Author: William E Burrell, Philip F. Durham, Anne N Hubbard, Yohan Akeakamai Walter, Hsinshun Terry Wu 👨🔬
Affiliation: Willis Knighton Cancer Center 🌍
Purpose: Linear accelerator (LINAC)-based single-isocenter, multi-target (SIMT) treatment has streamlined the stereotactic radiosurgery (SRS) workflow. However, off-isocenter treatment carries multiple additional difficulties compared to isocentric treatment, including increased sensitivity of delivered dose to rotational errors.
Room-mounted imaging systems carry the advantage of allowing fast, low-dose imaging at nonzero couch angles, which may combat the effects of table walkout and residual rotational errors. Here, we performed a series of end-to-end tests to determine if these corrections correlate with a measurable difference in delivered dose.
Methods: Ten treatment plans of increasing complexity were created in the Elements 4.0 treatment planning system (TPS, Brainlab AG). Plans were delivered on an Elekta Versa HD linear accelerator (Elekta AB) with an ExacTrac (ETX) planar oblique imaging system (Brainlab AG).
A CT scan of a StereoPHAN with SRS MapCHECK (Sun Nuclear) was imported into the TPS. Measured targets were contoured on the detector plane. Plans used 4-15 treatment arcs and 4-5 couch angles. ETX was used for initial phantom positioning. Dose measurements were performed for delivery with and without ETX-guided corrections at each table angle.
Results: The most dramatic difference in gamma pass rate (GPR) was observed for a case with a target 75 mm from isocenter. The GPR with and without ETX corrections were 99.5% and 87.9%, respectively, at 3%/1mm gamma criteria and 10% threshold. On average, peak dose discrepancies for each target improved marginally, by 0.13 ± 0.52%, and gamma pass rates improved by 2.18± 3.44%; however, the differences were not statistically significant (p=0.78 and 0.08, respectively).
Conclusion: Results demonstrate that repositioning corrections at each table angle may marginally improve agreement between planned and delivered dose. The test treatment plans in this study may be used for assessment of end-to-end treatment delivery accuracy for complex LINAC-based stereotactic radiotherapy procedures.