We investigated the intrafractional prostate motion during VMAT-SBRT for PCa. Calculated margins according to the Stroom formula were larger in the spacer group than the non-spacer group by (1.02, 0.45, 0.70) mm in the SI, LR, and AP directions, respectively. Based on the van Herk formula, calculated margins were larger in the spacer group than the non-spacer group by (1.13, 0.56, 0.77) mm in the SI, LR, and AP directions, respectively.
Errors related to the quality of radiotherapy are categorized into interfractional errors and intrafractional errors. While interfractional errors can be minimized by using high-precision techniques of patient positioning, intrafractional errors caused by organ motion depended on the patient himself and are difficult to be controlled.
Development of monitoring devices has contributed to knowing the intrafrational organ motions. Mah et al. (5) investigated the intrafractional prostate motion of 42 patients with prostate cancer using cine-MRI. They reported that the displacements of prostate (mean ± SD) were 0.0 ± 3.4 mm, 0.0 ± 1.5 mm, 0.2 ± 2.9 mm in the SI, LR, and AP dimensions, respectively. Willoughby et al. (9) used the Calypso 4D localization system which is real-time tracking system with implanted electromagnetic transponders to track the intrafractional shift of prostate. They showed that the average (± SD) of the maximum differences in 11 cases were 3.61 ± 3.13 mm, 0.91 ± 0.35 mm, 3.92 ± 4.32 mm in the SI, LR, and AP directions, respectively. Pinkawa et al. (11) demonstrated that the intrafractional displacements of prostate (mean ± SD) were 0.0 ± 2.0 mm, 0.2 ± 1.9 mm, 0.6 ± 2.2 mm in the SI, LR, and AP directions in 32 patients with prostate cancer by using transabdominal US tracking system. Comparable level of the intrafractional prostate motion with these studies was seen in our study. The average (± SE) of the maximum vector displacement was 2.24 ± 0.19 mm and 2.89 ± 0.62 mm in the non-spacer and the spacer group, respectively.
Shihono et al. (12) suggested the patient population-based margin according to the van Herk formula is as follows: 1.10 mm, 1.25 mm, and 1.33 mm in the SI, LR, and AP directions, respectively. They used the Clarity system just like our study. We demonstrated the larger margins calculated based on our population; 3.14 mm, 2.81 mm, 4.23mm in the non-spacer group and 4.27 mm, 3.37 mm, 5.00 mm in the spacer group. The difference is probably ascribed to the fact that Shihono et al. may have used mean intrafractional motion for margin calculation, whereas we used maximum intrafractional motion.
Knowledge about parameters related to intrafractional prostate motion is absolutely limited. Brown et al. (16) showed that there was no statistically significant relationship between intrafractional prostate motion and BMI by using linear regression analysis. Oates et al. (17) investigated a relationship between maximum rectal diameter (MRD) and intrafractional prostate motion. They showed with 90% confidence that for a MRD ≤ 3 cm, prostate displacement will be ≤ 5 mm and that for a MRD ≤ 3.5 cm, prostate displacement will be ≤ 5.5 mm. By prescribing a rectal enema and performing CBCT before each treatment session, the variety of MRD may have been minimized in our study. Rectum volume was smaller in the spacer group, which may be caused by the deformation of rectum by the pressure from the anterior direction by the injected hydrogel spacer (Fig. 3). However, rectum volume was not an independent risk factor for prostate displacement in the multivariate analysis. The displacement of prostate was shown to be smaller in step-and-shoot IMRT fractions than in VMAT fractions due to the shorter treatment time of VMAT by Ballhausen et al. (24). In the present study, we treated all patients with VMAT using flattening filter free (FFF) beams and monitoring time from the end of CBCT to the end of radiation was about 4.5 minutes. According to our study, monitoring time did not significantly affect prostate shift.
Picardi et al. (19) showed that hydrogel spacer injection into the recto-prostatic space did not significantly influence the interfraction prostate motion based on the analysis using implanted fiducial markers and CBCT. It was reported that hydrogel spacer insertion significantly reduced the intrafraction rotational shift in the AP direction on cine-MRI by Cuccia et al. (20) and they concluded that hydrogel spacer contributed to limiting prostate intrafractional motion. On the other hand, Juneja et al. (18) showed that the average of the mean intrafractional vector displacement of prostate was significantly larger in patients with hydrogel spacer than those without spacer by analyzing the implanted electromagnetic markers position on kV fluoroscopy. The difference between the two groups was 0.4 mm on their study. In our study, there was no significant difference in maximum VL, whereas superior and anterior MIDs were significantly larger in the spacer group in our study, and the difference between the two groups were 0.5 mm in the superior direction and 0.7 mm in the anterior direction.
The limitation of our study was the fact that the quality of our results depends on the accuracy of the Clarity system. Zhou et al. (24) investigated the discrepancy between the Clarity system and CBCT as the positioning device. The average (± SD) discrepancies were − 0.03 ± 5.22 mm, 0.18 ± 2.87 mm, and 0.31 ± 4.37 mm in the SI, LR, and AP directions, respectively when the Clarity was transperineally used.