Effect of a hydrogel spacer on the intrafractional prostate motion during CyberKnife treatment for prostate cancer

Abstract The purpose of this study was to evaluate the effect of a hydrogel spacer on intrafractional prostate motion during CyberKnife treatment. The retrospective study enrolled 24 patients (with the hydrogel spacer = 12, without the hydrogel spacer = 12) with two fiducial markers. Regarding intrafractional prostate motion, the offset values (mm) of three axes (X‐axis; superior [+] to inferior [−], Y‐axis; right [+] to left [−], Z‐axis; posterior [+] to anterior [−]) obtained from fiducial markers position between a digitally reconstructed radiographs images and live images in the Target Locating System were used, and extracted from generated log files. The mean values of the offset and each standard deviation were calculated for each patient, and both the groups were compared. For all the patients, a total of 2204 offset values and timestamps (without the hydrogel spacer group: 1065, with the hydrogel spacer group: 1139) were recorded for the X‐, Y‐, and Z‐axes, respectively. The offset values (mean ± standard deviation) for the X‐, Y‐, and Z‐axes were −0.04 ± 0.92 mm, −0.03 ± 0.97 mm (P = 0.66), 0.02 ± 0.51, −0.02 ± 0.49 mm (P = 0.50), and 0.56 ± 0.97 mm, 0.34 ± 1.07 mm (P = 0.14), in patients inserted without or with the hydrogel spacer, respectively. There was no effect of a hydrogel spacer on the intrafractional prostate motion in the three axes during CyberKnife treatment for prostate cancer.

stably reduced the rectal dose in all EBRT modalities. 13 Moreover, for high-precision EBRT, it is important to control inter-and intrafractional prostate motion. Among the several radiotherapy modalities, CyberKnife (Accuray, Sunnyvale, CA) can be used to acquire 2D images per 5-150 s by using 2-kV x-ray devices (target locating system [TLS]), resulting in high-precision image-guided radiotherapy. 14 Prostate SBRT using CyberKnife showed fewer grade 2 or worse genitourinary toxicities than other SBRT delivery methods. 15 However, larger intrafractional motion was observed in CyberKnife treatment than in conventional Linac treatment, because the irradiation time for CyberKnife treatment was likely to be long. Accordingly, we hypothesized that the use of a hydrogel spacer would help in fixing the position of the prostate, thereby reducing its movement.
To the best of our knowledge, no previous studies have compared prostate motion between patients with and without a hydrogel spacer during CyberKnife treatment. Therefore, the current study aimed to evaluate the effect of placing a hydrogel spacer on intrafractional prostate motion during CyberKnife treatment.

2.A | Patient data
This retrospective study was reviewed and approved by the Institutional Review Board of our institution. A total of 24 patients who underwent CyberKnife treatment for prostate cancer between March 2017 and May 2020 in our institution were enrolled in the study. Table 1 lists the patient characteristics. All the patients were implanted with one fiducial marker each in the left and right lobes of the prostate. The fiducial marker used was either the ball-shaped Gold Anchor (0.28 mm × 10.0 mm; Naslund Medical AB, Huddinge, Sweden) or the straight-shaped VISICOIL (0.5 mm × 5.0 mm; Radio-Med Corporation, Bartlett, TN). In addition, radiation oncologists inserted the hydrogel spacer between the prostate and rectum under transrectal ultrasound guidance in 12 patients.

2.C | Fiducial tracking
During the CyberKnife treatment of prostate cancer, the fiducial

2.D | Procedure
In the current study, prostate motion was defined using the offset values in each direction. Data in the log files of the 24 patients with prostate cancer treated with fiducial tracking were analyzed. The mean values and standard deviation of the offset value for each axis were obtained both with and without the hydrogel spacer. The t-test   column (a, b, c) show patients treated without a hydrogel spacer, and those in the right column (d, e, f) show the patients treated with a hydrogel spacer. The histogram is divided into specific time segments per 600 s, indicated using different colors. Each histogram includes the mean displacement (blue) and 95% confidential interval (red).
The offset values (mean ± standard deviation) for each axis. Both groups had larger mean value for the Z-axis than for the other axis.

| DISCUSSION
We hypothesized that using a hydrogel spacer would fix the position of the prostate and reduce its movement during the Cyber-Knife treatment. However, our results indicated that there were no significant differences between the groups, although the mean values were higher for the Z-axis than for the other axes. Several studies have investigated the prostate motion in the anterior-posterior (AP) dimension during CyberKnife treatment; however, these studies have not shown consistent results. 19,20 In contrast, a review article on prostate motion in general radiotherapy showed the tendency for the large motion in the AP dimension, which was consistent with our results. It was thought that bladder distension, rectal peristalsis, and anal contraction were the main factors that affected the prostate motion. 21,22 In addition, manual alignment to correct the prostate position was performed more frequently in those treated with a hydrogel spacer than in those treated without a hydrogel spacer. beam CT performed after weekly setup, irrespective of the use of a hydrogel spacer. 24 In contrast, the use of a hydrogel spacer reduced posterior displacement of the prostate when the distance was measured between the prostate and the anterior rectum on planning CT and CT scans obtained in the last week of radiation. 25  The current study has some limitations. Although the use of three or more fiducial markers in the prostate is recommended, only two markers were used in the current study, as only two are reimbursed by the Japanese medical insurance system. In addition, the effects of using a hydrogel spacer could not be compared in the same patient. Finally, in our study, it would be difficult to perfume this procedure with the same frequency as that of manual alignment in the two groups. Furthermore, this study investigated prostate motion only in the three axes. Future studies should be including pitch, roll, and yaw.

| CONCLUSION
We evaluated the effect of using a hydrogel spacer on intrafractional prostate motion in the three axes. No significant differences in prostate motion were observed between patients treated without or with a hydrogel spacer. Our study suggested that a hydrogel pacer could not affect on intrafractional prostate motion during CyberKnife treatment.

CONF LICT OF I NTEREST
There are no conflict of interest to declare.