^{1,a)}, Elena Vaccara

^{1}, Roberta Corvisiero

^{1}, Paolo Cavazzani

^{2}, Filippo Grillo Ruggieri

^{2}and Gianni Taccini

^{3}

### Abstract

In the authors’ hospital, stereotactic radiotherapy treatments are performed with a Varian Clinac 600C equipped with a BrainLAB micro-multileaf-collimator generally using the dynamic conformal arc technique. Patient immobilization during the treatment is achieved with a fixation mask supplied by BrainLAB, made with two reinforced thermoplastic sheets fitting the patient’s head. With this work the authors propose a method to evaluate treatment geometric accuracy and, consequently, to determine the amount of the margin to keep in the CTV-PTV expansion during the treatment planning. The reproducibility of the isocenter position was tested by simulating a complete treatment on the anthropomorphic phantom Alderson Rando, inserting in between two phantom slices a high sensitivity Gafchromic EBT film, properly prepared and calibrated, and repeating several treatment sessions, each time removing the fixing mask and replacing the film inside the phantom. The comparison between the dose distributions measured on films and computed by TPS, after a precise image registration procedure performed by a commercial piece of software (FILMQA, 3cognition LLC (Division of ISP), Wayne, NJ), allowed the authors to measure the repositioning errors, obtaining about 0.5 mm in case of central spherical PTV and about 1.5 mm in case of peripheral irregular PTV. Moreover, an evaluation of the errors in the registration procedure was performed, giving negligible values with respect to the quantities to be measured. The above intrinsic two-dimensional estimate of treatment accuracy has to be increased for the error in the third dimension, but the 2 mm margin the authors generally use for the CTV-PTV expansion seems adequate anyway. Using the same EBT films, a dosimetric verification of the treatment planning system was done. Measured dose values are larger or smaller than the nominal ones depending on geometric irradiation conditions, but, in the authors’ experimental conditions, always within 4%.

I. INTRODUCTION

II. MATERIALS AND METHODS

II.A. EBT Gafchromic Film

II.B. Measurement sensitivity

III. RESULTS AND DISCUSSION

III.A. Isocenter dose

III.B. Isocenter position

III.C. Extension of 80% isodose

IV. CONCLUSIONS

### Key Topics

- Dosimetry
- 28.0
- Computed tomography
- 23.0
- Medical imaging
- 19.0
- Image registration
- 8.0
- Radiosurgery
- 5.0

## Figures

The TaPo is fixed on the stereotactic frame during the treatments. The five paper sheets placed on it show the projection of the isocenter and the micro-MLC shape at the beginning and at the end of each arc. In the example considered in the figure a double isocenter treatment is shown. The laser is positioned on the first isocenter.

The TaPo is fixed on the stereotactic frame during the treatments. The five paper sheets placed on it show the projection of the isocenter and the micro-MLC shape at the beginning and at the end of each arc. In the example considered in the figure a double isocenter treatment is shown. The laser is positioned on the first isocenter.

The couch mount is rigidly fixed to the couch and, using the four screws shown in the figure, it is possible to slightly adjust the TaPo position and tilt without moving the couch.

The couch mount is rigidly fixed to the couch and, using the four screws shown in the figure, it is possible to slightly adjust the TaPo position and tilt without moving the couch.

Before each arc, after setting the right couch and collimator angles, we check if, with gantry angle set to 0°, the crosshair shadow coincides with the isocenter projection and, if not, we use the lateral and the longitudinal fine adjustment screws placed in the couch mount to correct such a misalignment. (a) After that, we set the right start and stop gantry angles and check if the field light coincides with the shape printed on the TaPo. (b) If not, we adjust the vertical screw, or, if it is not enough, a combination of the other screws (tilt, lateral, and longitudinal). When everything is okay, we treat the patient.

Before each arc, after setting the right couch and collimator angles, we check if, with gantry angle set to 0°, the crosshair shadow coincides with the isocenter projection and, if not, we use the lateral and the longitudinal fine adjustment screws placed in the couch mount to correct such a misalignment. (a) After that, we set the right start and stop gantry angles and check if the field light coincides with the shape printed on the TaPo. (b) If not, we adjust the vertical screw, or, if it is not enough, a combination of the other screws (tilt, lateral, and longitudinal). When everything is okay, we treat the patient.

Treatment plans for the three sets of measurements. (a) The first plan refers to sets 1 and 2, (b) while the second one refers to set 3. In both plans the arc positions are the same (couch at 330°, 300°, 90°, 60°, and 30°), while arc extensions are different in order to be suited to the two different geometric conditions. The micro-MLC leaves are optimized in order to cover the PTV periphery with the 80% isodose. (a) For sets 1 and 2 the PTV is spherical and placed in the middle of the skull, (b) while for set 3 the PTV is irregular and placed in the skull periphery.

Treatment plans for the three sets of measurements. (a) The first plan refers to sets 1 and 2, (b) while the second one refers to set 3. In both plans the arc positions are the same (couch at 330°, 300°, 90°, 60°, and 30°), while arc extensions are different in order to be suited to the two different geometric conditions. The micro-MLC leaves are optimized in order to cover the PTV periphery with the 80% isodose. (a) For sets 1 and 2 the PTV is spherical and placed in the middle of the skull, (b) while for set 3 the PTV is irregular and placed in the skull periphery.

“Grey level–dose” calibration for Gafchromic EBT films. The red channel (○) shows the highest sensitivity, especially up to 500 cGy.

“Grey level–dose” calibration for Gafchromic EBT films. The red channel (○) shows the highest sensitivity, especially up to 500 cGy.

A Gafchromic EBT film (a) before and (b) after registration with the corresponding (c) calculated dose matrix. Such film refers to the third set of measurements (irregular PTV, far from the middle).

A Gafchromic EBT film (a) before and (b) after registration with the corresponding (c) calculated dose matrix. Such film refers to the third set of measurements (irregular PTV, far from the middle).

(a) A pierced Gafchromic film fixed onto a phantom slice. In the upper right corner a magnified hole is shown. (b) The same film is entirely encompassed by a single CT scan. The five holes in the phantom slice, through which the film is pierced, are clearly visible.

(a) A pierced Gafchromic film fixed onto a phantom slice. In the upper right corner a magnified hole is shown. (b) The same film is entirely encompassed by a single CT scan. The five holes in the phantom slice, through which the film is pierced, are clearly visible.

Schematic representation of the relationship among FLE, FRE, and TRE. (a) A dose matrix with two target points (1 and 2) and four fiducials, indicated by small circles and supposed to be without localization error for simplicity, is shown. The reference system has its origin in the fiducial centroid. (b) A digitized film with the four fiducials, indicated by crosses and characterized by a localization error (schematized as arrows from the “correct” localizations) is shown. The reference system has its origin in the new fiducial centroid and is oriented differently from . In order to simplify any comparison with the dose matrix, the four correctly localized fiducials, the reference system, and the two target points are displayed. (c) The digitized film has been shifted and rotated to minimize the root mean square distance between homologous fiducials (linked by double tip arrows) that is coregistrated with the dose matrix. The root mean square residual error after coregistration is the FRE and its value is returned by the FILMQA software. The TRE is the localization error in special points of interest: If the point is close to the fiducial centroid (target 1), the overall error is quite small and mainly due to the translational component, while if the point is far from the centroid (target 2), the rotational component becomes large and makes the overall error larger.

Schematic representation of the relationship among FLE, FRE, and TRE. (a) A dose matrix with two target points (1 and 2) and four fiducials, indicated by small circles and supposed to be without localization error for simplicity, is shown. The reference system has its origin in the fiducial centroid. (b) A digitized film with the four fiducials, indicated by crosses and characterized by a localization error (schematized as arrows from the “correct” localizations) is shown. The reference system has its origin in the new fiducial centroid and is oriented differently from . In order to simplify any comparison with the dose matrix, the four correctly localized fiducials, the reference system, and the two target points are displayed. (c) The digitized film has been shifted and rotated to minimize the root mean square distance between homologous fiducials (linked by double tip arrows) that is coregistrated with the dose matrix. The root mean square residual error after coregistration is the FRE and its value is returned by the FILMQA software. The TRE is the localization error in special points of interest: If the point is close to the fiducial centroid (target 1), the overall error is quite small and mainly due to the translational component, while if the point is far from the centroid (target 2), the rotational component becomes large and makes the overall error larger.

Measurement technique of isocenter displacements. (a) A dose matrix image with four fiducials and the 80% isodose and (b) a digitized film with the corresponding four fiducials (supposed to be without localization error for simplicity) and the 80% isodose are shown. (c) After the image coregistration the homologous fiducials coincide, while the 80% isodose curves are slightly shifted because of geometric uncertainties. (d) and , the shifts necessary to make the two isodose curves coincident or concentric, are the quantities that we consider as repositioning errors.

Measurement technique of isocenter displacements. (a) A dose matrix image with four fiducials and the 80% isodose and (b) a digitized film with the corresponding four fiducials (supposed to be without localization error for simplicity) and the 80% isodose are shown. (c) After the image coregistration the homologous fiducials coincide, while the 80% isodose curves are slightly shifted because of geometric uncertainties. (d) and , the shifts necessary to make the two isodose curves coincident or concentric, are the quantities that we consider as repositioning errors.

80% computed (thick line) and measured (thin line) isodose curves (a) after registration with the corresponding dose matrix and (b) after shifting the film by 0.9 mm in the direction and by 1.1 mm in the direction. Such movements are necessary to make the curves concentric and represent the isocenter positioning error.

80% computed (thick line) and measured (thin line) isodose curves (a) after registration with the corresponding dose matrix and (b) after shifting the film by 0.9 mm in the direction and by 1.1 mm in the direction. Such movements are necessary to make the curves concentric and represent the isocenter positioning error.

Measurements of isocenter repositioning errors. (×) Set 1: spherical PTV in the middle, no TaPo. (○) Set 2: spherical PTV in the middle, TaPo mounted. (△) Set 3: irregular PTV in the periphery, TaPo mounted. The arrows represent the systematic errors, while the ellipse and semiaxes represent the standard deviations, that is, the random errors, in the same directions. The use of TaPo decreases both systematic and random errors, while the peripheral position of the PTV increases just the systematic error, leaving the random error unchanged.

Measurements of isocenter repositioning errors. (×) Set 1: spherical PTV in the middle, no TaPo. (○) Set 2: spherical PTV in the middle, TaPo mounted. (△) Set 3: irregular PTV in the periphery, TaPo mounted. The arrows represent the systematic errors, while the ellipse and semiaxes represent the standard deviations, that is, the random errors, in the same directions. The use of TaPo decreases both systematic and random errors, while the peripheral position of the PTV increases just the systematic error, leaving the random error unchanged.

## Tables

Characteristics of the three sets of measurements: PTV shape, PTV position in the skull, and use of TaPo.

Characteristics of the three sets of measurements: PTV shape, PTV position in the skull, and use of TaPo.

Translational and rotational components of the TRE and ratio TRE/FLE in the two different experimental conditions. In the first two measurement sets is negligible with respect to , while in the third set, characterized by a geometrical asymmetry, becomes comparable to . Their sum does not change much because of the increment of the fiducial number from 5 to 9 and so the TRE remains about 45% of the FLE in both configurations.

Translational and rotational components of the TRE and ratio TRE/FLE in the two different experimental conditions. In the first two measurement sets is negligible with respect to , while in the third set, characterized by a geometrical asymmetry, becomes comparable to . Their sum does not change much because of the increment of the fiducial number from 5 to 9 and so the TRE remains about 45% of the FLE in both configurations.

Registration errors (FRE, FLE, and TRE) in the three measurement sets. As TREs are always less than 0.05 mm, that is, much smaller than the quantities we want to measure, we are sure that the registration procedure does not affect the results of our measurements.

Registration errors (FRE, FLE, and TRE) in the three measurement sets. As TREs are always less than 0.05 mm, that is, much smaller than the quantities we want to measure, we are sure that the registration procedure does not affect the results of our measurements.

Average percentage shifts from expected dose value (5 Gy) and standard deviations of the five measurements in the three experimental conditions.

Average percentage shifts from expected dose value (5 Gy) and standard deviations of the five measurements in the three experimental conditions.

(LL), (AP), and radial isocenter displacements in the three measurement sets. Each quantity is expressed as mean value and standard deviation of the five measurements.

(LL), (AP), and radial isocenter displacements in the three measurement sets. Each quantity is expressed as mean value and standard deviation of the five measurements.

Differences between measured and computed radial extensions of reference isodose ( of 500 cGy). If we consider that the isocenter dose value can be slightly different from the nominal one and measure the same quantities referring to the 80% of the measured isocenter dose value, we obtain much closer values. Each quantity is expressed as mean value and standard deviation of the five measurements.

Differences between measured and computed radial extensions of reference isodose ( of 500 cGy). If we consider that the isocenter dose value can be slightly different from the nominal one and measure the same quantities referring to the 80% of the measured isocenter dose value, we obtain much closer values. Each quantity is expressed as mean value and standard deviation of the five measurements.

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