Stereotactic Planning Research Articles

Brachytherapy Versus Stereotactic Body Radiotherapy for Cervical Cancer Boost: A Dosimetric Comparison.

The standard treatment for locally advanced cervical cancer involves chemo-radiation followed by brachytherapy. However, some patients are unable to undergo brachytherapy intensification. Recent advancements in radiation technology have provided several techniques, with stereotactic body radiation therapy (SBRT) theoretically able to mimic the dose distribution of brachytherapy with a high dose gradient. We analyzed 20 high-dose-rate intra-cavity brachytherapy plans for women with cervical cancer and simulated an adjunctive stereotactic radiotherapy plan at the same doses used for brachytherapy (21 Gray [Gy] in three fractions). No planning tumoral volume (PTV) margin was added for SBRT dosimetry. We used the dose constraints for brachytherapy from the EMBRACE trial and the dose constraints for SBRT in three fractions. Dose distribution, maximum dose points on target volumes, bladder, rectum, and dose-volume histograms were compared between the two techniques. The mean volume of the high-riskclinical tumoral volume (CTV) was 64 cm3, and the mean volume of the intermediate-risk CTV was 93 cm3. The mean minimum dose received by 90% of the high-risk CTV (D90 CTV HR) was 17 Gy for brachytherapy versus 8.3 Gy for SBRT. The average minimum dose received by 90% of the intermediate-risk CTV (D90 CTV IR) was 7.5 Gy for brachytherapy versus 8.9 Gy for SBRT. The mean minimum dose delivered to 2cc of the bladder was 74.6 Gy for brachytherapy versus 84.7 Gy for SBRT. The mean minimum dose delivered to 2cc of the rectum was 71.8 Gy for brachytherapy versus 74.7 Gy for SBRT. We confirmed the dosimetric superiority of brachytherapy over SBRT in terms of target volume coverage and organ-at-risk sparing. Therefore, pending the results of further clinical studies, no current radiotherapy technique can replace brachytherapy for cervical cancer boost after external radiotherapy.

Evaluation of plan quality and treatment efficiency in cranial stereotactic radiosurgery treatment plans with a variable source-to-axis distance.

Radiotherapy deliveries with dynamic couch motions that shorten the source-to-axis distance (SAD) on a C-arm linac have the potential to increase treatment efficiency through the increase of the effective dose rate. In this investigation, we convert clinically deliverable volumetric modulated arc therapy (VMAT) and dynamic conformal arc (DCA) plans for cranial radiosurgery into virtual isocenter plans through implementation of couch trajectories that maintain the target at a shortened but variable SAD throughout treatment. A randomly sampled population of patients treated with cranial radiosurgery from within the last three years were separated into groups with one, two, and three lesions. All plans had a single isocenter (regardless of number of targets), and a single prescription dose. Patient treatment plans were converted from their original delivery at a standard isocenter to a dynamic virtual isocenter in MATLAB. The virtual isocenter plan featured a variable isocenter position based upon the closest achievable source-to-target distance (referred to herein as a virtual source-to-axis distance [vSAD]) which avoided collision zones on a TrueBeam STx platform. Apertures were magnified according to the vSAD and monitor units at a given control point were scaled based on the inverse square law. Doses were calculated for the plans with a virtual isocenter in the Eclipse (v13.6.23) treatment planning system (TPS) and were compared with the clinical plans. Plan metrics (MU, Paddick conformity index, gradient index, and the volume receiving 12 Gy or more), normal brain dose-volume differences, as well as maximum doses received by organs at risk (OARs) were assessed. The values were compared between standard and virtual isocenter plans with Wilcoxon Sign Ranked tests to determine significance. A subset of the plans were mapped to the MAX-HD anthropomorphic phantom which contained an insert housing EBT3 GafChromic film and a PTW 31010 microion chamber for dose verification on a linac. Delivering plans at a virtual isocenter resulted in an average reduction of 20.9% (p = 3×10-6 ) and 20.6% (p = 3.0×10-6 ) of MUs across all VMAT and all DCA plans, respectively. There was no significant change in OAR max doses received by plans delivered at a virtual isocenter. The low dose wash (1.0-2.0 Gy or 5-11% of the prescription dose) was increased (by approximately 20 cc) for plans with three lesions. This was equivalent to a 2.7%-3.8% volumetric increase in normal tissue receiving the respective dose level when comparing the plan with a virtual isocenter to a plan with a standard isocenter. Gamma pass rates with a 5%/1mm analysis criteria were 96.40% ± 2.90% and 95.07% ± 3.10% for deliveries at standard and virtual isocenter, respectively. Absolute point dose agreements were within -0.36% ± 3.45% and -0.55% ± 3.39% for deliveries at a standard and virtual isocenter, respectively. Potential time savings per arc were found to have linear relationship with the monitor units delivered per arc (savings of 0.009 s/MU with an r2 = 0.866 when fit to plans with a single lesion). Converting clinical plans at standard isocenter to a virtual isocenter design did not show any losses to plan quality while simultaneously improving treatment efficiency through MU reductions.

An anthropomorphic 3D printed inhomogeneity thorax phantom slab for SBRT commissioning and quality assurance.

Anthropomorphic phantoms with tissue equivalency are required in radiotherapy for quality assurance of imaging and dosimetric processes used in radiotherapy treatments. Commercial phantoms are expensive and provide limited approximation to patient geometry and tissue equivalency. In this study, a 5cm thick anthropomorphic thoracic slab phantom was designed and 3D printed using models exported from a CT dataset to demonstrate the feasibility of manufacturing anthropomorphic 3D printed phantoms onsite in a clinical radiotherapy department. The 3D printed phantom was manufactured with polylactic acid with an in-fill density of 80% to simulate tissue density and 26% to simulate lung density. A common radio-opacifier, barium sulfate (BaSO4), was added 6% w/w to an epoxy resin mixture to simulate similar HU numbers for bone equivalency. A half-cylindrical shape was cropped away from the spine region to allow insertion of the bone equivalent mixture. Two Gafchromic™ EBT3 film strips were inserted into the 3D printed phantom to measure the delivery of two stereotactic radiotherapy plans targeting lung and bone lesions respectively. Results were analysed within SNC Patient with a low dose threshold of 10% and a gamma criterion of 3%/2mm and 5%/1mm. The resulting gamma pass rate across both criterions for lung and bone were ≥ 95% and approximately 85% respectively. Results shows that a cost-effective anthropomorphic 3D printed phantom with realistic heterogeneity simulation can be fabricated in departments with access a suitable 3D printer, which can be used for performing commissioning and quality assurance for stereotactic type radiotherapy to lesions in the presence of heterogeneity.

Evaluation of stereotactic VMAT lung treatment plans for small moving targets

PurposeThe aim of this study is to perform patient quality controls and end-to-end tests for stereotactic VMAT lung treatment plans and to investigate the influence of various parameters on the results. Method18 plans were defined by an experimental design methodology to cover a large variety of stereotactic VMAT lung treatments including different doses per fraction, target diameters, target movements and respiratory parameters. Plans were first controlled using portal dosimetry and a homogeneous static cylindrical phantom. End-to-end tests were then performed in a dynamic respiratory thorax phantom. Measurements were conducted with ionization chamber and films. Calculations were performed with the AcurosXB and AAA algorithms in 6 FFF. ResultsPortal dosimetry gave excellent gamma pass rates (greater than 97.1 %) and dose deviations between measurement and calculations in a homogeneous static phantom were smaller than 2 %. The methodology followed for comparing calculated and measured doses in a moving target was validated in static fields (largest deviation smaller than 2 %). End-to-end tests showed mean deviations of 1.9 %, 3.3 % and 6.6 % for the 3, 2 and 1 cm diameter’s target respectively. Deviations increased for larger movements for the 1 cm lesion. ConclusionEnd-to-end tests revealed that stereotactic VMAT lung treatment plans for moving targets can be delivered within 5 % for 3 and 2 cm diameter targets and amplitudes up to 1.5 cm. The AcurosXB and AAA algorithms however tend to underestimate the dose to the target. Even with satisfactory patient quality controls like portal dosimetry, extra care should be taken for GTV lesions smaller than 2 cm.

Analysis of CyberKnife intracranial treatment plans using ICRU 91 dose reporting: A retrospective study.

ICRU 91, published in 2017, is an international standard for prescribing, recording, and reporting stereotactic treatments. Since its release, there has been limited research published on the implementation and impact of ICRU 91 on clinical practice. This work provides an assessment of the recommended ICRU 91 dose reporting metrics for their use in clinical treatment planning. A set of 180 intracranial stereotactic treatment plans for patients treated by the CyberKnife (CK) system were analyzed retrospectively using the ICRU 91 reporting metrics. The 180 plans comprised 60 trigeminal neuralgia (TGN), 60 meningioma (MEN), and 60 acoustic neuroma (AN) cases. The reporting metrics included the planning target volume (PTV) near-minimum dose ( ), near-maximum dose ( ), and median dose ( ), as well as the gradient index (GI) and conformity index (CI). The metrics were assessed for statistical correlation with several treatment plan parameters. In the TGN plan group, owing to the small targets, was greater than in 42 plans, whereas both metrics were not applicable in 17 plans. The metric was predominantly influenced by the prescription isodose line (PIDL). The GI was significantly dependent on target volume in all analyses performed, where the variables were inversely related. The CI was only dependent on target volume in treatment plans for small targets. The ICRU 91 and metrics breakdown in plans for small target volumes below 1 cm3 ; the Min and Max pixel should be reported in such cases. The metric is of limited use for treatment planning. Given their volume dependence, the GI and CI metrics could potentially serve as plan evaluation tools in the planning of the sites analyzed in this study, which would ultimately improve treatment plan quality.

Evaluation of flattening-filter-free and flattening filter dosimetric and radiobiological criteria for lung SBRT: A volume-based analysis.

The use of volumetric modulated arc therapy (VMAT) with flattening-filter-free (FFF) beams is becoming more prevalent in lung cancer stereotactic body radiotherapy (SBRT). The aim in this study was to assess the impact of dosimetric and radiobiological differences between FFF and flattening filter (FF) beams for lung SBRT based on the target volume. A total of 198 lung stereotactic body radiation therapy treatment plans with FFF beams and FF beams were retrospectively selected for this study. For all plans, the prescribed dose was 50 Gy/5 fractions, and the dose volume histogram (DVH) for the target and organs at risk (OAR) and the normal tissue complication probability (NTCP) of the lung were recorded and compared. Moreover, monitor units (MUs), the beam on-time and the treatment time were evaluated. The study was performed following the Radiation Therapy Oncology Group (RTOG) 0813 and 0915 protocols. No significant differences in D90, coverage rate (CR) or conformity index (CI) of the target were observed between FFF beams and FF beams (p>0.05). The D2, R50% and gradient index (GI) for the target improved with FFF beams compared with FF beams (p<0.05). FFF beams also significantly reduced the dose for the lung, heart, spinal cord, esophagus and NTCP of the lung (p<0.05), compared with FF beams. However, there was no significant difference in sparing of the trachea (p>0.05). The mean MUs, beam on-time and treatment time were 1871 ± 278 MUs, 3.2 ± 0.2min and 3.9 ± 0.3min for FFF beams, and 1890 ± 260 MUs, 4.2 ± 0.3min and 4.8 ± 0.4min for FF beams, respectively. The FFF beam technique for lung SBRT with VMAT results in a better dose fall-off, better dose-sparing of OAR, lower NTCP of the lung and a shorter beam on-time compared with the FF beam technique. Additionally, the improvement in target and OAR-sparing for FFF beams was increased with increasing target volume.

Potential advantages of gEUD optimisation as compared with conventional physical optimisation for stereotactic treatment planning

Abstract Introduction: A small number of studies have confirmed the advantage of generalised equivalent uniform dose (gEUD) optimisation for some standard clinical scenarios; however, its performance with complicated stereotactic treatments is yet to be explored. Therefore, this study compared two planning optimisation methods, gEUD and Physical dose, in stereotactic treatments for several complex anatomical locations. Methods: Thirty patients were selected, ten each for sites of brain, lung and spine. Two stereotactic plans were generated for each case using the gEUD objective and Physical objective cost functions. Within each of the three sites, dosimetric indices for conformity, gradient and homogeneity, along with parameters of monitor units and dose–volume histograms (DVHs), were compared for statistical significance. Additionally, patient-specific quality assurance was conducted using portal dosimetry, and the gamma passing rate between the two plans was evaluated. Results: Optimisation was better with a gEUD objective as compared with Physical objective, notably sparing critical organs. Overall, the differences in mean values for six critical organs at risk favoured gEUD-based over Physical-based plans (all six 2-tailed p-values were &lt; 0·0002). Furthermore, all differences in mean values for DVH parameters favoured gEUD-based plans: GTVmean, GTVmax, PTVD100V, homogeneity index, gradient index and monitor unit (treatment time) (each 2-tailed p &lt; 0·05). Conclusions: gEUD optimisation in stereotactic treatment plans has a clear and general statistical advantage over Physical dose optimisation.

Can the use of knowledge-based planning systems improve stereotactic radiotherapy planning? A systematic review

Abstract Introduction: This study aimed to systematically review the literature to synthesise and summarise whether using knowledge-based planning (KBP) can improve the planning of stereotactic radiotherapy treatments. Methods: A systematic literature search was carried out using Medline, Scopus and Cochrane databases to evaluate the use of KBP planning in stereotactic radiotherapy. Three hundred twenty-five potential studies were identified and screened to find 25 relevant studies. Results: Twenty-five studies met the inclusion criteria. Where a commercial KBP was used, 72.7% of studies reported a quality improvement, and 45.5% reported a reduction in planning time. There is evidence that when used as a quality control tool, KBP can highlight stereotactic plans that need revision. In studies that use KBP as the starting point for radiotherapy planning optimisation, the radiotherapy plans generated are typically equal to or superior to those planned manually. Conclusions: There is evidence that KBP has the potential to improve the quality and speed of stereotactic radiotherapy planning. Further research is required to accurately quantify such systems’ quality improvements and time savings. Notably, there has been little research into their use for prostate, spinal or liver stereotactic radiotherapy, and research in these areas would be desirable. It is recommended that future studies use the ICRU 91 level 2 reporting format and that blinded physician review could add a qualitative assessment of KBP system performance.

Evaluation of a complementary metal oxide semiconductor detector as a tool for stereotactic body radiotherapy plan quality assurance.

A sub-mm resolution Complementary Metal Oxide Semiconductor sensor has been developed for stereotactic radiotherapy quality assurance. Herein we evaluate its basic dosimetric performance and its application for linac C-arm stereotactic body radiotherapy (SBRT) plan quality assurance. The detector was integrated into its accompanying phantom or in Water Equivalent Plastic (WEP). The measurement reproducibility, stability, dose linearity and dependence on angularity, dose rate and field size were investigated. Clinical plan measurements were compared to our radiotherapy treatment planning system and radiochromic film. Sensitivity to introduced Multi Leaf Collimator (MLC) offsets was evaluated by simulating single MLC offsets in SBRT plans and comparing measurements to expected doses. Signal reproducibility was within±0.1% and output calibration was stable over a 6month period. Detector showed good linearity with dose (r2=1). Signal decreased by 5% when dose rate was decreased from 1300 MU/min to 300 MU/min. Output factors agreed within 0.5% of chamber measurements for 1x1 cm field sizes or greater. Angularity measurements showed good agreement with reference. For measurement of planned clinical doses, gamma pass-rates were 98.5%±2.3% (treatment planning system reference, 2%/2mm) and 99.2%±1.0% (film reference, 2%,2mm). The detector also showed sensitivity to errors of 1mm offsets in MLC positioning. The detector performed well when used for pre-treatment SBRT plan quality assurance, offering a good alternative to radiochromic film.

Dose Prescription to Isodose Lines in Static Multi-Beam Stereotactic Body Radiotherapy for Lung Tumors: Which Line Is Optimal?

This study investigated the appropriate dose prescription method in static multi-beam stereotactic body radiotherapy for lung tumors. Static multi-beam stereotactic body radiotherapy is a mainstream treatment in Japan. Based on the hypothesis that dose prescription to lower isodose lines may improve planning target volume dose coverage and decrease doses to organs at risk, we investigated changes in dose-volume histograms with prescription to various isodose lines for planning target volume in static multi-beam stereotactic body radiotherapy. In all treatment plans, 45 Gy in 4 fractions were prescribed to 95% of the planning target volume. By adjusting the leaf margins of each beam, various prescription isodose lines encompassing 95% volume of the planning target volume were generated. The prescription isodose lines investigated were 40, 50, 60, 70, 80 and 90% lines relative to the maximum dose of each planning target volume. The conformity index, homogeneity index, mean lung dose, and V5-V40 of the lung were evaluated. The dose was calculated by the adaptive convolve algorithm. The conformity index was lowest in the 70% or 80% isodose plan. The mean lung doses and V10-V40 of the lung decreased steeply from the 90% to the 70% isodose plan, and was lowest in the 60% and 70% isodose plans. These indices increased in the 40% and 50% isodose plans. The optimal stereotactic body radiotherapy plans appeared to be dose prescription to the 60% or 70% isodose line. Further investigation is warranted to clarify the advantage of using this method clinically.

Effects of reconstruction methods on dose distribution for lung stereotactic body radiotherapy treatment plans.

The aim of the present study was to investigate the effect of tumour motion on various imaging strategies as well as on treatment plan accuracy for lung stereotactic body radiotherapy treatment (SBRT) cases. The ExacTrac gating phantom and paraffin were used to investigate respiratory motion and represent a lung tumour, respectively. Four-dimensional computed tomography (4DCT) imaging was performed, while the phantom was moving sinusoidally with 4s cycling time with three different amplitudes of 8, 16, and 24mm. Reconstructions were done with maximum (MIP) and average intensity projection (AIP) methods. Comparisons of target density and volume were performed using two reconstruction techniques and references values. Volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) were planned based on reconstructed computed tomography (CT) sets, and it was examined how density variations affect the dose-volume histogram (DVH) parameters. 4D cone beam computed tomography (CBCT) was performed with the Elekta Versa HD linac imaging system before irradiation and compared with 3D CBCT. Thus, various combinations of 4DCT reconstruction methods and treatment alignment methods have been investigated. Point measurements as well as 2 and 3D dose measurements were done by optically stimulated luminescence (OSL), gafchromic films, and electronic portal imaging devices (EPIDs), respectively. The mean volume reduction was 7.8% for the AIP and 2.6% for the MIP method. The obtained Hounsfield Unit (HU) values were lower for AIP and higher for MIP when compared with the reference volume density. In DVH analysis, there were no statistical differences for D95%, D98%, and Dmean (p > 0.05). However, D2% was significantly affected by HU changes (p < 0.01). A positional variation was obtained up to 2mm in moving direction when 4D CBCT was applied after 3D CBCT. Dosimetric measurements showed that the main part of the observed dose deviation was due to movement. In lung SBRT treatment plans, D2% doses differ significantly according to the reconstruction method. Additionally, it has been observed that setups based on 3D imaging can cause a positional error of up to 2mm compared to setups based on 4D imaging. It is concluded that MIP has advantages over AIP in defining internal target volume (ITV) in lung SBRT applications. In addition, 4D CBCT and 3D EPID dosimetry are recommended for lung SBRT treatments.

Step-by-Step Stereotactic Radiotherapy Planning of Glomus Jugulare: A Guide to Radiation Oncologists—Dr Kanhu’s ROSE (Radiation Oncology from Simulation to Execution)

Background Glomus jugulare is a rare, slow-growing tumor that arise within the jugular foramen of the temporal bone. In the past, surgery was the primary modality of treatment for glomus Jugulare, but it leads to many complications and increased mortality. Radiotherapy was indicated in adjuvant setting in post-operative residual disease. But, with the advent of highly conformal radiation planning, stereotactic radiosurgery (SRS), is now one of the main modalities of radiation treatment in glomus jugulare. Objective To describe the procedural steps for radiation planning of SRS of glomus jugulare. Methods The step-by-step procedure for stereotactic planning of glomus jugulare has been described using a clinical scenario of glomus jugulare. Results The stereotactic radiation planning of glomus jugulare starts with the basic history and relevant clinical evaluation, that is, visual testing. Computed tomography (CT) scan and magnetic resonance imaging (MRI) of the brain is the imaging modality of choice. The radiation planning of glomus jugulare starts with CT simulation. MRI of brain should be done in the prescribed format to achieve uniformity in radiation planning. After CT and MRI image fusion, contouring of target, organs at risk (OAR) and radiation planning should be done. The plan evaluation includes target and OAR coverage index, conformity, homogeneity and gradient index, and beam arrangement. After radiation plan evaluation, treatment is delivered after quality assurance and dry run. Conclusion The paper highlights the sequential process of radiation planning for SRS in glomus jugulare—starting from simulation, planning, evaluation of plan, and treatment.

Step-by-Step Stereotactic Radiotherapy Planning of Brain Metastasis: A Guide to Radiation Oncologists—the ROSE Case (Radiation Oncology from Simulation to Execution)

Brain metastasis is seen in 10% to 20% of all adult cancer patients. One of the main modalities of treatment is stereotactic radiosurgery (SRS). Here, we describe the step by step procedure for stereotactic planning of brain metastasis by using a clinical scenario. The management of brain metastasis starts with the clinical evaluation of the patient followed by imaging and SRS treatment in the present case. The paper highlights the sequential process of radiation planning for SRS—starting from simulation, planning, evaluation of plan, and treatment.

RADT-06. SPARING OF MOTOR STRUCTURES IN ADJUVANT RADIATION THERAPY AFTER RESECTION OF BRAIN METASTASES: APPLICATION OF NTMS-DERIVED DTI-BASED MOTOR FIBER TRACKING IN ADJUVANT STEREOTACTIC RT TREATMENT PLANNING

Abstract BACKGROUND Resection of brain metastases (BM) close to the motor cortex and the corticospinal tract (CST) bears a significant risk for treatment-related morbidity. Navigated transcranial mapping (nTMS) combined with diffusion-tensor-imaging (DTI) based fiber tracking (DTI-FTTMS) is a valuable tool to guide the neurosurgeon along the CST to preserve motor function. This study aims to proof the practicability of DTI-FTTMS in local adjuvant stereotactic RT planning in the management of BM. Method: Pre-surgical generated DTI-FTTMS-based CST reconstructions of 24 patients with 25 resected BM were incorporated into the RT planning system and elastic fused with planning imaging. The CST was delineated as the planning risk volume (PRV-FTTMS). Fractionated stereotactic intensity-modulated RT (IMRT) plans (7 x 5 Gy) were retrospectively calculated and then optimized to preserve PRV-FTTMS. Areas covered by the planning target volume (PTV) were not spared (overlap). RESULTS In regular plans mean dose (Dmean) of complete PRV-FTTMS was 5.4 ± 2.5 Gy. Regarding PRV-FTTMS portions within the 8.75 Gy (25% of prescription dose) isodose level Dmean was 18.2 ± 4.3 Gy and after plan optimization 13.1 ± 3.8Gy (-28.0%, p &amp;lt; 0.001). Within the 17.5 Gy (50%) isodose line PRV-FTTMS Dmean was reduced by 31.7% from 24.3 ± 3 Gy to 16.6 ± 4.8 Gy (p&amp;lt; 0.001). There was no decline of the effective treatment dose, PTV Dmean in regular plans was 36.9 ± 0.7 Gy vs. 37.7 ± 1.4Gy (p=0.013) after adaption. PTV coverage (V35Gy(%)*100) did not change with plan optimization: 0.99 vs. 0.99 (p=0.43). Dose constraints of organs at risk were all met both in regular and optimized plans. CONCLUSION DTI-FTTMS based motor tracts could be implemented in the adjuvant stereotactic RT planning of cavities after resection of BM. A significant dose reduction of motor structures within critical dose levels seems possible without reducing PTV treatment dose. However, the functional benefit needs to be investigated prospectively within clinical trials.

Dosimetric potential of knowledge-based planning model trained with HyperArc plans for brain metastases.

Dosimetric potential of knowledge-based RapidPlan planning model trained with HyperArc plans (Model-HA) for brain metastases has not been reported. We developed a Model-HA and compared its performance with that of clinical volumetric modulated arc therapy (VMAT) plans. From 67 clinical stereotactic radiosurgery (SRS) HyperArc plans for brain metastases, 47 plans were used to build and train a Model-HA. The other 20 clinical HyperArc plans were recalculated in RapidPlan system with Model-HA. The model performance was validated with the 20 plans by comparing dosimetric parameters for normal brain tissue between clinical plans and model-generated plans. The 20 clinical conventional VMAT-based SRS or stereotactic radiotherapy plans (CL-VMAT) were reoptimized with Model-HA (RP) and HyperArc system (HA), respectively. The dosimetric parameters were compared among three plans (CL-VMAT vs. RP vs. HA) in terms of planning target volume (PTV), normal brain excluding PTVs (Brain-PTV), brainstem, chiasm, and both optic nerves. In model validation, the optimization performance of Model-HA was comparable to that of HyperArc system. In comparison to CL-VMAT, there were no significant differences among three plans with respect to PTV coverage (p>0.17) and maximum dose for brainstem, chiasm, and optic nerves (p>0.40). RP provided significantly lower V20Gy , V12Gy , and V4Gy for Brain-PTV than CL-VMAT (p<0.01). The Model-HA has the potential to significantly reduce the normal brain dose of the original VMAT plans for brain metastases.

Dosimetric impact of mechanical movements of the Linac gantry during treatments with small fields.

Current accepted linac Quality Assurance (QA) guidelines used for Volumetric Modulated Arc Therapy (VMAT) suggest a mechanical isocentre tolerance level of 1mm. However, this tolerance level has not been well-established for the specific case of small field stereotactic VMAT. This study aims to evaluate the clinical impact of mechanical uncertainty on this treatment modality by modelling systematic gantry sag derived isocentre variance in the Treatment Planning System (TPS). A previously reported dataset of gantry sag values in the literature served as a starting point for this study. Using an in-house developed VMAT arc splitting algorithm, isocentre shifts were applied at a Control Point (CP) level to DICOM-RT treatment plans. Dose distributions for varying isocentre shift magnitudes were calculated for a set of 29 stereotactic VMAT plans using the Eclipse Acuros XB dose algorithm. These plans had a range of Planning Target Volume (PTV) sizes. A quantitative comparison of each plan was conducted by evaluating five Dose Volume Histogram (DVH)-derived plan quality metrics. All metrics exhibited a deterioration in plan quality with increasing magnitudes of isocentre shift. At small PTV sizes, these effects were amplified, exhibiting significant changes at 1mm of average shift when typical targets and tolerances were considered. For plans with PTVs between 0 and 5 cm3, a 1mm shift reduced PTV coverage by 6.6 ± 2.2% and caused a 12.1 ± 3.8% deterioration in the conformity index. Based on the results of this study, the prevalent tolerance of 1mm may not be suitable for treatments of small PTVs with small fields. In contrast to commonly accepted values, an absolute mechanical isocentre of 0.5mm with action level at 0.75mm is recommended for stereotactic VMAT of PTV sizes below 10 cm3.

Virtual non-contrast reconstructions improve differentiation between vascular enhancement and calcifications in stereotactic planning CT scans of cystic intracranial tumors

PurposeTo assess the diagnostic value of spectral-detector CT (SDCT) derived virtual non-contrast images (VNC) for differentiation between vascular enhancement and wall calcifications of cystic intracranial tumors in contrast-enhanced stereotactic planning examinations. Method48 patients with cystic intracranial tumors who underwent stereotactic SDCT examinations between 02/2017 and 02/2020 were retrospectively included. In each patient, two separate hyperattenuating structures along the cyst wall were defined as either enhancement or calcification, respectively, using reference MRI examinations. Quantitative analysis was performed ROI-based in conventional images (CI) and VNC. In the subjective analysis, two radiologists diagnosed the predefined peri-cystic structures in binary decisions as either enhancement or calcification using CI and the combination of CI and VNC, and rated diagnostic confidence, image noise and removal of iodine in VNC. Moreover, a potential diagnostic benefit of VNC was indicated. ResultsAttenuation in CI was higher as compared to VNC across all assessed ROI (all p < 0.01). In VNC, CNR between calcification and white matter was significantly higher as compared to CNR between vascular enhancement and white matter (2.6 vs 1.3, p < 0.01), while there was no significant difference in CI. In the qualitative assessment, diagnostic accuracy was significantly higher using both VNC and CI compared to using CI alone. Raters reported less image noise in VNC as compared to CI. An additional diagnostic benefit of VNC was indicated in 84.4 % of all cases. ConclusionsSDCT-derived VNC images facilitate differentiation between peri-cystic contrast enhancement in blood vessels and calcifications in stereotactic planning scans of cystic intracranial tumors.

Feasibility of Treating Brain Lesions with a Horizontal Beam Line and a Couch with Adjustable Backrest Angle

<h3>Purpose/Objective(s)</h3> A gantry enabling beam delivery from any angle is technically complex and expensive. This is especially true for particle beams. An alternative is a horizontal beam line and upright positioning of the patient. This requires a vertical CT scanner and dedicated positioning tools for accurate delivery. We propose a couch with an adjustable backrest in order to mimic a large range of gantry and table rotation combinations. In this work, we investigated which dose distributions are achievable with such a unit and compared it with those achieved with a gantry and a conventional couch. <h3>Materials/Methods</h3> Ten stereotactic treatment plans of cerebral metastases (1.8 cm³ to 26.5 cm³) comprising 8-10 beams were investigated. The treatments were re-planned by mimicking backrest angles of 0°, 30° and 60° by gantry angles of 90°, 120° and 150° respectively. Ten beams with meaningful isocentric table angles were set. The beam shapes and weights were optimized. The calculations were performed with a treatment planning system, simulating a 6MV FFF photon beam. Dose-volume parameters of the clinically used plan were compared to the one mimicking the horizontal beam line. <h3>Results</h3> For the target and a 2 mm shell around the target, approximately identical dose distributions were achieved. The mean dose to the shell decreased by 0.5% on average. The coverage index enhanced from a value of 1.41 ± 0.2 to 1.38 ± 0.2 on average. Only the gradient index got worse with an average of 3.8 ± 0.7 compared to 3.0 ± 0.3. <h3>Conclusion</h3> Despite the restriction to only 3 backrest angles, for photon beams, comparable dose distributions were achieved with the proposed couch unit. For protons we expect at least similar results, as the range of possible angles of incidence is even larger. For the head, displacement of anatomical structures is not expected. Therefore, saving a gantry and a huge amount of radiation shielding, the proposed device is a cost-effective treatment option for brain lesions. Conventional CT and supportive image data could be used.

Monte Carlo-based independent dose verification of radiosurgery HyperArc plans

PurposeTo investigate the feasibility of using the free PRIMO Monte Carlo software for independent dose check of cranial SRS plans designed with the Varian HyperArc (HA) technique. Materials and methodsIn this study, the PRIMO Monte Carlo software v. 0.3.64.1800 was used with the phase-space files (v. 2, Feb. 27, 2013) provided by Varian for 6 MV flattening-filter-free (FFF) photon beams from a Varian TrueBeam linear accelerator (linac), equipped with a Millennium 120 multileaf collimator (MLC). This configuration was validated by comparing the percentage depth doses (PDDs), lateral profiles and relative output factors (OFs) simulated in a water phantom against measurements for field sizes from 1 × 1 to 40 × 40 cm2. The agreement between simulated and experimental relative dose curves was evaluated using a global (G) gamma index analysis. In addition, the accuracy of PRIMO to model the MLC was investigated (dosimetric leaf gap, tongue and groove, leaf transmission and interleaf leakage).Thirty-five HA SRS plans computed in the Eclipse treatment planning system (TPS) were simulated in PRIMO. The Acuros XB algorithm v. 16.10 (dose to medium) was used in Eclipse. Sixty targets with diameters ranging from 6 to 33 mm were included. Agreement between the dose distributions given by Eclipse and PRIMO was evaluated in terms of 3D global gamma passing rates (GPRs) for the 2 %/2 mm criteria. ResultsAverage GPR greater than 95 % with the 2 %(G)/1 mm criteria were obtained over the PDD and profiles of each field size. Differences between PRIMO calculated and measured OFs were within 0.5 % in all fields, except for the 1 × 1 cm2 with a discrepancy of 1.5 %. Regarding the MLC modeling in PRIMO, an agreement within 3 % was achieved between calculated and experimental doses. Excellent agreement between PRIMO and Eclipse was found for the 35 HA plans. The 3D global GPRs (2 %/2 mm) for the targets and external patient contour were 99.6 % ± 1.1 % and 99.8 % ± 0.5 %, respectively. ConclusionsAccording to the results described in this study, the PRIMO Monte Carlo software, in conjunction with the 6X FFF Varian phase-space files, can be used as secondary dose calculation software to check stereotactic radiosurgery plans from Eclipse using the HyperArc technique.

Dosimetric evaluation of respiratory gating on a 0.35-T magnetic resonance-guided radiotherapy linac.

The commercial 0.35-T magnetic resonance imaging (MRI)-guided radiotherapy vendor ViewRay recently introduced upgraded real-time imaging frame rates based on compressed sensing techniques. Furthermore, additional motion tracking algorithms were made available. Compressed sensing allows for increased image frame rates but may compromise image quality. To assess the impact of this upgrade on respiratory gating accuracy, we evaluated gated dose distributions pre- and post-upgrade using a motion phantom and radiochromic film. Seven motion waveforms (four artificial, two patient-derived free-breathing, and one breath-holding) were used to drive an MRI-compatible motion phantom. A treatment plan was developed to deliver a 3-cm diameter spherical dose distribution typical of a stereotactic body radiotherapy plan. Gating was performed using 4-frames per second (fps) imaging pre-upgrade on the "default" tracking algorithm and 8-fps post-upgrade using the "small mobile targets" (SMT) and "large deforming targets" (LDT) tracking algorithms. Radiochromic film was placed in a moving insert within the phantom to measure dose. The planned and delivered dose distributions were compared using the gamma index with 3%/3-mm criteria. Dose-area histograms were produced to calculate the dose to 95% (D95) of the sphere planning target volume (PTV) and two simulated gross tumor volumes formed by contracting the PTV by 3 and 5mm, respectively. Gamma pass rates ranged from 18% to 93% over the 21 combinations of breathing trace and gating conditions examined. D95 ranged from 206 to 514cGy. On average, the LDT algorithm yielded lower gamma and D95 values than the default and SMT algorithms. Respiratory gating at 8fps with the new tracking algorithms provides similar gating performance to the original algorithm with 4fps, although the LDT algorithm had lower accuracy for our non-deformable target. This indicates that the choice of deformable image registration algorithm should be chosen deliberately based on whether the target is rigid or deforming.