A novel phantom and procedure providing submillimeter accuracy in daily QA tests of accelerators used for stereotactic radiosurgery*

[Development of a Phantom and Analysis Program for Assessment of Positional Accuracy of Image-guided Radiation Therapy]

PURPOSE

We created a phantom and analysis program for the assessment of IGRT positional accuracy. We verified the accuracy of analysis and the practicality of this evaluation method at several facilities.

METHOD

End-to-end test was performed using an in-house phantom, and EPID images were acquired after displacement by an arbitrary amount using a micrometer, with after image registration as the reference. The difference between the center of the target and the irradiated field was calculated using our in-house analysis program and commercial software. The end-to-end test was conducted at three facilities, and the IGRT positional accuracy evaluation was verified.

RESULT

The maximum difference between the displacement of the target determined from the EPID image and the arbitrary amount of micrometer displacement was 0.24 mm for the in-house analysis program and 0.30 mm for the commercial software. The maximum difference between the center of the target and the irradiation field on EPID images acquired at the three facilities was 0.97 mm.

CONCLUSION

The proposed evaluation method using our in-house phantom and analysis program can be used for the assessment of IGRT positional accuracy.

Automating QA analysis for a six-degree-of-freedom (6DOF) couch using image displacement and an accelerometer sensor

The purpose of this study is to develop an approach for automating quality assurance (QA) analysis for a six-degree-of-freedom (6DOF) couch using image displacement and an accelerometer sensor. A cubic phantom was fabricated using 3D printing and the accelerometer sensor was embedded in the phantom to measure the couch in the pitch and roll directions. The accuracy and reliability of image displacement and the accelerometer sensor were investigated prior to their practical use for 6DOF couch QA. Image displacement performance had an accuracy and reliability of 0.026 ± 0.026 mm for the translation direction and 0.021 ± 0.016° for the rotation direction. Accelerometer sensor performance had an accuracy and reliability of 0.023 ± 0.018° for pitch rotation and 0.051 ± 0.024° for roll rotation. Automating QA analysis was used to perform 6DOF couch QA, and the couch position errors measured using image displacement were less than 0.99 mm, 0.91 mm, 0.82 mm for the vertical, longitudinal, lateral translation in range between ±20 mm, and 0.07°, 0.23°, and 0.2° for pitch, roll, and yaw rotation in range between ±3° whereas the couch position errors measured using the accelerometer sensor were less than 0.1° and 0.19° for the pitch and roll rotation in range between ±3°.

The current status and shortcomings of stereotactic radiosurgery

Background

Stereotactic radiosurgery (SRS) is a common treatment for intracranial lesions. This work explores the state of SRS treatment delivery to characterize current treatment accuracy based on treatment parameters.

Methods

NCI clinical trials involving SRS rely on an end-to-end treatment delivery on a patient surrogate (credentialing phantom) from the Imaging and Radiation Oncology Core (IROC) to test their treatment accuracy. The results of 1072 SRS phantom irradiations between 2012 and 2020 were retrospectively analyzed. Univariate analysis and random forest models were used to associate irradiation conditions with phantom performance. The following categories were evaluated in terms of how they predicted outcomes: year of irradiation, TPS algorithm, machine model, energy, and delivered field size.

Results

Overall, only 84.6% of irradiations have met the IROC/NCI acceptability criteria. Pass rate has remained constant over time, while dose calculation accuracy has slightly improved. Dose calculation algorithm (P < .001), collimator (P = .024), and field size (P < .001) were statistically significant predictors of pass/fail. Specifically, pencil beam algorithms and cone collimators were more likely to be associated with failing phantom results. Random forest modeling identified the size of the field as the most important factor for passing or failing followed by algorithm.

Conclusion

Constant throughout this retrospective study, approximately 15% of institutions fail to meet IROC/NCI standards for SRS treatment. In current clinical practice, this is particularly associated with smaller fields that yielded less accurate results. There is ongoing need to improve small field dosimetry, beam modeling, and QA to ensure high treatment quality, patient safety, and optimal clinical trials.

Evaluation of patient-specific MR distortion correction schemes for improved target localization accuracy in SRS

PURPOSE

This work aims at promoting target localization accuracy in cranial stereotactic radiosurgery (SRS) applications by focusing on the correction of sequence-dependent (also patient induced) magnetic resonance (MR) distortions at the lesion locations. A phantom-based quality assurance (QA) methodology was developed and implemented for the evaluation of three distortion correction techniques. The same approach was also adapted to cranial MR images used for SRS treatment planning purposes in single or multiple brain metastases cases.

METHODS

A three-dimensional (3D)-printed head phantom was filled with a 3D polymer gel dosimeter. Following treatment planning and dose delivery, volumes of radiation-induced polymerization served as hypothetical lesions, offering adequate MR contrast with respect to the surrounding unirradiated areas. T1-weighted (T1w) MR imaging was performed at 1.5 T using the clinical scanning protocol for SRS. Additional images were acquired to implement three distortion correction methods; the field mapping (FM), mean image (MI) and signal integration (SI) techniques. Reference lesion locations were calculated as the averaged centroid positions of each target identified in the forward and reverse read gradient polarity MRI scans. The same techniques and workflows were implemented for the correction of contrast-enhanced T1w MR images of 10 patients with a total of 27 brain metastases.

RESULTS

All methods employed in the phantom study diminished spatial distortion. Median and maximum distortion magnitude decreased from 0.7 mm (2.10 ppm) and 0.8 mm (2.36 ppm), respectively, to <0.2 mm (0.61 ppm) at all target locations, using any of the three techniques. Image quality of the corrected images was acceptable, while contrast-to-noise ratio slightly increased. Results of the patient study were in accordance with the findings of the phantom study. Residual distortion in corrected patient images was found to be <0.3 mm in the vast majority of targets. Overall, the MI approach appears to be the most efficient correction method from the three investigated.

CONCLUSIONS

In cranial SRS applications, patient-specific distortion correction at the target location(s) is feasible and effective, despite the expense of longer imaging time since additional MRI scan(s) need to be performed. A phantom-based QA methodology was developed and presented to reassure efficient implementation of correction techniques for sequence-dependent spatial distortion.

Dosimetric impact of rotational errors on the quality of VMAT-SRS for multiple brain metastases: Comparison between single- and two-isocenter treatment planning techniques

Purpose

In the absence of a 6D couch and/or assuming considerable intrafractional patient motion, rotational errors could affect target coverage and OAR‐sparing especially in multiple metastases VMAT‐SRS cranial cases, which often involve the concurrent irradiation of off‐axis targets. This work aims to study the dosimetric impact of rotational errors in such applications, under a comparative perspective between the single‐ and two‐isocenter treatment techniques.

Methods

Ten patients (36 metastases) were included in this study. Challenging cases were only considered, with several targets lying in close proximity to OARs. Two multiarc VMAT plans per patient were prepared, involving one and two isocenters, serving as the reference plans. Different degrees of angular offsets at various orientations were introduced, simulating rotational errors. Resulting dose distributions were evaluated and compared using commonly employed dose‐volume and plan quality indices.

Results

For single‐isocenter plans and 1^⁰ rotations, plan quality indices, such as coverage, conformity index and D_95%, deteriorated significantly (>5%) for distant targets from the isocenter (at> 4–6 cm). Contrarily, for two‐isocenter plans, target distances to nearest isocenter were always shorter (≤4 cm), and, consequently, 1^⁰ errors were well‐tolerated. In the most extreme case considered (2^⁰ around all axes) conformity index deteriorated by on‐average 7.2%/cm of distance to isocenter, if one isocenter is used, and 2.6%/cm, for plans involving two isocenters. The effect is, however, strongly associated with target volume. Regarding OARs, for single‐isocenter plans, significant increase (up to 63%) in D_max and D_0.02cc values was observed for any angle of rotation. Plans that could be considered clinically unacceptable were obtained even for the smallest angle considered, although rarer for the two‐isocenter planning approach.

Conclusion

Limiting the lesion‐to‐isocenter distance to ≤4 cm by introducing additional isocenter(s) appears to partly mitigate severe target underdosage, especially for smaller target sizes. If OAR‐sparing is also a concern, more stringent rotational error tolerances apply.

Feasibility of virtual starshot analysis providing submillimeter radiation isocenter accuracy: A long-term multi-institutional analysis

PURPOSE

We developed a technique to calculate the offset between room lasers and the radiation isocenter using a digital Winston-Lutz (WL) test with a starshot technique. We have performed isocenter localization quality assurance (QA) with submillimeter accuracy for a long period. Here we evaluated the feasibility and accuracy of this virtual starshot (VS) analysis for isocenter localization QA.

METHODS

A 6-MV photon beam with a square multileaf collimator field was used to irradiate a WL sphere positioned at the intersection of the room lasers. Images were acquired using an electronic portal imaging device. A four-field WL test was performed, and the path of each beam was calculated from the offset between the beam and sphere. Virtual starshot analysis was used to analyze the radiation isocenter, which calculates the center of the beam paths by using a least-squares method, similar to the starshot analysis. Then, eight coplanar and 12 noncoplanar beams were irradiated to evaluate isocenter localization accuracy.

RESULTS

Several VS analyses, using different WL spheres, were performed at three institutions, and the calculated accuracies were within 0.1 mm at all institutions. Long-term analysis showed that the isocenter localization accuracy was appropriately managed with three-dimensional accuracy within ± 0.5 mm for 90 months after the first laser adjustments. The offset between each beam and the room laser was within 0.6 mm and within 1.0 mm for eight coplanar and 12 noncoplanar beams, respectively, for 90 months. Cone-beam computed tomography images, acquired after verification beams, showed that the offset between the radiation isocenter and the imaging center was within 0.66 mm for 90 months. The isocenter localization accuracy within 1 mm was kept for long period at other four institutions.

CONCLUSIONS

Long-term analysis showed the feasibility of VS analysis for isocenter localization QA, including room laser re-alignment, noncoplanar irradiation verification, and image guidance accuracy.

Efficient quality assurance method with automated data acquisition of a single phantom setup to determine radiation and imaging isocenter congruence

We developed a quality assurance (QA) method to determine the isocenter congruence of Optical Surface Monitoring System (OSMS, Varian, CA, USA), kilovoltage (kV), and megavoltage (MV) imaging, and the radiation isocenter using a single setup of the OSMS phantom for frameless Stereotactic Radiosurgery (SRS) treatment. After aligning the phantom to the OSMS isocenter, a cone‐beam computed tomography (CBCT) of the phantom was acquired and registered to a computed tomography (CT) scan of the phantom to determine the CBCT isocenter. Without moving the phantom, MV and kV images were simultaneously acquired at four gantry angles to localize MV and kV isocenters. Then, Winston‐Lutz (W‐L) test images of the central BB in the phantom were acquired to analyze the radiation isocenter. The gantry and couch were automatically controlled using the TrueBeam Developer Mode during MV, kV, and W‐L image acquisition. All the images were acquired weekly for 17 weeks to track the congruence of all the imaging modalities' isocenter in six‐dimensional (6D) translations and rotations, and the radiation isocenter in three‐dimensional (3D) translations. The shifts of isocenters of all imaging modalities and the radiation isocenter from the OSMS isocenter were within 0.2 mm and 0.2° on average over 17 weeks. The maximum discrepancy between OSMS and other imaging modalities or radiation isocenters was 0.8 mm and 0.3°. However, systematic shifts of radiation isocenter anteriorly and laterally relative to the OSMS isocenter were observed. The measured discrepancies were consistent from week‐to‐week except for two weeks when the isocenter discrepancies of 0.8 mm were noted due to drifts of the OSMS isocenter. Once recalibration was performed on OSMS, the discrepancy was reduced to 0.3 mm and 0.2°.By performing the proposed QA on a weekly basis, the isocenter congruencies of multiple imaging systems and radiation isocenter were validated for a linear accelerator.

Stereotactic radiosurgery with MLC-defined arcs: Verification of dosimetry, spatial accuracy, and end-to-end tests

Purpose

To measure dosimetric and spatial accuracy of stereotactic radiosurgery (SRS) delivered to targets as small as the trigeminal nerve (TN) using a standard external beam treatment planning system (TPS) and multileaf collimator‐(MLC) equipped linear accelerator without cones or other special attachments or modifications.

Methods

Dosimetric performance was assessed by comparing computed dose distributions to film measurements. Comparisons included the γ‐index, beam profiles, isodose lines, maximum dose, and spatial accuracy. Initially, single static 360° arcs of MLC‐shaped fields ranging from 1.6 × 5 to 30 × 30 mm² were planned and delivered to an in‐house built block phantom having approximate dimensions of a human head. The phantom was equipped with markings that allowed accurate setup using planar kV images. Couch walkout during multiple‐arc treatments was investigated by tracking a ball pointer, initially positioned at cone beam computed tomography (CBCT) isocenter, as the couch was rotated. Tracks were mapped with no load and a 90 kg stack of plastic plates simulating patient treatment. The dosimetric effect of walkout was assessed computationally by comparing test plans that corrected for walkout to plans that neglected walkout. The plans involved nine 160° arcs of 2.4 × 5 mm² fields applied at six different couch angles. For end‐to‐end tests that included CT simulation, target contouring, planning, and delivery, a cylindrical phantom mimicking a 3 mm lesion was constructed and irradiated with the nine‐arc regimen. The phantom, lacking markings as setup aids was positioned under CBCT guidance by registering its surface and internal structures with CTs from simulation. Radiochromic film passing through the target center was inserted parallel to the coronal and the sagittal plane for assessment of spatial and dosimetric accuracy.

Results

In the single‐arc block phantom tests computed maximum doses of all field sizes agreed with measurements within 2.4 ± 2.0%. Profile widths at 50% maximum agreed within 0.2 mm. The largest targeting error was 0.33 mm. The γ‐index (3%, 1 mm) averaged over 10 experiments was >1 in only 1% of pixels for field sizes up to 10 × 10 mm² and rose to 4.4% as field size increased to 20 × 20 mm². Table walkout was not affected by load. Walkout shifted the target up to 0.6 mm from CBCT isocenter but, according to computations shifted the dose cloud of the nine‐arc plan by only 0.16 mm. Film measurements verified the small dosimetric effect of walkout, allowing walkout to be neglected during planning and treatment. In the end‐to‐end tests average and maximum targeting errors were 0.30 ± 0.10 and 0.43 mm, respectively. Gamma analysis of coronal and sagittal dose distributions based on a 3%/0.3 mm agreement remained <1 at all pixels. To date, more than 50 functional SRS treatments using MLC‐shaped static field arcs have been delivered.

Conclusion

Stereotactic radiosurgery (SRS) can be planned and delivered on a standard linac without cones or other modifications with better than 0.5 mm spatial and 5% dosimetric accuracy.