Evaluation of kV Triggered Fiducial Tracking for Prostate SBRT Intrafraction Motion Management.

Abstract

Stereotactic body radiation therapy (SBRT) is a safe and effective treatment for localized prostate cancer. Given the high dose per fraction, a high degree of precision is needed during SBRT treatment delivery. Fiducial marker tracking is a means of monitoring intrafraction motion, ensuring precise treatment delivery and potentially allowing for decreased planning target volume (PTV) margins. We present a 5-year single institution experience to characterize the performance of fiducial tracking. We analyzed 132 consecutive patients treated at our institution with prostate SBRT between July 2017 and November 2022 using fiducial tracking with the TrueBeam Auto Beam Hold feature. Patients were treated using a 5-fraction dose escalation protocol, with many receiving a simultaneous integrated boost to the GTV up to 45 Gy. All patients underwent pre-treatment cone-beam CT and planar kV imaging with rigid registration to the fiducial markers. Cine kV images were acquired every 3-5 seconds during treatment delivery and the beam was paused when a fiducial deviated from a given 4 mm diameter tolerance region. Additional intrafraction 2D or 3D imaging techniques were then used to realign the fiducials. Data was retrieved from the record and verify system including: number and type of intrafraction images, number of treatment interruptions, number of and magnitude of shifts, and total treatment time. The average number of cine kV images acquired per fraction and for the total 5 fraction SBRT course were 86.6 ± 26.8 and 425.6 ± 136.8 respectively. Assuming 1.3 mGy/kV image, the fiducial tracking process delivered an average additional 55.3 cGy over the full treatment course. On average, the beam was paused 5.0 ± 2.5 times per fraction (range 1-21 interruptions) and shifts were applied 4.1 ± 2.1 times (range 0-10.2 times) per fraction. Over the 2560 total shifts applied for all the patients, the average(max) translations were 0.6 (12) mm vrt, 0.7 (22) mm lng, and 0.9 (5) mm lat and the average(max) rotations were 0.19 (5.9)° pitch, 0.01 (2.4)° roll, and 0.16 (6.4)° rtn. Overall, 2.4% of shifts were greater than the 4 mm PTV margin and 4.2% of vertical shifts were greater than the 2.5 mm posterior margin. Average fraction treatment time was 27 ± 9.7 minutes. Fiducial tracking affords the potential to increase daily treatment precision when delivering prostate SBRT. Our data indicate that the beam is typically paused several times during a prostate SBRT fraction, with occasional large shifts required, suggesting the importance of intrafraction motion management. Downsides of kV triggered fiducial tracking include extension of treatment times and delivery of additional radiation dose to the patient. We are investigating if this method of motion management might allow for a reduction in PTV margins to reduce normal tissue dosing.