertrophy.

We sought to determine the most useful echocardiographic measurements for assessment of diastolic function in patients with left ventricular hypertrophy (LVH) and normal systolic function. We compared myocardial Doppler velocities of the basal inferoposterior wall with mitral inflow pulsed wave Doppler velocities in 11 healthy volunteers (age, 36 +/- 6 years), 25 patients (age, 64 +/- 14 years) without LVH, and 37 patients (age, 67 +/- 14 years) with LVH and otherwise normal echocardiograms. The discriminatory measurements were myocardial A-wave duration (120 +/- 18 versus 98 +/- 20 and 92 +/- 12 ms, P <.0001), myocardial isovolumetric relaxation time (124 +/- 45 versus 95 +/- 48 and 78 +/- 25 ms, P =.0035), mitral A-wave velocity (0.98 +/- 0.37 versus 0.73 +/- 0.28 m/s and 0.61 +/- 0.22 m/s, P =.009), and mitral E-wave deceleration time (257 +/- 93 versus 201 +/- 85 ms and 184 +/- 83 ms, P =.015), which were significantly increased, and myocardial E-wave velocity (0.84 +/- 0.04 m/s versus 0.13 +/- 0.03 m/s and 0.14 +/- 0.03 m/s, P <.0001), which was significantly decreased, in patients with LVH compared with patients without LVH and normal volunteers, respectively. Left ventricular posterior wall thickness correlated with myocardial isovolumetric relaxation time (r = 0.52, P <.0001) and myocardial A-wave duration (r = 0.59, P <.0001), negatively with myocardial E wave (r = -0.43, P <.0001), and showed no correlation with mitral inflow parameters except mitral inflow A wave (r = 0.43, P =.002). On multivariate analysis using these variables, myocardial isovolumetric relaxation time (P =.0014) and A-wave duration (P =.001) were the only 2 variables that correlated with posterior wall thickness (multiple R = 0.71). In the presence of LVH and preserved left ventricular systolic function, myocardial relaxation time and velocities are more sensitive than mitral Doppler inflow parameters in detecting abnormal left ventricular relaxation.

Myocardial Doppler tissue imaging: findings in inferior myocardial infarction and left ventricular hypertrophy--wall motion assessment.

BACKGROUND: Because of the geometry of the basal inferior wall and its relation with the posterior medial papillary muscle, differentiating abnormal from normal basal inferior wall motion can be challenging. METHODS: We performed pulsed wave Doppler echocardiography of the basal inferior wall and basal interventricular septum in 26 patients (63 +/- 14 years) with a normal echocardiogram, 33 patients (67 +/- 14 years) with inferior myocardial infarction (MI) associated with hypokinesis to dyskinesis of the basal inferior wall, and 38 patients (67 +/- 14 years) with left ventricular hypertrophy (LVH). RESULTS: Systolic velocity was significantly lower in the basal interventricular septum (0.071 +/- 0.013 m/s versus 0.084 +/- 0.023 m/s) and basal inferior wall (0.075 +/- 0.014 m/s versus 0.085 +/- 0.019 m/s) in the MI group compared with the LVH group, and both were significantly lower compared with normal values at the interventricular septum (0.090 +/- 0.023 m/s, P <.001, analysis of variance) and basal inferior wall (0.095 +/- 0.014 m/s, P <.0001, analysis of variance). The sum of the systolic (S), early diastolic (E'), and late diastolic (A') velocities of 0.30 m/s at the basal inferior wall had 91%, 76%, and 84% sensitivity, specificity, and accuracy, respectively, for the differentiation of a normal wall from an infarcted basal inferior wall, and 76%, 73%, and 75% sensitivity, specificity, and accuracy, respectively, for the differentiation of a normal wall from a hypertrophied basal inferior wall. The sum of systolic and diastolic velocities of 0.25 m/s at the basal interventricular septum had 70%, 66%, and 68% sensitivity, specificity, and accuracy, respectively, for the differentiation of an infarcted from a hypertrophied basal interventricular septum. Mitral inflow early-filling wave deceleration time by pulsed wave Doppler was the most sensitive parameter for the differentiation of LVH from MI (P <.0001). CONCLUSION: Doppler tissue imaging velocities of the basal inferior wall and basal interventricular septum may help differentiate normal from infarcted and hypertrophied myocardium.