(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: To evaluate the positional variation of anatomic landmarks through pre and post-treatment CBCT image analysis for deep inspiration breath hold (DIBH) liver stereotactic body radiotherapy (SBRT).
Methods: Eleven liver cancer patients were treated using DIBH SBRT with a dose of 27.5–50 Gy in 5 fractions with VMAT plans. Prior to every treatment fraction (n = 55) and immediately after treatment, DIBH CBCT guidance was performed. Soft-tissue matching of the CBCT was performed by a radiation therapist immediately prior to treatment delivery. Patients were shifted and a second CBCT was acquired. The residual shifts in the anterior-posterior (AP), superior-inferior (SI) and left–right (LR), directions were used to calculate population based PTV margins (considered interfractional margins). The post-treatment CBCT shifts were used to quantify the implications for target margins due to intrafraction motion. Population based PTV margins were computed from systematic (Σ) and random (σ) positional errors from the pre- and post-treatment CBCT shifts.
Results: The pre-treatment systematic residual errors were ΣAP= 0.9, ΣSI = 0.7, ΣLT = 0.5 mm for AP, SI, and LT directions, respectively. The pre-treatment random residual errors were σ AP= 1.4, σ SI = 1.4, σ LT = 1.4 mm. This yields a population-based PTV margin of 4.9 mm. The post-treatment systematic residual errors were ΣAP= 2.2, ΣSI = 2.2, ΣLT = 0.8 mm and random residual errors were σ AP= 2.5, σ SI = 3.6, σ LT = 1.8 mm, yielding a PTV margin of 10.8 mm. The PTV margin computed from the combined data of the pre- and post-treatment CBCT shifts was 8.4 mm.
Conclusion: Intrafraction liver positional changes can be significant. This study demonstrated the need to incorporate these errors in PTV margins.
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