Elekta linacs are capable of delivering reproducible and stable PLRT plans. A prospective clinical study employing PLRT is currently in development.
Purpose: Brainlab's Elements Multiple Brain Mets SRS (MBMS) is a dedicated treatment planning system for single-isocenter multi-target (SIMT) cranial stereotactic radiosurgery (SRS) treatments. The purpose of this study is to present the commissioning experience of MBMS on an Elekta Versa HD. Methods: MBMS was commissioned for 6 X, 6 FFF, and 10 FFF. Beam data collected included: output factors, percent depth doses (PDDs), diagonal profiles, collimator transmission, and penumbra. Beam data were processed by Brainlab and resulting parameters were entered into the planning system to generate the beam model. Beam model accuracy was verified for simple fields. MBMS plans were created on previously treated cranial SRS patient data sets. Plans were evaluated using Paddick inverse conformity (ICI), gradient indices (GI), and cumulative volume of brain receiving 12 Gy. Dosimetric accuracy of the MBMS plans was verified using microDiamond, Gafchromic film, and SRS Mapcheck measurements of absolute dose and dose profiles for individual targets. Finally, an end-to-end (E2E) test was performed with a MR-CT compatible phantom to validate the accuracy of the simulation-to-delivery process. Results: For square fields, calculated scatter factors were within 1.0% of measured, PDDs were within 0.5% past dmax, and diagonal profiles were within 0.5% for clinically relevant off-axis distances (<10 cm). MBMS produced plans with ICIs < 1.5 and GIs < 5.0 for targets > 10 mm. Average point doses of the MBMS plans, measured by microDiamond, were within 0.31% of calculated (max 2.84%). Average perfield planar pass rates were 98.0% (95.5% minimum) using a 2%/1 mm/10% threshold relative gamma analysis. E2E point dose measurements were within 1.5% of calculated and Gafchromic film pass rates were 99.6% using a 5%/1 mm/10% threshold gamma analysis. Conclusion: The experience presented can be used to aid the commissioning of the Versa HD in the Brainlab MBMS treatment planning system, to produce safe and accurate SIMT cranial SRS treatments.
Purpose: The intra‐fraction variability of target position during the prostate cancer radiotherapy may cause dose discrepancy between planned and delivered dose, especially with longer hypo‐fractionated treatments. We report our clinical experience with real‐time 4D ultrasound imaging (4D‐US) to monitor intrafraction prostate motion. Methods: Three prostate patients were treated on an IRB‐approved protocol delivering 51 Gy in 10 fractions using single arc volumetric modulated arc therapy (VMAT). Each patient had three gold markers implanted and had simultaneous CT and 4D‐US simulation, followed by an MRI scan. Target and normal organs were delineated on MR images. During setup simultaneous cone‐beam CT (CBCT) and continuous 4D‐US were acquired, and during VMAT delivery (about 2 min) 4D‐US was acquired. The prostate 4D‐US position was compared to the CBCT average position, and movement during treatment was characterized. Results: The median (range) of mean intra‐fraction prostatic motion in the right‐left(RL), anterior‐posterior(AP) and superior‐inferior(SI) directions were 0.1 mm (−1.6 to 0.8 mm), 0 mm (−1.8 to 1.3 mm), and −0.1 mm (−2.2 to 1.4 mm), with respective median (range) of standard deviation were 0.2 mm (0 to 0.8 mm), 0.2 mm (0 to 1.2 mm), and 0.2 mm (0 to 0.7mm). There were 9/27 fractions with shifts >=2 mm in any direction, with an average duration of 23% of treatment time, with a single fraction having a shift greater than 3mm. The discrepancy between 4D‐US and CBCT shifts were 0.6±1.6 mm, −0.2±1.4 mm and −0.4±0.7 mm in the RL, AP and SI directions. There was one instance of flatulence during treatment setup where vertical shifts >=3 mm (up to 6.1 mm) persisted for 108 sec. Conclusion: Real‐time imaging is essential for tracking hypo‐fractionated prostate motion to reduce dosimetric uncertainty. 4D ultrasound imaging during treatment improves accuracy of dose delivery, and may allow a reduction of treatment margins.
Purpose: Develop a framework for the spatial accuracy assessment of multi‐modality deformable image registration (DIR) based on precise and reproducible deformations of a novel prostate phantom. Methods: An existing, tissue‐like prostate phantom has been adapted to allow quantifiable, reproducible, and variable degrees of volumetric deformation. This phantom is sequentially deformed into 11 stationary deformation states that can be individually imaged using CT, MRI, US and CBCT. The deformation is induced programmatically by changing the water volume of a balloon using a precise syringe pusher (5ml increments), effectively displacing and deforming the prostate and surrounding tissue. To demonstrate the phantom's utility as a DIR assessment tool, selected DIR algorithms (CT<‐>MR: Normalized Mutual Information, MR‐MR: Mutual Information, CT‐CT: Local Cross‐Correlation+Mutual Information) were applied to intra and cross‐imaging modalities depicting varying degrees of deformation. DIR accuracy was assessed via the DICE overlap score, calculated between the deformed prostate contour (computed by warping the manually delineated contour on the initial deformation state) and the manually delineated contour on the target deformation state. Results: DICE coefficients show excellent agreement of individual deformation states for MR and CT, hence validating the reproducibility of the induced deformation (< 1.0 + 0.1 %). Intra‐modality DIR (MR‐MR, CT‐CT) shows good performance (0.93–0.98). CT‐to‐MR performance is acceptable only for the closest 5–6 deformation states (> 0.94), while the DIR solutions for farthest 6 deformation states (0.78–0.92) are erroneous or artifactual. MR‐to‐CT DIR performance is demonstrably weaker (0.0–0.96) yielding asymmetrical errors w.r.t. the initial deformation state (no‐deformation vs. maxdeformation). Moreover, the first 3 deformed contours show zero overlap with the manual contours. Conclusion: We have developed a novel prostate phantom capable of gradual and reproducible deformation, and demonstrated its ability to assess multi‐modality (CT<‐>MR) DIR algorithm performance and range. Due to the phantom multi‐imaging capabilities, other cross‐imaging DIR algorithms can now be explored.
Purpose:Online adaptive aperture morphing is desirable over translational couch shifts to accommodate not only the target position variation but also anatomic changes (rotation, deformation, and relation of target to organ‐atrisks). We proposed quick and reliable method for adapting segment aperture leaves for IMRT treatment of prostate.Methods:The proposed method consists of following steps: (1) delineate the contours of prostate, SV, bladder and rectum on kV‐CBCT; (2) determine prostate displacement from the rigid body registration of the contoured prostate manifested on the reference CT and the CBCT; (3) adapt the MLC segment apertures obtained from the pre‐treatment IMRT planning to accommodate the shifts as well as anatomic changes. The MLC aperture adaptive algorithm involves two steps; first move the whole aperture according to prostate translational/rotational shifts, and secondly fine‐tune the aperture shape to maintain the spatial relationship between the planning target contour and the MLC aperture to the daily target contour. Feasibility of this method was evaluated retrospectively on a seven‐field IMRT treatment of prostate cancer patient by comparing dose volume histograms of the original plan and the aperture‐adjusted plan, with/without additional segments weight optimization (SWO), on two daily treatment CBCTs selected with relative large motion and rotation.Results:For first daily treatment, the prostate rotation was significant (12degree around lateral‐axis). With apertureadjusted plan, the D95 to the target was improved 25% and rectum dose (D30, D40) was reduced 20% relative to original plan on daily volumes. For second treatment‐fraction, (lateral shift = 6.7mm), after adjustment target D95 improved by 3% and bladder dose (D30, maximum dose) was reduced by 1%. For both cases, extra SWO did not provide significant improvement.Conclusion:The proposed method of adapting segment apertures is promising in treatment position correction, including target translational displacement, rotation and deformation. Additional SWO could improve ROIs dose distribution.
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