Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_3_3143 - Distribution of CT Values Along Longitudinal in Adaptive Radiation Therapy Using CBCT

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Distribution of CT Values Along Longitudinal in Adaptive Radiation Therapy Using CBCT
T. Taniguchi1, T. Hara2, T. Shimozato3, M. Matsuo4, O. Tanaka5, and K. Ono6; 1Murakami Memorial Hospital Asahi University, Gifu, Japan, 2Department of Medical Technology, Nakatsugawa Municipal General Hospital, Nakatsugawa, Japan, 3Department of Radiological Technology, School of Health Sciences, Gifu University of Medical Science, Gifu, Japan, 4Department of Radiation Oncology, Gifu University Graduate School of Medical Sciences, Gifu, Japan, 5Murakami Memorial Hospital, Gifu City, Japan, 6Murakami Memorial Hospital, Gifu, Japan

Purpose/Objective(s): While performing adaptive radiotherapy using cone beam computed tomography (CBCT), it is preferable to have the same CT value as planning CT (PCT) three dimensionally to calculate dose accurately. However, compared to PCT, it is more difficult to obtain accurate CT values in CBCT owing to the influence of scattered radiation. Therefore, we evaluated the distribution of CT values along longitudinal, which is considered to have a significant scattering effect in CBCT.

Materials/Methods: We measured the CT values along longitudinal using a CBCT device mounted on a linear accelerator. An electron density (ED) phantom was scanned at 5 locations, i.e., the isocenter and positions that were ± 4 cm and ± 8 cm off-center along longitudinal. CT values were compared with PCT values for equivalent materials in the lung (0.634 electrons/cc), soft tissue (3.261 electrons/cc), and bone (4.862 electrons/cc). In addition, image acquisition parameters, such as collimation (small, medium, large), with or without filter (F1 or F0), and tube voltage (100 kV, 120 kV), were changed for the scan mode with reference to the default setting of the device. CT values were examined for three different scan modes, i.e., the head mode (small, F0, 100 kV), abdomen mode (medium, F1, 120 kV), and pelvic mode (large, F1, 120 kV). As the ED phantom was 25 cm in length, scattered radiation along longitudinal could be considered sufficiently.

Results: Significant error was observed between the values measured off-center and at the isocenter. The influence of scattering was strong for the bone material. At the isocenter, the CT value for the bone material scanned in the head mode was 120.1 HU higher than the PCT value. Table 1 CT value along longitudinal for equivalent materials electron density.
Materials Scan mode -8 cm off-center (HU) -4 cm off-center (HU) Isocenter (HU) 4 cm off-center (HU) 8 cm off-center (HU)
Lung Head 38.7 25.8 4.9 14.1 27.7
Abdomen -21.2 -55.7 12.0 -31.5 -58.8
Pelvic -2.9 -22.8 9.4 -6.4 -50.6
Soft tissue Head 33.7 11.3 -13.0 2.5 36.6
Abdomen 71.8 60.3 26.2 70.0 80.0
Pelvic 12.7 10.0 -11.0 27.6 43.6
Bone Head 177.9 153.2 120.1 154.2 218.8
Abdomen 469.0 520.9 1.9 527.9 519.0
Pelvic 320.4 370.6 35.6 436.5 458.9

Conclusion: We demonstrated that the CT value changes considerably along longitudinal during CBCT. Therefore, adaptive radiotherapy using CBCT may have a risk of significant error in dose calculation outside the central cross section. As CT value of the CBCT device used in this study is corrected using only the values of air and water, it is considered that the error becomes large for the bone material with high ED. Hence, it is necessary to include high ED for CT value correction. In addition, the CT values for the bone material scanned in the head mode (with filter) are higher than the PCT values because the scattering correction of the same default setting is used, regardless of the scan mode. It is necessary to appropriately perform scattering correction depending on the case of with or without filter.

Author Disclosure: T. Taniguchi: None. T. Hara: None. M. Matsuo: None. K. Ono: None.

Takuya Taniguchi, MS, RT

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