Development of a phantom to evaluate the positioning accuracy of patient immobilization systems using thermoplastic mask and polyurethane cradle.

PURPOSE: The purpose of this study was to develop a new phantom to evaluate the positioning accuracy of patient immobilization systems. METHODS: The phantom was made of papers formed into a human shape, paper clay, and filling rigid polyester. Acrylonitrile butadiene styrene (ABS) pipes were inserted at anterior-posterior (A-P) and right-left (R-L) directions in the phantom to give static load by pulling ropes through the pipes. First, the positioning precision of the phantom utilizing a target locating system (TLS) was evaluated by moving the phantom on a couch along inferior-superior (I-S), A-P, and R-L directions in a range from -5 mm to +5 mm. The phantom's positions detected with the TLS were compared with values measured by a vernier caliper. Second, the phantom movements in a tensile test were chosen from patient movements determined from 15 patients treated for intracranial lesions and immobilized with a thermoplastic mask and polyurethane cradle. The phantom movement was given by minimum or maximum values of patient movements in each direction. Finally, the relationship between phantom movements and the static load in the tensile test was characterized from measurements using the new phantom and the TLS. RESULTS: The differences in all positions between the vernier caliper measurement and the TLS detected values were within 0.2 mm with frequencies of 100%, 95%, and 90% in I-S, A-P, and R-L directions, respectively. The phantom movements according to patient movements in clinical application in I-S, A-P, and R-L directions were within 0.58 mm, 0.94 mm, and 0.93 mm from the mean value plus standard deviation, respectively. The regression lines between the phantom movements and static load were given by y = 0.359x, y = 0.241x, and y = 0.451x in I-S, A-P, and R-L directions, respectively, where x is the phantom movement (mm) and y is the static load (kgf). The relationship between the phantom movements and static load may represent the performance of inhibiting patient movements, so the accuracy of the immobilization system in the intracranial lesion will be estimated in advance by basic tensile test on the new phantom. CONCLUSIONS: The newly developed phantom was useful to evaluate the accuracy of immobilization systems for a Cyberknife system for intracranial lesions.

Estimation of imaging intervals and intrafraction displacement in CyberKnife image-guided radiotherapy for intracranial lesions.

PURPOSE: A recent report by the American Association of Physicists in Medicine Task Group 75 and 180 provided imaging dose estimates for image-guided CyberKnife radiotherapy. However, to our knowledge, there have been no concrete demonstrations of imaging intervals that are directly linked to exposure dose. We hypothesized that setting a rational standard may be clearer through a balance of treatment accuracy and reducing imaging doses if the margin of the planned treatment volume is controlled through the imaging interval. This study was conducted to simulate the association between the imaging interval and intrafraction displacement and to estimate a reasonable internal margin (IM). METHODS: We retrospectively analyzed data from 21 shell-fixed heads of patients treated with CyberKnife G3 using our dedicated monitoring system. This system comprises pressure sensors that can monitor head displacement every 0.2 s in the absence of any imaging dose. First, the root sum square of head displacements was calculated in 76 treatment fractions with an imaging interval of 10-1440 s. The cumulative frequency of a root sum square displacement (which was less than the IM) was evaluated in image verifications that were undertaken 546 274 times for every imaging interval. RESULTS: We found that the mean values and SDs of the displacement were larger in proportion to the imaging interval (p < 0.002) and that the maximum displacements did not correlate in any combination within 720 s (p > 0.056). The cumulative frequencies of displacement of 0.6 and 1.4 mm (i.e., less than an IM) were 99.2% and 99.1% for imaging intervals of 10 and 360 s, respectively. CONCLUSIONS: In the current study, we found that imaging intervals were directly proportional to intrafraction displacement and that there was no correlation in any combination within 720 s. Imaging intervals for an IM of 0.6 and 1.4 mm were 10 and 360 s, respectively, with a 99% confidence interval of intrafraction displacement. With CyberKnife M6 or a previous version of this system, the imaging dose could be reduced by 0.4760 mSv per 24-min treatment as the imaging dose ranged from 0.4896 to 0.0136 mSv for imaging intervals of 10 and 360 s with an IM of 0.6 and 1.4 mm, respectively. A rational method that includes X-ray imaging guidance may be achieved with modulation of the imaging interval via the CyberKnife system.

Relationship between pressure levels on the occipital region and intrafraction motion using an individualized head support for intracranial treatment.

The purpose of this study was to evaluate the relationship between pressure on the occipital region and intrafraction motion using an individualized vacuum pillow and a thermoplastic mask for intracranial treatment. We calculated head displacement during treatment from 8811 image verifications in 59 patients and divided them into two groups according to the magnitude of the mean and standard deviation (SD) of the displacement in the 59 patients. Pressure was compared between the small (n = 29) and large (n = 30) displacement groups using Welch's t-test for the mean and SD of displacement. The mean head displacement in the small and large groups was (0.3, 0.3, 0.4) and (0.5, 0.6, 0.7) (unit: mm) for the vector length and 10 mm and 30 mm radius targets, respectively. The mean SD of head displacement in the small and large groups was (0.2, 0.2, 0.2) and (0.3, 0.3, 0.4) (unit: mm) for the vector length and 10 mm and 30 mm radius targets, respectively. Significant differences were observed in the SD of the displacement in the vector length and 10 mm radius target between the two groups. The SD of the displacement under a pressure of 15 kPa was smaller than that under a pressure of 11 kPa. The intrafraction motion under a high-pressure level on the occipital region was less than that under a low-pressure level. Management of pressure on the occipital region may result in less intrafraction motion in clinical practice.

Development of a real-time monitoring system for intra-fractional motion in intracranial treatment using pressure sensors.

This study developed a dedicated real-time monitoring system to detect intra-fractional head motion in intracranial radiotherapy using pressure sensors. The dedicated real-time monitoring system consists of pressure sensors with a thickness of 0.6 mm and a radius of 9.1 mm, a thermoplastic mask, a vacuum pillow, and a baseplate. The four sensors were positioned at superior-inferior and right-left sides under the occipital area. The sampling rate of pressure sensors was set to 5 Hz. First, we confirmed that the relationship between the force and the displacement of the vacuum pillow follows Hook's law. Next, the spring constant for the vacuum pillow was determined from the relationship between the force given to the vacuum pillow and the displacement of the head, detected by Cyberknife target locating system (TLS) acquisitions in clinical application. Finally, the accuracy of our system was evaluated by using the 2 × 2 confusion matrix. The regression lines between the force, y, and the displacement, x, of the vacuum pillow were given by y = 3.8x, y = 4.4x, and y = 5.0x when the degree of inner pressure was -12 kPa,-20 kPa, and -27 kPa, respectively. The spring constant of the vacuum pillow was 1.6 N mm(-1) from the 6D positioning data of a total of 2999 TLS acquisitions in 19 patients. Head motions of 1 mm, 1.5 mm, and 2 mm were detected in real-time with the accuracies of 67%, 84%, and 89%, respectively. Our system can detect displacement of the head continuously during every interval of TLS with a resolution of 1-2 mm without any radiation exposure.

Observation of intrafraction prostate displacement through the course of conventionally fractionated radiotherapy for prostate cancer.

PURPOSE: Intrafraction prostate displacement (IFPD) through the course of conventionally fractionated radiotherapy was observed by real-time tracking. MATERIALS AND METHODS: IFPD was observed by using a CyberKnife real-time tracking system over 39 serial fractions in two patients. Stereoscopic X-ray images tracking the implanted fiducial markers were obtained with mean intervals of 58 s. In preparation for treatment, urination was performed routinely 1 h before treatment and rectal gas was evacuated if necessary. Patients were immobilized by a thermoplastic body shell. RESULTS: The maximal absolute values of IFPD in all 78 fractions were 7.9, 2.1, and 11.5 mm in cranio-caudal (CC), left-right (LR), and antero-posterior (AP) direction, respectively. Only in 5 % of fractions (4/78 fractions), the maximal absolute values of IFPD were 5.0 mm or larger. In these fractions, large IFPD was temporary or persistent. IFPD of ≥3 mm was detected in only ~2-3 % of all obtained tracking images. CONCLUSIONS: Daily maximal IFPD changed day by day. Although maximal IFPD was more than 10 mm, IFPD of ≥3 mm was observed in a comparatively small proportion of treatment time. Through the course of conventionally fractionated radiotherapy, fractions with IFPD of ≥5 mm were infrequent.