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The feasibility of estimating patient-specific dose verification results directly from linear accelerator (linac) log files has been investigated for prostate cancer patients who undergo volumetric modulated arc therapy (VMAT). Twenty-six patients who underwent VMAT in our facility were consecutively selected. VMAT plans were created using Monaco treatment planning system and were transferred to an Elekta linac. During the beam delivery, dynamic machine parameters such as positions of the multi-leaf collimator and the gantry were recorded in the log files; subsequently, root mean square (rms) values of control errors, speeds and accelerations of the above machine parameters were calculated for each delivery. Dose verification was performed for all the plans using a cylindrical phantom with diodes placed in a spiral array. The gamma index pass rates were evaluated under 3%/3 mm and 2%/2 mm criteria with a dose threshold of 10%. Subsequently, the correlation coefficients between the gamma index pass rates and each of the above rms values were calculated. Under the 2%/2 mm criteria, significant negative correlations were found between the gamma index pass rates and the rms gantry angle errors (<i>r</i> = 0.64, p < 0.001) as well as the pass rates and the rms gantry accelerations (<i>r</i> = 0.68, p < 0.001). On the other hand, the rms values of the other dynamic machine parameters did not significantly correlate with the gamma index pass rates. We suggest that the VMAT quality assurance (QA) results can be directly estimated from the log file thereby providing potential to simplify patient-specific prostate VMAT QA procedure.

Volumetric modulated arc therapy (VMAT) is a high precision beam delivery technique that dynamically varies multi-leaf collimator (MLC) leaf positions and dose rates during gantry rotation on a linear accelerator (linac) [

Several groups previously reported VMAT plan complexity indices by referring to each treatment plan file [

Tyagi et al. reported the correlation between MU-weighted integral field aperture errors and dose errors in a target and organs at risk, where the integral field aperture errors were calculated by comparing planned and measured field apertures by way of linac mechanical data monitoring whereas the dose errors were calculated by comparing planned and reconstructed doses [

Previous reports mainly focused on field aperture or leaf position errors and few papers investigated correlation between gantry angle errors and the dose verification results. For example, Agnew et al. reported that the correlations between the gantry angle errors and the dose accuracy were not observed [

Twenty-six prostate cancer patients who underwent VMAT in our institute were retrospectively and consecutively selected. Computer tomography (CT) imaging with a slice thickness of 2 mm was performed in supine position with a full bladder. This study was approved by the institutional review board.

Gross tumor volume (GTV) consisted of the entire prostate and the seminal vesicles. Clinical target volume (CTV) was identical to the GTV. Planning target volume (PTV) was defined by adding a margin of 10 mm to the CTV in all directions, except a posterior margin of 5 mm. Rectum, bladder, small bowel and large bowel were contoured as organs at risk.

VMAT plans were created by Monaco v.3.3 treatment planning system (TPS) (Elekta AB, Stockholm, Sweden) using a 10 MV photon beam. All VMAT plans consisted of a single clockwise full arc. A dose of 76 Gy in 38 fractions was prescribed to the 95% volume of the PTV excluding rectum overlap. A grid size of 3 mm was selected for the Monaco Monte Carlo dose calculation.

A Synergy linac (Elekta AB, Stockholm, Sweden) equipped with an MLC with a leaf width of 1 cm was employed. The maximum gantry speed was 6 degree/sec, and the maximum leaf speed was 2 cm/sec. The dose rate was variable by a factor of two in six steps.

The VMAT plans were delivered in service mode, and gantry angles and MLC leaf positions along with cumulative monitor units (MU) and dose rates were recorded in log files with a sampling interval of 250 msec. The log file data were analyzed using an in-house software created by Visual Basic 2010 (Microsoft, Redmond, USA), resulting in control errors, speeds, and accelerations of the gantry angle and the MLC leaf positions in every 250 msec, based on the following equations where i denotes the current sample number:

To simplify the above leaf parameter calculation, only central six leaf pairs that may cover the prostate PTV were considered.

A cylindrical phantom with an array detector, ArcCHECK (Sun Nuclear Corp., FL) was used to verify the patient-specific delivery accuracy. The ArcCHECK has 1386 diodes with a measurement diameter of 21 cm. All treatment plans were transferred to the phantom for dose recalculation in Monaco. Gamma index pass rates between the calculated and the measured dose distributions were evaluated under two different criteria of 3%/3 mm and 2%/2 mm with a dose threshold of 10%.

A root mean square (rms) value of each parameter during each VMAT delivery was calculated. Subsequently, Pearson’s correlation coefficients (r) between gamma index pass rates and each of the above rms values were calculated by SPSS v.19 (IBM, New York, U.S.A.). Statistical significance was defined as a p value of <0.05.

mean (SD) | max | min | |
---|---|---|---|

rms leaf position error (mm) | 0.57 (0.18) | 0.83 | 0.27 |

rms leaf speed (cm/sec) | 0.65 (0.04) | 0.77 | 0.60 |

rms leaf acceleration (cm/sec^{2}) | 0.39 (0.07) | 0.52 | 0.27 |

rms gantry angle error (degree) | 0.37 (0.11) | 0.60 | 0.15 |

rms gantry speed (degree/sec) | 3.13 (0.33) | 3.81 | 2.13 |

rms gantry acceleration (degree/sec^{2}) | 7.50 (3.29) | 14.05 | 3.15 |

Abbreviation: SD = standard deviation.

gamma index criteria | mean (SD) | max | min |
---|---|---|---|

3%/3 mm | 99.6 (0.55) | 100.0 | 97.3 |

2%/2 mm | 97.4 (2.24) | 99.4 | 89.9 |

error, and b) the rms gantry acceleration against the rms gantry angle error. These parameters were calculated using a log file recorded during VMAT delivery. The correla-

tion coefficients of the rms gantry angle error versus each of the rms gantry speed and the rms gantry acceleration were 0.74, 0.66 respectively. Significant positive correlations were observed.

gamma index criteria | ||||
---|---|---|---|---|

3%/3 mm | 2%/2 mm | |||

r | p value | r | p value | |

rms leaf position error (mm) | 0.15 | 0.481 | 0.31 | 0.126 |

rms leaf speed (cm/sec) | −0.24 | 0.246 | −0.13 | 0.540 |

rms leaf acceleration (cm/sec^{2}) | −0.19 | 0.360 | −0.17 | 0.399 |

rms gantry angle error (degree) | −0.49 | 0.011 | −0.64 | <0.001 |

rms gantry speed (degree/sec) | −0.18 | 0.374 | −0.36 | 0.069 |

rms gantry acceleration (degree/sec^{2}) | −0.47 | 0.017 | −0.68 | <0.001 |

tion. The correlation became stronger for stricter gamma index criteria of 2%/2 mm. On the other hand, other dynamic machine parameters related to the MLC did not significantly correlated with the gamma index pass rates.

In this study, we investigated the correlation between the gamma index pass rate and each rms of leaf position errors, leaf speeds and leaf accelerations. The scatter plots in

We also investigated the correlation between the gamma index pass rates and each rms of the gantry angle errors, the gantry speeds and the gantry accelerations. It was demonstrated in

We also demonstrated in

The purpose of this study was to pursue the feasibility of estimating patient-specific dose verification results directly from linac log files.

curve fitting to the plots of a) the gamma index pass rates against the rms gantry angle error and b) the gamma index pass rates against the gantry acceleration, both given in ^{2}, the 95% confidence interval for the gamma passing rate may be between 97.0% and 99.6%; on the other hand, if the rms gantry acceleration is 12 degree/sec^{2}, the 95% confidence interval may be between 90.6% and 100%. A possible cause of the larger variations in the gamma index pass rates may be that the impact of a given large rms gantry angle error on dose distributions depends on treatment plans, whereas the doses were always evaluated at the same detector positions.

The present study shows a possibility of estimating gamma index pass rates from dynamic parameters by analyzing the linac log files acquired during VMAT delivery. It was reported that log file analysis should not be solely relied upon for QA as it does not detect systematic machine errors resulting from incomplete calibration [

Our study was performed under binned dose rate control, where the dose rate varied by a factor of two in six steps due to an earlier linac system design. Because the dose rate is not continuously variable, the gantry speed needs to be extensively varied in order to meet the MU/degree prescribed in the treatment plan thereby possibly resulting in frequent gantry acceleration and deceleration. A more recent linac controller system allows continuously variable dose rate in 256 steps [

A limitation of this study lies in that only prostate VMAT plans were examined. Evaluation for other tumor sites may allow us to analyze the linac dynamic parameters more extensively, thereby possibly resulting in more comprehensive perspective.

We have demonstrated that the patient-specific prostate VMAT QA results may be directly estimated from gantry angle errors or gantry accelerations by analyzing the log files, thereby providing potential to simplify patient-specific prostate VMAT QA pro- cedure by improving efficiency and laborsaving. Evaluation for other tumor sites is needed for further comprehensive insights.

The authors thank all the radiological technologists from the Department of Radiological Technology, Hyogo College of Medicine College Hospital (Nishinomiya, Hyogo, Japan) for their general guidance and encouragement for this work, and they also acknowledge Kiyoshi Yoda from Elekta KK for his invaluable comments on this research.

Kosaka, K., Tanooka, M., Doi, H., Inoue, H., Tarutani, K., Suzuki, H., Takada, Y., Fujiwara, M., Kamikonya, N. and Hirota, S. (2016) Feasibility of Estimating Patient-Specific Dose Verification Results Directly from Linear Accelerator Log Files in Volumetric Modulated Arc The- rapy. International Journal of Medical Phy- sics, Clinical Engineering and Radiation On- cology, 5, 317-328. http://dx.doi.org/10.4236/ijmpcero.2016.54031