Estimation of patient-specific imaging dose for real-time tumour monitoring in lung patients during respiratory-gated radiotherapy.

Purpose: To quantify the patient-specific imaging dose for real-time tumour monitoring in the lung during respiratory-gated stereotactic body radiotherapy (SBRT) in clinical cases using SyncTraX.

Methods And Materials: Ten patients who underwent respiratory-gated SBRT with SyncTraX were enrolled in this study. The imaging procedure for real-time tumour monitoring using SyncTraX was simulated using Monte Carlo. We evaluated the dosimetric effect of a real-time tumour monitoring in a critical organ at risk (OAR) and the planning target volume (PTV) over the course of treatment. The relationship between skin dose and gating efficiency was also investigated.

Results: For all patients, the mean D50 to the PTV, ipsilateral lung, liver, heart, spinal cord and skin was 118.3 (21.5-175.9), 31.9 (9.5-75.4), 15.4 (1.1-31.6), 10.1 (1.3-18.1), 25.0 (1.6-101.8), and 3.6 (0.9-7.1) mGy, respectively. The mean D2 was 352.0 (26.5-935.8), 146.4 (27.3-226.7), 90.7 (3.6-255.0), 42.2 (4.8-82.7), 88.0 (15.4-248.5), and 273.5 (98.3-611.6) mGy, respectively. The D2 of the skin dose was found to increase as the gating efficiency decreased.

Conclusions: The additional dose to the PTV was at most 1.9% of the prescribed dose over the course of treatment for real-time tumour monitoring. For OARs, we could confirm the high dose region, which may not be susceptible to radiation toxicity. However, to reduce the skin dose from SyncTraX, it is necessary to increase the gating efficiency.