Reliability of contour-based volume calculation for radiosurgery

Benchmarking Tests of Contemporary SRS Platforms: Have Technological Developments Resulted in Improved Treatment Plan Quality?

PURPOSE

Stereotactic radiosurgery treatment delivery can be performed with a range of devices, each of which have evolved over recent years. We sought to evaluate the differences in performance of contemporary stereotactic radiosurgery platforms and also to compare them with earlier platform iterations from a previous benchmarking study.

METHODS AND MATERIALS

The following platforms were selected as "state of the art" in 2022: Gamma Knife Icon (GK), CyberKnife S7 (CK), Brainlab Elements (Elekta VersaHD and Varian TrueBeam), Varian Edge with HyperArc (HA), and Zap-X. Six benchmarking cases were used from a 2016 study. To reflect the evolution of increasing numbers of metastases treated per patient, a 14-target case was added. The 28 targets among the 7 patients ranged from 0.02 to 7.2 cc in volume. Participating centers were sent images and contours for each patient and asked to plan them to the best of their ability. Although some variation in local practice was allowed (eg, margins), groups were asked to prescribe a specified dose to each target and tolerance doses to organs at risk were agreed upon. Parameters compared included coverage, selectivity, Paddick conformity index, gradient index (GI), R50%, efficiency index, doses to organs at risk, and planning and treatment times.

RESULTS

Mean coverage for all targets ranged from 98.2% (Brainlab/Elekta) to 99.7% (HA-6X). Paddick conformity index values ranged from 0.722 (Zap-X) to 0.894 (CK). GI ranged from a mean of 3.52 (GK), representing the steepest dose gradient, to 5.08 (HA-10X). The GI appeared to follow a trend with beam energy, with the lowest values from the lower energy platforms (GK, 1.25 MeV; Zap-X, 3 MV) and the highest value from the highest energy (HA-10X). Mean R50% values ranged from 4.48 (GK) to 5.98 (HA-10X). Treatment times were lowest for C-arm linear accelerators.

CONCLUSIONS

Compared with earlier studies, newer equipment appears to deliver higher quality treatments. CyberKnife and linear accelerator platforms appear to give higher conformity whereas lower energy platforms yield a steeper dose gradient.

Evaluation of a dedicated software for semi-automated VMAT planning of spine Stereotactic Body Radiotherapy (SBRT)

PURPOSE

To determine whether SBRT of spinal metastasis using a dedicated treatment planning system (TPS) and delivered with a gantry-based LINAC could provide plans of similar quality to the Cyberknife technology. Additional comparison was also done with other commercial TPS used for volumetric modulated arc therapy (VMAT) planning.

MATERIALS AND METHODS

Thirty Spine SBRT patients, previously treated in our institution with CyberKnife (Accuray, Sunnyvale) using Multiplan TPS, were replanned in VMAT with an dedicated TPS (Elements Spine SRS, Brainlab, Munich) and our clinical TPS (Monaco, Elekta LTD, Stockholm), using exactly the same arc geometry. The comparison was done by assessing differences in dose delivered to PTV, CTV and spinal cord, calculating modulation complexity scores (MCS) and performing quality control (QA) of the plans.

RESULTS

Regardless of the vertebra level, in general, no statistical difference was found in PTV coverage between all TPS. Conversely, PTV and CTV D_50% were found significantly higher for the dedicated TPS compared to others. In addition, the dedicated TPS also resulted in better gradient index (GI) than clinical VMAT TPS, whatever the vertebral level, and better GI than Cyberknife TPS for the thoracic level only. The D_2% to the spinal cord was generally significantly lower with the dedicated TPS compared with others. No significant difference was found in the MCS between both VMAT TPS. All QA were clinically acceptable.

CONCLUSION

The Elements Spine SRS TPS offers very effective and user-friendly semi-automated planning tools and is secure and promising for gantry-based LINAC spinal SBRT.

Impact of MRI resolution for Linac-based stereotactic radiosurgery

Objective

Magnetic resonance imaging (MRI) is a standard imaging modality in intracranial stereotactic radiosurgery (SRS) for defining target volumes. However, wide disparities in MRI resolution exist, which could directly impact accuracy of target delineation. Here, sequences with various MRI resolution were acquired on phantoms to evaluate the effect on volume definition and dosimetric consequence for cranial SRS.

Materials/Methods

Four T1-weighted MR sequences with increasing 3D resolution were compared, including two Spin Echo (SE) 2D acquisitions with 5mm and 3mm slice thickness (SE5mm, SE3mm) and two gradient echo 3D acquisitions (TFE, BRAVO). The voxel sizes were 0.4×0.4×5.0, 0.5×0.5×3.0, 0.9×0.9×1.25, and 0.4×0.4×0.5 mm³, respectively. Four phantoms with simulated lesions of different shape and volume (range, 0.53-25.0 cm³) were imaged, resulting in 16 total sets of MRIs. Four radiation oncologists provided contours on individual MR image set. All observer contours were compared with ground truth, defined on CT image according to the absolute dimensions of the target structure, using Dice similarity coefficient (DSC), Hausdorff distance (HD), mean distance-to-agreement (MDA), and the ratio between reconstructed and true volume (Ratio_vol ). For dosimetric consequence, SRS plans targeting observer volumes were created. The true Paddick conformity index ( C I p a d d i c k t r u e ), calculated with true target volume, was correlated with quality of observer volume.

Results

All measures of observer contours improved as increasingly higher MRI resolution was provided from SE5mm to BRAVO. The improvement in DSC, HD and MDA was statistically significant (p<0.01). Dosimetrically, C I p a d d i c k t r u e   strongly correlated with DSC of the planning observer volume (Pearson's r=0.94, p<0.00001).

Conclusions

Significant improvement in target definition and reduced inter-observer variation was observed as the MRI resolution improved, which also improved the quality of SRS plans. Results imply that high resolution 3D MR sequences should be used to minimize potential errors in target definition, and multi-slice 2D sequences should be avoided.

Accuracy of dose-volume metric calculation for small-volume radiosurgery targets

PURPOSE

For stereotactic radiosurgery (SRS), accurate evaluation of dose-volume metrics for small structures is necessary. The purpose of this study was to compare the DVH metric capabilities of five commercially available SRS DVH analysis tools (Eclipse, Elements, Raystation, MIM, and Velocity).

METHODS

DICOM RTdose and RTstructure set files created using MATLAB were imported and evaluated in each of the tools. Each structure set consisted of 50 randomly placed spherical targets. The dose distributions were created on a 1-mm grid using an analytic model such that the dose-volume metrics of the spheres were known. Structure sets were created for 3, 5, 7, 10, 15, and 20 mm diameter spheres. The reported structure volume, V100% [cc], and V50% [cc], and the RTOG conformity index and Paddick Gradient Index, were compared with the analytical values.

RESULTS

The average difference and range across all evaluated target sizes for the reported structure volume was - 4.73%[-33.2,0.2], 0.11%[-10.9, 9.5], -0.39%[-12.1, 7.0], -2.24%[-21.0, 1.3], and 1.15%[-15.1,0.8], for TPS-A through TPS-E, respectively. The average difference and range for the V100%[cc] (V20Gy[cc]) was - 0.4[-24.5,9.8], -2.73[-23.6, 1.1], -3.01[-23.6, 0.6], -3.79[-27.3, 1.3], and 0.26[-6.1,2.6] for TPS-A through TPS-E, respectively. For V50%[cc](V10Gy[cc]) in TPS-A through TPS-E the average and ranger were - 0.05[-0.8,0.4], -0.18[-1.2, 0.5], -0.44[-1.4, 0.3], -0.26[-1.8, 2.6], and 0.09[-1.4,2.7].

CONCLUSION

This study expanded on the previously published literature to quantitatively compare the DVH analysis capabilities of software commonly used for SRS plan evaluation and provides freely available and downloadable analytically derived set of ground truth DICOM dose and structure files for the use of radiotherapy clinics. The differences between systems highlight the need for standardization and/or transparency between systems, especially when evaluating plan quality for multi-institutional clinical trials.

NCTN Assessment on Current Applications of Radiomics in Oncology

Radiomics is a fast-growing research area based on converting standard-of-care imaging into quantitative minable data and building subsequent predictive models to personalize treatment. Radiomics has been proposed as a study objective in clinical trial concepts and a potential biomarker for stratifying patients across interventional treatment arms. In recognizing the growing importance of radiomics in oncology, a group of medical physicists and clinicians from NRG Oncology reviewed the current status of the field and identified critical issues, providing a general assessment and early recommendations for incorporation in oncology studies.

Stereotactic radiosurgery for benign brain tumors: Results of multicenter benchmark planning studies

PURPOSE

Stereotactic radiosurgery (SRS) is strongly indicated for treatment of surgically inaccessible benign brain tumors. Various treatment platforms are available, but few comparisons have included multiple centers. As part of a national commissioning program, benchmark planning cases were completed by all clinical centers in the region.

METHODS AND MATERIALS

Four benign cases were provided, with images and structures predelineated, including intracanalicular vestibular schwannoma (VS), larger VS, skull base meningioma, and secreting pituitary adenoma. Centers were asked to follow their local practice, and plans were reviewed centrally using metrics for target coverage, selectivity, gradient falloff, and normal tissue sparing.

RESULTS

Sixty-eight plans were submitted using 18 different treatment platforms. Fourteen plans were subsequently revised following feedback, and review of 5 plans led to a restriction of service on 2 platforms (2 centers). Prescription doses were consistent for VS and meningioma submissions, but a wide range of doses were used for the pituitary case. All centers prioritized coverage, with the prescription isodose covering ≥95% of 78/82 target volumes. Lower values may be expected next to air cavities when using advanced algorithms, and in general may be acceptable for some benign lesions. Selectivity was much more variable, and in some cases this was combined with high gradient index and/or >1 mm margin, resulting in large volumes of normal tissue being irradiated. Normal tissue doses were more variable across linear accelerator (LINAC)-based plans than with Gamma Knife or CyberKnife, and dose spillage seemed independent of prescription isodose (inhomogeneity). This may reflect the variety of LINAC-based approaches represented or the necessary tradeoff between different objectives.

CONCLUSIONS

These benchmarking exercises have highlighted areas of different clinical practice and priorities and potential for improvement. The subsequent sharing of plan data and margin philosophies between the neurosurgery and oncology communities allowed for meaningful comparison between centers and their peers.

Stereotactic radiosurgery for multiple brain metastases: Results of multicenter benchmark planning studies

PURPOSE

Stereotactic radiosurgery is indicated for treatment of multiple brain metastases. Various treatment platforms are available, but most comparisons are limited to single-center studies. As part of a national commissioning program, benchmark planning cases were completed by 21 clinical centers, providing a unique dataset of current practice across a large number of providers and equipment platforms.

METHODS AND MATERIALS

Two brain metastases cases were provided, with images and structures predrawn, involving 3 and 7 lesions. Centers produced plans according to their local practice, which were reviewed centrally using metrics for target coverage, selectivity, gradient fall-off, and normal tissue sparing.

RESULTS

Fifty plans were submitted, using 24 treatment platforms. Eleven plans were revised following feedback, including 2 centers that acquired a new platform; 1 other center accepted a restriction of service. All centers prioritized coverage, with the prescription isodose covering ≥95% of 233 of 235 target volumes. Selectivity was much more variable, especially for smaller lesions, and when combined with poor gradient indices resulted in large volumes of normal tissue being irradiated. Tomotherapy submissions were outliers for either selectivity or gradient index, but other platforms could produce plans with relatively low gradient indices for larger lesion volumes. There was more variation among Varian and Elekta LINAC plans than for Gamma Knife and CyberKnife, and larger differences for smaller targets, both inter- and intratreatment platform. Doses to normal brain and brainstem were highest when margins were applied, but improvements were possible by replanning alone.

CONCLUSIONS

Multicenter benchmarking exercises have highlighted some variation in clinical practice and priorities, with a few outliers. Most platforms are able to achieve comparable plans, except for the smallest volumes and when larger planning margins are used. The data will be used to advance standardization and quality improvement of national services and can provide useful guidance for centers worldwide.

Dependence of volume calculation and margin growth accuracy on treatment planning systems for stereotactic radiosurgery

OBJECTIVE

Uncertainties in radiotherapy target structures are partly dependent on differences between volume calculation and margin growing methods in treatment planning systems (TPS). These uncertainties are exacerbated with very small structures such as those common in stereotactic radiosurgery.

METHODS

Data from a national commissioning programme for SRS was used to assess variation in reported volumes for six benchmark cases, including malignant and benign indications. Reported volumes were compared both with and without any margins added according to local practice.

RESULTS

137 plans were submitted, with a total of 311 structures and covering seven TPS. For volumes < 1 cm³ agreement was within 0.05 cm³, and for volumes > 1 cm³ agreement was within 5%. Systematic differences were seen between TPS, partly because of different methods for calculating the end slice volume. About one third of structures had a margin added, of 1-2 mm. Most TPS over-grew the volumes, compared to the approximation of a perfect sphere, especially Pinnacle and Eclipse.

CONCLUSION

Differences between volume calculation methods may lead to 5-10% variation in reported volumes from different TPS. This should be taken into account when comparing multicentre studies, and it is recommended that a minimum volume of 0.05 cm³ be used for any near-point doses to allow more consistent comparisons. When margins are added to small structures, there may be up to 40% difference to nominal margin size. Such differences are still small compared to interobserver variation in delineation. Advances in knowledge: This study quantifies the potential uncertainties in clinical volume calculation and margin growth with small radiosurgical targets.

Standardization of terminology in stereotactic radiosurgery: Report from the Standardization Committee of the International Leksell Gamma Knife Society

Object

This report has been prepared to ensure more uniform reporting of Gamma Knife radiosurgery treatment parameters by identifying areas of controversy, confusion, or imprecision in terminology and recommending standards.

Methods

Several working group discussions supplemented by clarification via email allowed the elaboration of a series of provisional recommendations. These were also discussed in open session at the 16th International Leksell Gamma Knife Society Meeting in Sydney, Australia, in March 2012 and approved subject to certain revisions and the performance of an Internet vote for approval from the whole Society. This ballot was undertaken in September 2012.

Results

The recommendations in relation to volumes are that Gross Target Volume (GTV) should replace Target Volume (TV); Prescription Isodose Volume (PIV) should generally be used; the term Treated Target Volume (TTV) should replace TVPIV, GTV in PIV, and so forth; and the Volume of Accepted Tolerance Dose (VATD) should be used in place of irradiated volume. For dose prescription and measurement, the prescription dose should be supplemented by the Absorbed Dose, or DV% (for example, D95%), the maximum and minimum dose should be related to a specific tissue volume (for example, D2% or preferably D1 mm3), and the median dose (D50%) should be recorded routinely. The Integral Dose becomes the Total Absorbed Energy (TAE). In the assessment of planning quality, the use of the Target Coverage Ratio (TTV/ GTV), Paddick Conformity Index (PCI = TTV2/[GTV · PIV]), New Conformity Index (NCI = [GTV · PIV]/TTV2), Selectivity Index (TTV/PIV), Homogeneity Index (HI = [D2% –D98%]/D50%), and Gradient Index (GI = PIV0.5/PIV) are reemphasized. In relation to the dose to Organs at Risk (OARs), the emphasis is on dose volume recording of the VATD or the dose/volume limit (for example, V10) in most cases, with the additional use of a Maximum Dose to a small volume (such as 1 mm3) and/or a Point Dose and Mean Point Dose in certain circumstances, particularly when referring to serial organs. The recommendations were accepted by the International Leksell Gamma Knife Society by a vote of 92% to 8%.

Conclusions

An agreed-upon and uniform terminology and subsequent standardization of certain methods and procedures will advance the clinical science of stereotactic radiosurgery.