Background Fiducial markers are frequently used before treatment for image‐guided patient setup in radiation therapy (RT), but can also be used during treatment for image‐guided intrafraction motion detection. This report describes our implementation of automatic marker detection with periodic kV imaging (TrueBeam v2.5) to monitor and correct intrafraction motion during prostate RT. Methods We evaluated the reproducibility and accuracy of software fiducial detection using a phantom with 3 implanted fiducial markers. Clinical implementation for patients with intraprostatic fiducials receiving volumetric modulated arc therapy (VMAT) utilized periodic on‐board kV imaging with 10 s intervals during treatment delivery. For each image, the software automatically identified fiducial locations and determined whether their distance relative to planned locations were within a 3 mm tolerance. Motion was corrected if either ≥2 fiducials in a single image or ≥1 fiducial in sequential images were out of tolerance. Results Phantom studies demonstrated poorer performance of linear fiducials compared to collapsible fiducials, and wide variability to accurately detect fiducials across eight software settings. For any given setting, results were relatively reproducible and precise to ~0.5 mm. Across 17 patients treated with a median of 20 fractions, the software recommended a shift in 44% of fractions, and a shift was actually implemented after visual confirmation of movement greater than the 3 mm threshold in 20% of fractions. Adjustment of our approach led to improved accuracy for the latter (n = 7) patient subset. On average, table repositioning added 3.0 ± 0.3 min to patient time on table. Periodic kV imaging increased skin dose by an estimated 1 cGy per treatment arc. Conclusions Periodic kV imaging with automatic detection of motion during VMAT prostate treatments is commercially available, and can be successfully implemented to mitigate effects of intrafraction motion with careful attention to software settings.
Purpose: To monitor intrafraction motion during spine stereotactic body radiotherapy(SBRT) treatment delivery with readily available technology, we implemented triggered kV imaging using the on-board imager(OBI) of a modern medical linear accelerator with an advanced imaging package. Methods: Triggered kV imaging for intrafraction motion management was tested with an anthropomorphic phantom and simulated spine SBRT treatments to the thoracic and lumbar spine. The vertebral bodies and spinous processes were contoured as the image guided radiotherapy(IGRT) structures specific to this technique. Upon each triggered kV image acquisition, 2D projections of the IGRT structures were automatically calculated and updated at arbitrary angles for display on the kV images. Various shifts/rotations were introduced in x, y, z, pitch, and yaw. Gantry-angle-based triggering was set to acquire kV images every 45°. A group of physicists/physicians(n = 10) participated in a survey to evaluate clinical efficiency and accuracy of clinical decisions on images containing various phantom shifts. This method was implemented clinically for treatment of 42 patients(94 fractions) with 15 second time-based triggering. Result: Phantom images revealed that IGRT structure accuracy and therefore utility of projected contours during triggered imaging improved with smaller CT slice thickness. Contouring vertebra superior and inferior to the treatment site was necessary to detect clinically relevant phantom rotation. From the survey, detectability was proportional to the shift size in all shift directions and inversely related to the CT slice thickness. Clinical implementation helped evaluate robustness of patient immobilization. Based on visual inspection of projected IGRT contours on planar kV images, appreciable intrafraction motion was detected in eleven fractions(11.7%). Discussion: Feasibility of triggered imaging for spine SBRT intrafraction motion management has been demonstrated in phantom experiments and implementation for patient treatments. This technique allows efficient, non-invasive monitoring of patient position using the OBI and patient anatomy as a direct visual guide.
Complexity in MLC‐based radiosurgery treatment delivery can be characterized by the efficiency of monitor unit (MU) utilization and the average MLC leaf separation distance for a treatment plan. A reduction in plan complexity may be desirable if plan quality is not impacted. In this study, a number of strategies are explored to determine how plan quality is affected by efforts to reduce plan complexity. Ten radiosurgery cases of varying complexity are retrospectively planned using six optimization strategies: an unconstrained volumetric modulated arc therapy (VMAT) technique, a MU‐constrained VMAT technique, three techniques using various strengths of the aperture shape controller (ASC), and a hybrid technique consisting of a final‐stage VMAT optimization applied to a dynamic conformal arc leaf sequence (ODCA). The plans are compared in terms of MU efficiency, MLC leaf‐separation, conformity index (CI), gradient index (GI), and QA measurement results. The five VMAT techniques exhibited only minor differences in CI and GI values, though the ASC and MU‐constrained techniques did require 6–20% fewer MU and had mean field apertures 5–19% larger. On average, the ODCA technique had CI values 3.5% lower and GI values 1.0–2.5% higher than the VMAT techniques, but also had a mean field aperture 24–47% larger and required 16–32% fewer MU. The QA measurement results showed a 0.61% variation in mean per‐field 2%/1 mm gamma passing rates across all techniques (range 96.81%–97.42%), with no observed correlation between passing rate and technique. For simple targets, the ODCA technique achieved CI results that were equivalent to the unconstrained VMAT technique with an average 30% reduction in required MU, an average 50% increase in mean leaf separation distance, and brain V12 Gy values within 0.38 cc of the VMAT technique for targets up to approximately 2 cm diameter. For MLC‐based single‐target radiosurgery, plan complexity can often be significantly reduced without an equivalent reduction in plan quality.
modulated radiation therapy (IMRT) with dose-escalated simultaneous integrated boost (SIB) to involved lymph nodes (LNs). Materials/Methods: Between 2005 and 2017, 59 patients with clinically LN-positive cervical cancer were treated with definitive concurrent cisplatin, IMRT, and LN-directed SIB, followed by intracavitary brachytherapy boost to achieve an EQD2 of 85-90 Gy. The elective clinical target volume (CTVelect) included the cervix, uterus, and pelvic LNs, with the addition of para-aortic (PA) LNs for patients with involved PA LNs. The dose-escalated SIB target volume (CTVsib) included clinically positive LNs + 5-10 mm cropped from gastrointestinal (GI) or genitourinary (GU) organs. Small bowel (SB) dose constraints, V52Gy[cc]<0.03cc and V45Gy[cc]<195cc, were prioritized over target coverage. Acute and late adverse effects (AEs) were recorded per common toxicity criteria for adverse effects. Kaplan-Meier method was used to estimate overall survival (OS), progression free survival (PFS), and in-field locoregional control (LRC). Results: Median age was 49 years (IQR 37-56). Squamous histology was most common (81%). FIGO stage was I (31%), II (39%), III (29%), and IVa (2%), with positive LNs present in the pelvis (56%) or pelvis and PA (44%). Median number of positive LN was 3 (IQR 2-6), with a median of 3 (IQR 2-4) pelvic, and for those with PA LN+, a median of 2 (IQR 1-4) PA. Most common CTVelect dose was 45 Gy (EQD2: 44.3 Gy, 89%) and CTVsib dose was 56.25 Gy (EQD2: 57.4 Gy, 66%) delivered in 25 fractions. Median treatment duration, including brachytherapy, was 45 days (IQR 42-50). Median follow-up was 30 months. Estimated 2 year LRC, PFS, and OS were 91% (95% CI 82-100), 63% (95% CI 49-78), and 79% (95% CI 67-92), respectively. PA nodal involvement was associated with worse 2 year PFS (HR 2.51; 95% CI 1.05-5.99; pZ0.04). Risk of disease progression was most pronounced for 4 positive LNs (HR 2.79; 95% CI 1.16-6.71; pZ0.02). Twenty (34%) patients had disease recurrence. Five (8%) patients had in-field locoregional recurrence, including cervix (1), cervix and elective PA LN (1), and boosted pelvic (1) or PA LN (2). Two (3%) patients had out-of-field isolated PA LN recurrence. Thirteen (22%) patients had distant metastasis. Acute GI/GU grade 3 or higher AE rate was 5%. Late GI/GU complications included fistula (4), and ureteral stenosis requiring stenting (7). No patients developed late small bowel obstruction. Conclusion: LN-directed SIB as part of definitive chemoradiation for clinically LN-positive cervical cancer is well tolerated and is associated with excellent oncologic outcomes.
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