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A novel technique to enable experimental validation of deformable dose accumulation
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/content/aapm/journal/medphys/39/2/10.1118/1.3676185
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Figures

Image of FIG. 1.

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FIG. 1.

Pictures of the actuation device used to apply breath-mimicking sinusoidal motions to deform gel dosimeters during irradiation. In particular, the sample chamber and the motor unit are illustrated in more details.

Image of FIG. 2.

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FIG. 2.

An illustration of the motion characterization process in morfeus. The primary and secondary CBCT images were extracted from the 4D CBCT images acquired at the time of treatment to represent the gel at the undeformed and fully deformed states, respectively. The locations of the three seeds implanted in each deformable gel are indicated as white circles on the CBCT images. Gel contours on the primary and secondary CBCT images were converted into meshes, and the displacement profile at every node was then calculated using the contact surface technique in MORFEUS.

Image of FIG. 3.

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FIG. 3.

A flowchart summarizing the experiment procedure and the dose analysis methods to compare the dose accumulated in MORFEUS with the dose measured by the deformable gels under the same deformation.

Image of FIG. 4.

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FIG. 4.

R2-dose calibration curves obtained from Conf1 and Conf2. The linear fit equations and the coefficients of determinants (r2) are also displayed.

Image of FIG. 5.

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FIG. 5.

An axial snapshot of the STATIC dose for Conf1 (c) and the dose measured by Conf1 (d). The dose comparisons between the STATIC distribution and the distribution measured by the control gels were performed in the volume enclosed by the 200 cGy isodose surface on the STATIC dose map, which is outlined in black in (a) and (b).

Image of FIG. 6.

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FIG. 6.

An axial snapshot of the dose measured by Conf3 (a) and the dose accumulated by MORFEUS for Conf3 (b). The dose comparisons between the dose measured by the deformable gels and the accumulated dose were performed in the volume enclosed by the 200 cGy isodose surface on the gel dose map, which is outlined in black in (a) and (b).

Tables

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TABLE I.

Composition of the gel dosimeters.

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TABLE II.

Summary of the designed treatment for the five gel dosimeters fabricated in this study, including the purpose they serve, the magnitude of deformation applied to them, the treatment plan, and the number of gold seeds implanted to serve as internal fiducials.

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TABLE III.

The six phases (ф) into which the motion applied to the deformable gels was divided and the time weighting factor (τф ) associated with each phase used in the MORFEUS dose accumulation computation.

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TABLE IV.

Summary of the absolute accuracy of the deformation map calculated by the MORFEUS deformable image registration method with the contact surface technique integrated. For each deformable gel, accuracy was assessed by comparing the displacements of the three implanted gold seeds estimated by MORFEUS with the seed displacements measured from the sorted 4D CBCT images of that gel. The mean and standard deviation of the absolute vector errors and the absolute errors in the LR, AP, and SI direction are shown, as well as the magnitude of deformation applied to each gel.

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TABLE V.

The mean and standard deviation of the voxel-by-voxel dose differences between the dose measured by each control gel and the corresponding STATIC distribution calculated in the TPS. Dose difference is reported in terms of both dose level (cGy) and as a percentage (%) of the STATIC dose, which was the benchmark distribution against which the dose measured by the gel was compared.

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TABLE VI.

The mean and standard deviation of the distances between corresponding isodose surfaces on the dose distribution measured by the control gels (Conf1 and Conf2) and on the STATIC distribution calculated in the TPS.

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TABLE VII.

The percentage of the voxels of the dose distribution measured by each control gel that have passed the gamma index test using the 2%/2 mm and 3%/3 mm criteria.

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TABLE VIII.

The mean and standard deviation of the voxel-by-voxel differences between the dose measured by each deformable gel and the dose accumulated in MORFEUS for the same gel. Dose difference is reported in terms of both dose level (cGy) and as a percentage (%) of the gel-measured dose, which was the benchmark distribution against which the accumulated dose was compared. The magnitude of deformation applied to each gel during irradiation is also included.

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TABLE IX.

The mean and standard deviation of the distances between corresponding isodose surfaces on the dose distribution measured by each deformable gel (Conf3, Conf4, and Conf5) and on the dose accumulated in MORFEUS for the same gel.

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TABLE X.

The percentage of the voxels of the accumulated doses for the deformable gels (Conf3, Conf4, and Conf5) that have passed the gamma index test using the 11.8 cGy/2 mm (3 mm) and 4.7%/2 mm (3 mm) criteria. The dose difference criteria of 11.8 cGy and 4.7% reflect the gel dosimeters’ dose measurement precision of the conformal plan and were determined from the results of the voxel-by-voxel comparison between the control gels and the corresponding STATIC distributions.

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/content/aapm/journal/medphys/39/2/10.1118/1.3676185
2012-01-18
2014-04-17

Abstract

Purpose:

To propose a novel technique to experimentally validate deformable dose algorithms by measuring 3D dose distributions under the condition of deformation using deformable geldosimeters produced by a novel gel fabrication method.

Method:

Five geldosimeters, two rigid control gels and three deformable gels, were manufactured and treated with the same conformal plan that prescribed 400 cGy to the isocenter. The control gels were treated statically; the deformable gels were treated while being compressed by an actuation device to simulate breathing motion (amplitude of compression = 1, 1.5, and 2 cm, respectively; frequency = 16 rpm). Comparison between the dose measured by the control gels and the corresponding static dose distribution calculated in the treatment planning system (TPS) has determined the intrinsic dose measurement uncertainty of the geldosimeters.Doses accumulated using MORFEUS, a biomechanical model-based deformable registration and dose accumulation algorithm, were compared with the doses measured by the deformable geldosimeters to verify the accuracy of MORFEUS using dose differences at each voxel as well as the gamma index test. Flexible plastic wraps were used to contain and protect the deformable gels from oxygen infiltration, which inhibits the gels’ dose sensitizing ability. Since the wraps were imperfect oxygen barrier, dose comparison between MORFEUS and the deformable gels was performed only in the central region with a received dose of 200 cGy or above to exclude the peripheral region where oxygen penetration had likely affected dose measurements.

Results:

Dose measured with the control gels showed that the intrinsic dose measurement uncertainty of the geldosimeters was 11.8 cGy or 4.7% compared to the TPS. The absolute mean voxel-by-voxel dose difference between the accumulated dose and the dose measured with the deformable gels was 4.7 cGy (SD = 36.0 cGy) or 1.5% (SD = 13.4%) for the three deformable gels. The absolute mean vector distance between the 250, 300, 350, and 400 cGy isodose surfaces on the accumulated and measured distributions was 1.2 mm (SD < 1.5 mm). The gamma index test that used the dose measurement precision of the control gels as the dose difference criterion and 2 mm as the distance criterion was performed, and the average pass rate of the accumulated dose distributions for all three deformable gels was 92.7%. When the distance criterion was relaxed to 3 mm, the average pass rate increased to 96.9%.

Conclusion:

This study has proposed a novel technique to manufacture deformable volumetric geldosimeters. By comparing the doses accumulated in MORFEUS and the doses measured with the dosimeters under the condition of deformation, the study has also demonstrated the potential of using deformable geldosimetry to experimentally validate algorithms that include deformations into dose computation. Since dose less than 200 cGy was not evaluated in this study, future investigations will focus more on low dose regions by either using bigger geldosimeters or prescribing a lower dose to provide a more complete experimental validation of MORFEUS across a wider dose range.

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Scitation: A novel technique to enable experimental validation of deformable dose accumulation
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/39/2/10.1118/1.3676185
10.1118/1.3676185
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