2943 - Dosimetric Comparisons of Coplanar VMAT Using a C-arm Linear Accelerator and Non-Coplanar VMAT and BROAD-RT Using an O-Ring Linear Accelerator In Localized Prostate Cancer

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 11

POSTER

Presenter(s)

Fumiya Baba, MD, PhD - Narita Memorial Hospital, Toyohashi, Aichi

F. Baba¹, M. Ichihara², M. Iwana³, K. Saito⁴, N. Tomita⁵, and A. Hiwatashi⁵; ¹Department of Radiology, Narita Memorial Hospital, Toyohashi, Japan, ²Department of Radiation Technology, Narita Memorial Hospital, Toyohashi, Japan, ³Department of Radiotherapy, Nagoya City University West Medical Center, Nagoya, Japan, ⁴Department of Radiation Technology, Nagoya City University West Medical Center, Nagoya, Japan, ⁵Department of Radiology, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan

Purpose/Objective(s):

Radiotherapy device using an O-ring-type linear accelerator system enables diverse irradiation trajectories through biaxially rotational dynamic radiation therapy (BROAD-RT) where the O-ring gantry rotates while the ring itself swivels horizontally. In this study, we performed dosimetric comparisons between coplanar VMAT (C-VMAT) using a conventional C-arm linear accelerator (C-arm LA), and non-coplanar VMAT (NC-VMAT) and BROAD-RT using the O-ring linear accelerator (O-ring LA) in patients with localized prostate cancer.

Materials/Methods:

Ten patients with localized prostate cancer (T2a–T3aN0M0) were included in this simulation study. The prescribed dose was set at 60 Gy in 20 fractions to the PTV D50%. The dose constraints were defined as follows: (1) PTV: D95% = 57.0 Gy and D2% = 63.0 Gy; (2) Rectum: D0.03cc = 60.0 Gy, V48Gy = 15%, and V32Gy = 30%; (3) Bladder D0.03cc = 60.0 Gy, V54Gy = 20%, and V32Gy = 50%; (4) Small bowel: D0.03cc = 52.0 Gy and V49Gy = 1 cc; (4) Bilateral femoral heads: Dmax = 32.0 Gy. Treatment plans were generated starting from identical initial conditions to satisfy these constraints without using dummy contours. For all plans, a 6 MV photon energy and two arcs (gantry rotation from 182° to 178°) were used. Collimator angles were fixed at 15° and 75° for C-VMAT with C-ram LA and at 0° for NC-VMAT and BROAD-RT with O-ring LA. Ring angles were set at 20° and 340° for NC-VMAT with O-ring LA, while they were varied from 20° to 340° for BROAD-RT with O-ring LA. A paired t-test was used for statistical comparison.

Results:

For the PTV, all planning constraints were met; no significant differences were observed in D2%, D95%, or the conformity index (CI). The mean (± standard deviation) values of V54Gy (%) for the rectum was significantly reduced in the BROAD-RT (6.72 ± 2.08) than in C-VMAT (7.52 ± 1.96) (p=0.025). In the bladder, the V54Gy (%) increased significantly in NC-VMAT (13.65 ± 4.72) than in C-VMAT (12.62 ± 5.10) (p=0.043). The values of V12Gy (%) for the left and right femoral heads were significantly lower in NC-VMAT (0.29±5.82 and 5.72±5.54) and BROAD-RT (30.29±16.74 and 18.01±10.22) compared to C-VMAT (46.35±16.34 and 46.89±11.33), respectively (all p<0.001). Conversely, the V12Gy (%) values for the rectum, bladder, and small bowel were significantly higher in NC-VMAT than in C-VMAT (p<0.001, p<0.001 and p<0.018, respectively). However, no such significant increases in the V12Gy (%) values for these organs were observed with BROAD-RT.

Conclusion:

BROAD-RT, a technique specific to the O-ring LA, enables dose reduction in arbitrary lateral regions of the PTV while preserving PTV coverage, without increasing the dose to organs at risk seen with NC-VMAT, and further reducing the high-dose volume in the rectum.