In this study, we evaluated the image distortion of three magnetic resonance imaging (MRI) systems with magnetic field strengths of 0.4 T, 1.5 T and 3 T, during stereotactic irradiation of the brain. A quality assurance phantom for MRI image distortion in radiosurgery was used for these measurements of image distortion. Images were obtained from a 0.4-T MRI (APERTO Eterna, HITACHI), a 1.5-T MRI (Signa HDxt, GE Healthcare) and a 3-T MRI (Signa HDx 3.0 T, GE Healthcare) system. Imaging sequences for the 0.4-T and 3-T MRI were based on the 1.5-T MRI sequence used for stereotactic irradiation in the clinical setting. The same phantom was scanned using a computed tomography (CT) system (Aquilion L/B, Toshiba) as the standard. The results showed mean errors in the Z direction to be the least satisfactory of all the directions in all results. The mean error in the Z direction for 1.5-T MRI at －110 mm in the axial plane showed the largest error of 4.0 mm. The maximum errors for the 0.4-T and 3-T MRI were 1.7 mm and 2.8 mm, respectively. The errors in the plane were not uniform and did not show linearity, suggesting that simple distortion correction using outside markers is unlikely to be effective. The 0.4-T MRI showed the lowest image distortion of the three. However, other items, such as image noise, contrast and study duration need to be evaluated in MRI systems when applying frameless stereotactic irradiation.

SummaryIn standard external beam radiotherapy dosimetry, which is based on absorbed dose by water, the absorbed dose at any calibration depth is calculated using the same beam quality conversion factor, regardless of the presence or absence of a waterproofing sleeve. In this study, we evaluated whether there were differences between absorbed doses at calibration depths calculated using a beam quality conversion factor including a wall correction factor that corresponds to a waterproofing sleeve thickness of 0.3 mm, and without a waterproofing sleeve. The Japan Society of Medical Physics (JSMP) has reported that the uncertainty of the results using a beam quality conversion factor that included a wall correction factor corresponding to a waterproofing sleeve thickness of 0.3 mm, regardless of the presence or absence of the sleeve, was 0.2%. This uncertainty proved to be in agreement with the reported range.

A comparison of absorbed doses to water at a calibration depth determined by Japan Society of Medical Physics (JSMP) 12 and 01 was conducted, using a farmer type ionization chamber. The absorbed dose to water calibration factor (ND,W,Q0) and beam quality conversion factor (kQ,Q0) for JSMP 12 were smaller than the absorbed dose to water calibration factor and beam quality conversion factor for JSMP 01. Differences in absorbed doses at a calibration depth were -0.78% for 6 MV photon beam and -0.94% for 10 MV photon beam. In the present experiment, absorbed doses at a calibration depth were measured, using a farmer type ionization chamber. Further experiments at a larger number of facilities should be conducted to reveal the status of measurement of absorbed doses at a calibration depth using JSMP 12.

High tissue contrast in magnetic resonance imaging (MRI) allows better radiotherapy planning. However, geometric distortion in MRI induces inaccuracies affecting such planning, making it necessary to evaluate the characteristics of such geometric distortion. Although many studies have considered geometric distortion, most of these involved measurements performed only a few times. In this study, we evaluated MRI device-specific geometric distortion over long term and measured its variation by using an automatic analysis tool. The result showed that geometric distortion increased with distance from the center along both lateral and longitudinal directions. Specifically, the average distortion rate and average diameter error over the full measurement period increased by up to 1.02% and 1.96 mm, respectively, when using T1 weighted Image (WI) 3D fast spoiled gradient echo (FSPGR) at R15. In the case of T2 WI 2D fast spin echo (FSE) at R15, the standard deviation of the distortion rate and diameter error increased up to 0.38%, 0.72 mm, respectively. We conclude that periodic quality assurance of geometric distortion should be performed in order to maintain geometric distortion within allowable values.

Purpose: Several clinical reference dosimetry protocols for absorbed‐dose to water have recently been published: The American Association of Physicists in Medicine (AAPM) published an Addendum to the AAPM's TG‐51 (Addendum TG‐51) in April 2014, and the Japan Society of Medical Physics (JSMP) published the Japan Society of Medical Physics 12 (JSMP12), a clinical reference dosimetry protocol, in September 2012. This investigation compared and evaluated the absorbed‐dose to water of high‐energy photon beams according to Addendum TG‐51, International Atomic Energy Agency Technical Report Series No. 398 (TRS‐398), and JSMP12. Methods: Differences in the respective beam quality conversion factors with Addendum TG‐51, TRS‐398, and JSMP12 were analyzed and the absorbed‐dose to water using 6‐ and 10‐MV photon beams was measured according to the protocols recommended in Addendum TG‐51, TRS‐398, and JSMP12. The measurements were conducted using two Farmer‐type ionization chambers, Exradin A12 and PTW 30013. Results: The beam quality conversion factors for both the 6‐ and 10‐MV photon beams with Addendum TG‐51 were within 0.6%, in agreement with the beam quality conversion factors with TRS‐398 and JSMP12. The Exradin A12 provided an absorbed‐dose to water ratio from 1.003 to 1.006 with TRS‐398 / Addendum TG‐51 and from 1.004 to 1.005 with JSMP 12 / Addendum TG‐51, whereas the PTW 30013 provided a ratio of 1.001 with TRS‐398 / Addendum TG‐51 and a range from 0.997 to 0.999 with JSMP 12 / Addendum TG‐51. Conclusion: Despite differences in the beam quality conversion factor, no major differences were seen in the absorbed‐dose to water with Addendum TG‐51, TRS‐398, and JSMP12. However, Addendum TG‐51 provides the most recent data for beam quality conversion factors based on Monte Carlo simulation and greater detail for the measurement protocol. Therefore, the absorbed‐dose to water measured with Addendum TG‐51 is an estimate with less uncertainty.