The impact of the three degrees‐of‐freedom fiducial marker‐based setup compared to soft tissue‐based setup in hypofractionated intensity‐modulated radiotherapy for prostate cancer

Abstract Purpose We evaluated the setup accuracy of a three‐degree‐of‐freedom fiducial marker (3DOF‐FM)‐based setup compared to a soft tissue (ST)‐based setup in hypofractionated intensity‐modulated radiotherapy (IMRT) for prostate cancer. Materials and Methods We analyzed the setup accuracy for 17 consecutive prostate cancer patients with three implanted FMs who underwent hypofractionated IMRT. The 3DOF‐ST‐based setup using cone‐beam computed tomography (CT) was performed after a six DOF‐bony structure (BS)‐based setup using an ExacTrac x‐ray system. The 3DOF‐FM‐based matching using the ExacTrac x‐ray system was done during the BS‐ and ST‐based setups. We determined the mean absolute differences and the correlation between the FM‐ and ST‐based translational shifts relative to the BS‐based setup position. The rotational mean shifts detected by the ExacTrac x‐ray system were also evaluated. Results The mean differences in the anterior‐posterior (AP), superior‐inferior (SI), and left‐right (LR) dimensions were 0.69, 0.0, and 0.30 mm, respectively. The Pearson correlation coefficients for both shifts were 0.92 for AP, 0.91 for SI, and 0.68 for LR. The percentages of shift agreements within 2 mm were 85% for AP, 93% for SI, and 99% for LR. The absolute values of rotational shifts were 0.1° for AP, 0.3°, and 1.2° for LR. Conclusions The setup accuracy of the 3DOF‐FM‐based setup has the potential to be interchangeable with a ST‐based setup. Our data are likely to be useful in clinical practice along with the popularization of the hypofractionated IMRT in prostate cancer.


2.A | Patient selection
We conducted an institutional review board (IRB)-approved study (approval no. 2145) for prostate cancer patients in whom three FMs were implanted and who had undergone hypofractionated IMRT at our hospital between April 2014 and March 2015. A total of 17 consecutive prostate cancer patients were provided their informed consent under our IRB concerning the use of their data for research purposes.
Among the 17 patients, one patient was ineligible for this analysis because the FM implantation was not successful. Ten patients received 70 Gy in 28 fractions 4 days per week, and the other six patients received 62 Gy in 20 fractions four days per week. In total, the shift data for 400 fractions were analyzed. The patient characteristics are summarized in Table 1.  2.C | Image guidance system and isocenter setup accuracy All patients were treated on a Novalis-Tx system (Varian), with both a Varian kV on-board-imager (OBI) and an ExacTrac x-ray system.

2.B | Treatment planning
The CBCT images were obtained with the follow clinical protocol: field of view (FOV) 20 cm, matrix size 512 mm × 512 mm, slice thickness 1.0 mm, and 'full-fan' acquisition. 17 The ExacTrac x-ray system images were obtained with the follow conditions: 120 kV, 160 mA, and 160 ms. The isocenter setup accuracy of each system had been ensured within 0.3 mm in each direction in a daily quality assurance protocol.

2.D | Patient setup
The patient position was imaged daily in accordance with the proto- detected by ExacTrac x-ray system was also recorded.

2.E | Evaluation index and statistical analysis
We  Table 2: 41%, 50%, and 97% of the dataset in the AP, SI, and LR axes resulted within 2 mm for both the FM-based and ST-based shifts, respectively. There were no data in opposed shifts over 2 mm relative to the position of the BS-based setup.

FM-based setup
The absolute values of the rotational mean shifts in the ExacTrac xray system after the BS-based setup were 0.1°(SD 1.2°) for the AP axis, 0.3°(SD 1.8°) for the SI axis, and 1.2°(SD 3.8°) for the LR axis, as shown in Fig. 3. The percentages over 3°were remarkably larger in the LR (39%) axis compared to the AP (2.6%) and SI (6.2%) axes.

ST-based shifts
The Pearson's correlation coefficients (r) between the 3DOF-FMbased and ST-based shifts were 0.92 for the AP axis, 0.91 for the SI axis, and 0.68 for the LR axis, as shown in Fig. 4    or AP, 1.8°for SI, and 3.8°for LR). 26 It is assumed that the large percentage of >3°in LR direction is caused by filling the rectum which exerts pressure on the prostate from the posterior direction.
We consider the control of rectum condition are important for 3DOF-FM-based setup.
The results of the average rotational shifts <2°in all axes and good shift agreements with the ST-based setup with CBCT in this study confirm the relevance of the 3DOF-FM-based setup using the ExacTrac x-ray system. Considering the rotational shift verification based on the 6DOF-FM-based setup, there is a risk that unnecessarily large shifts may be calculated because the distance among fiducial markers are very close compared to the size of the entire pelvis.
Therefore, we think that it is unwise to take the results of the rotational shift on faith, and further investigations are needed to clarify this issue. In light of the overestimation risk for the patient setup, a 6DOF-FM-based setup should be performed based on careful consideration.
Although the IGRT technique has been gradually developed in order to improve setup accuracy, the optimal modalities for setup in prostate cancer are not clinically well established. A 2010 survey of IGRT in the U.S. revealed that the most common disease site was genitourinary tumors in all sites for with the IGRT technique, 16 and the survey showed that the most commonly used IGRT modalities in genitourinary tumors were volumetric-based technologies including CBCT (55.3%). We reported that an