ESTRO 2024 - Abstract Book

S5482

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

(SGRT) was used to analyse motion during stereotactic body radiotherapy (SBRT) in lung cancer patients and to explore the margin of the planning target volume (PTV).

Material/Methods:

12 lung SBRT patients were selected. The residual errors of the SGRT (AlignRT, London) were evaluated based on grayscale CBCT registration results before each treatment. AlignRT log file was used to analyse the correlation between the frequency, longest duration of errors (larger than 2 mm and lasting longer than 2 seconds) and maximum error with age and treatment duration. The last SGRT displacement value at the end of treatment (D1), the SGRT average displacement value (D2), and the 95% probability density SGRT displacement interval (D3) were defined as intrafraction errors. PTV1 (D1), PTV2 (D2), PTV3 (D3) were calculated using Van Herk formula (PTV 1 & 2) or Z score analysis (PTV 3).

Results:

Total of 59 sets of CBCT images and intra-fractional data were analysed. The inter-fraction residual errors were 0.06±0.18 cm, -0.03±0.19 cm, 0.02±0.15 cm, 0.23±0.7°, 0.1±0.69° and -0.02±0.79° in vertical, longitudinal, lateral, pitch, roll and yaw directions. The frequency, longest duration and maximum error in vertical direction were correlated with treatment duration (r = 0.404, 0.353, 0.283, P < 0.05, respectively). In longitudinal direction, the frequency was correlated with age and treatment duration (r = 0.376, 0.283, P < 0.05, respectively), maximum error was correlated with age (r = 0.4, P < 0.05). The margins of PTV1, PTV2, PTV3 were 2 mm, 4 mm, 2 mm; 2 mm, 2 mm, 2 mm; 3 mm, 5 mm, 3 mm, in vertical, longitudinal, and lateral axes respectively. After replacing the original PTV margins with PTV3, mean lung dose (MLD), 2-cm3 chest wall dose (CD) and lung V20 decreased by 0.2 Gy, 2.1 Gy and 0.5% respectively (P < 0.05).

Conclusion:

AlignRT can be used for lung SBRT inter- and intrafraction treatment. It is recommended to use upper and lower limits of the 95% probability density interval as intrafraction error calculations.

Keywords: surface guided radiotherapy, PTV, Setup accuracy

References:

1. Anita Vloet, Winnie Li, Meredith Giuliani, et al. Comparison of residual geometric errors obtained for lung SBRT under static beams and VMAT techniques: Implications for PTV margins [J]. Phys Medica, 2018, 52: 129–132

2. Daniel N, Rebeca R, Jad F, et al. Reproducibility of surface-based deep inspiration breath-hold technique for lung stereotactic body radiotherapy on a closed-bore gantry linac [J]. Phys Imaging Radiat Oncol, 2023, 26: 100448.

3. Freislederer P, Kügele M, Öllers M, et al. Recent advanced in surface guided radiation therapy [J]. Radiat Oncol, 2020,15:187-197.