Dosimetric accuracy of the Convolution algorithm for Leksell Gamma Plan radiosurgery treatment planning: Evaluation in the presence of clinically relevant inhomogeneities.

PURPOSE: The Leksell Gamma Plan Convolution algorithm (LGP-Convolution) has not been widely adopted. This mainly stems from the higher calculated beam-on times relative to the standard ray tracing-based LGP-TMR10 dose calculation algorithm. This study aims to evaluate the accuracy of the LGP-Convolution in scenarios where the treated lesions are in the vicinity of or encompassed by bone and/or air inhomogeneities. METHODS: The solid water dosimetry phantom provided by the vendor was modified to include bone and air inhomogeneities. Two treatment planning scenarios were investigated involving a single shot and multiple shots, respectively. Treatment planning and dose prescription were performed using the LGP-Convolution algorithm. Triple channel film dosimetry was performed using GafChromic EBT3 films calibrated in terms of absorbed dose to water in a 60 Co beam. Monte Carlo (MC) simulation dosimetry was also performed in the inhomogeneous experimental geometry using the EGSnrc MC platform and a previously validated sector-based phase-space source model. MC simulations were also employed to determine correction factors required for converting EBT3 measurements at points within the bone and air inhomogeneities from dose-to-water values to the corresponding dose to medium values. RESULTS AND CONCLUSIONS: EBT3 dose to medium correction factors ranged with field size (4, 8, or 16 mm) within 0.941-0.946 for bone and 0.745-0.749 for air inhomogeneities. An excellent agreement was found between the LGP-Convolution calculations with corresponding EBT3 and MC dose to medium results at all measurement points, except those located inside the air inhomogeneity. The latter is of no clinical importance and excluding them yielded gamma index passing rates of nearly 100% for 3% local dose difference and 1 mm distance-to-agreement criteria. The excellent agreement observed between LGP-Convolution calculations and film as well as MC results of dose to medium indicates that the latter is the quantity reported by the LGP-Convolution.

Target localization accuracy in frame-based stereotactic radiosurgery: Comparison between MR-only and MR/CT co-registration approaches.

PURPOSE: In frame-based Gamma Knife (GK) stereotactic radiosurgery two treatment planning workflows are commonly employed; one based solely on magnetic resonance (MR) images and the other based on magnetic resonance/computed tomography (MR/CT) co-registered images. In both workflows, target localization accuracy (TLA) can be deteriorated due to MR-related geometric distortions and/or MR/CT co-registration uncertainties. In this study, the overall TLA following both clinical workflows is evaluated for cases of multiple brain metastases. METHODS: A polymer gel-filled head phantom, having the Leksell stereotactic headframe attached, was CT-imaged and irradiated by a GK Perfexion unit. A total of 26 4-mm shots were delivered at 26 locations directly defined in the Leksell stereotactic space (LSS), inducing adequate contrast in corresponding T2-weighted (T2w) MR images. Prescribed shot coordinates served as reference locations. An additional MR scan was acquired to implement the "mean image" distortion correction technique. The TLA for each workflow was assessed by comparing the radiation-induced target locations, identified in MR images, with corresponding reference locations. Using T1w MR and CT images of 15 patients (totaling 81 lesions), TLA in clinical cases was similarly assessed, considering MR-corrected data as reference. For the MR/CT workflow, both global and region of interest (ROI)-based MR/CT registration approaches were studied. RESULTS: In phantom measurements, the MR-corrected workflow demonstrated unsurpassed TLA (median offset of 0.2 mm) which deteriorated for MR-only and MR/CT workflows (median offsets of 0.8 and 0.6 mm, respectively). In real-patient cases, the MR-only workflow resulted in offsets that exhibit a significant positive correlation with the distance from the MR isocenter, reaching 1.1 mm (median 0.6 mm). Comparable results were obtained for the MR/CT-global workflow, although a maximum offset of 1.4 mm was detected. TLA was improved with the MR/CT-ROI workflow resulting in median/maximum offsets of 0.4 mm/1.1 mm. CONCLUSIONS: Subpixel TLA is achievable in all workflows. For the MR/CT workflow, a ROI-based MR/CT co-registration approach could considerably increase TLA and should be preferred instead of a global registration.

Impact of systematic MLC positional uncertainties on the quality of single-isocenter multi-target VMAT-SRS treatment plans.

PURPOSE: To study the impact of systematic MLC leaf positional uncertainties (stemming from mechanical inaccuracies or sub-optimal MLC modeling) on the quality of intracranial single-isocenter multi-target VMAT-SRS treatment plans. An estimation of appropriate tolerance levels is attempted. METHODS: Five patients, with three to four metastases and at least one target lying in close proximity to organs-at-risk (OARs) were included in this study. A single-isocenter multi-arc VMAT plan per patient was prepared, which served as the reference for dosimetric impact evaluation. A range of leaf offsets was introduced (±0.03 mm up to ±0.30 mm defined at the MLC plane) to both leaf banks, by varying the leaf offset MLC modeling parameter in Monaco for all the prepared plans, in order to simulate projected leaf offsets of ±0.09 mm up to ±0.94 mm at the isocenter plane, respectively. For all offsets simulated and cases studied, dose distributions were re-calculated and compared with the corresponding reference ones. An experimental dosimetric procedure using the SRS mapCHECK diode array was also performed to support the simulation study results and investigate its suitability to detect small systematic leaf positional errors. RESULTS: Projected leaf offsets of ±0.09 mm were well-tolerated with respect to both target dosimetry and OAR-sparing. A linear relationship was found between D95% percentage change and projected leaf offset (slope: 12%/mm). Impact of projected offset on target dosimetry was strongly associated with target volume. In two cases, plans that could be considered potentially clinically unacceptable (i.e., clinical dose constraint violation) were obtained even for projected offsets as small as 0.19 mm. The performed experimental dosimetry check can detect potential small systematic leaf errors. CONCLUSIONS: Plan quality indices and dose-volume metrics are very sensitive to systematic sub-millimeter leaf positional inaccuracies, projected at the isocenter plane. Acceptable and tolerance levels in systematic MLC uncertainties need to be tailored to VMAT-SRS spatial and dosimetric accuracy requirements.

A Holographic Augmented Reality Interface for Visualizing of MRI Data and Planning of Neurosurgical Procedures.

The recent introduction of wireless head-mounted displays (HMD) promises to enhance 3D image visualization by immersing the user into 3D morphology. This work introduces a prototype holographic augmented reality (HAR) interface for the 3D visualization of magnetic resonance imaging (MRI) data for the purpose of planning neurosurgical procedures. The computational platform generates a HAR scene that fuses pre-operative MRI sets, segmented anatomical structures, and a tubular tool for planning an access path to the targeted pathology. The operator can manipulate the presented images and segmented structures and perform path-planning using voice and gestures. On-the-fly, the software uses defined forbidden-regions to prevent the operator from harming vital structures. In silico studies using the platform with a HoloLens HMD assessed its functionality and the computational load and memory for different tasks. A preliminary qualitative evaluation revealed that holographic visualization of high-resolution 3D MRI data offers an intuitive and interactive perspective of the complex brain vasculature and anatomical structures. This initial work suggests that immersive experiences may be an unparalleled tool for planning neurosurgical procedures.

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° rotations, plan quality indices, such as coverage, conformity index and D95% , 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° errors were well-tolerated. In the most extreme case considered (2° 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 Dmax and D0.02cc 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.

Relative profile measurements in 1.5T MR-linacs: investigation of central axis deviations.

Objective. In 1.5 T MR-linacs, the absorbed dose central axis (CAX) deviates from the beam's CAX due to inherent profile asymmetry. In addition, a measured CAX deviation may be biased due to potential lateral (to the beam) effective point of measurement (EPOML) shifts of the detector employed. By investigating CAX deviations, the scope of this study is to determine a set ofEPOMLshifts for profile measurements in 1.5 T MR-linacs.Approach. The Semiflex 3D ion chamber and microDiamond detector (PTW, Germany) were considered in the experimental study while three more detectors were included in the Monte Carlo (MC) study. CAX deviations in the crossline and inline profiles were calculated based on inflection points of the 10×10 cm2field, at five centers. In MC simulations, the experimental setup was reproduced. A small water voxel was simulated to calculate CAX deviation without the impact of the detector-specificEPOMLshift.Main results. All measurements were consistent among the five centers. MC-based and experimental measurements were in agreement within uncertainties. Placing the microDiamond in the vertical orientation does not appear to affect the detector'sEPOML, which is on its central longitudinal axis. For the Semiflex 3D in the crossline direction, the CAX deviation was 2.3 mm, i.e. 1 mm larger than the ones measured using the microDiamond and simulated considering the ideal water detector. Thus, anEPOMLshift of 1 mm is recommended for crossline profile measurements under both Semiflex 3D orientations. For the inline profile, anEPOMLshift of -0.5 mm was determined only for the parallel configuration. In the MC study, CAX deviations were found detector- and orientation-dependent. The dead volume is responsible for theEPOMLshift only in the inline profile and under the parallel orientation.Significance. This work contributes to data availability on the correction or mitigation of the magnetic field-induced changes in the detectors' response.

Dose-response dependencies of OSL dosimeters in conventional linacs and 1.5T MR-linacs: an experimental and Monte Carlo study.

Objective. This work focuses on the optically stimulated luminescence dosimetry (OSLD) dose-response characterization, with emphasis on 1.5T MR-Linacs.Approach. Throughout this study, the nanoDots OSLDs (Landauer, USA) were considered. In groups of three, the mean OSLD response was measured in a conventional linac and an MR-Linac under various irradiation conditions to investigate (i) dose-response linearity with and without the 1.5T magnetic field, (ii) signal fading rate and its dependencies, (iii) beam quality, detector orientation and dose rate dependencies in a conventional linac, (iii) potential MR imaging related effects on OSLD response and (iv) detector orientation dependence in an MR-Linac. Monte Carlo calculations were performed to further quantify angular dependence after rotating the detector around its central axis parallel to the magnetic field, and determine the magnetic field correction factors,kB,Q,for all cardinal detector orientations.Main results. OSLD dose-response supralinearity in an MR-Linac setting was found to agree within uncertainties with the corresponding one in a conventional linac, for the axial detector orientation investigated. Signal fading rate does not depend on irradiation conditions for the range of 3-30 d considered. OSLD angular (orientation) dependence is more pronounced under the presence of a magnetic field. OSLDs irradiated with and without real-time T2w MR imaging enabled during irradiation yielded the same response within uncertainties.kB,Qvalues were determined for all three cardinal orientations. Corrections needed reached up to 6.4%. However, if OSLDs are calibrated in the axial orientation and then irradiated in an MR-Linac placed again in the axial orientation (perpendicular to the magnetic field), then simulations suggest thatkB,Qcan be considered unity within uncertainties, irrespective of the incident beam angle.Significance. This work contributes towards OSLD dose-response characterization and relevant correction factors availability. OSLDs are suitable for QA checks in MR-based beam gating applications andin vivodosimetry in MR-Linacs.

Dosimetry in 1.5 T MR-Linacs: Monte Carlo determination of magnetic field correction factors and investigation of the air gap effect.

BACKGROUND: In Magnetic Resonance-Linac (MR-Linac) dosimetry formalisms, a new correction factor, kB,Q , has been introduced to account for corresponding changes to detector readings under the beam quality, Q, and the presence of magnetic field, B. PURPOSE: This study aims to develop and implement a Monte Carlo (MC)-based framework for the determination of kB,Q correction factors for a series of ionization chambers utilized for dosimetry protocols and dosimetric quality assurance checks in clinical 1.5 T MR-Linacs. Their dependencies on irradiation setup conditions are also investigated. Moreover, to evaluate the suitability of solid phantoms for dosimetry checks and end-to-end tests, changes to the detector readings due to the presence of small asymmetrical air gaps around the detector's tip are quantified. METHODS: Phase space files for three irradiation fields of the ELEKTA Unity 1.5 T/7 MV flattening-filter-free MR-Linac were provided by the manufacturer and used as source models throughout this study. Twelve ionization chambers (three farmer-type and nine small-cavity detectors, from three manufacturers) were modeled (including their dead volume) using the EGSnrc MC code package. kB,Q values were calculated for the 10 × 10 cm2 irradiation field and for four cardinal orientations of the detectors' axes with respect to the 1.5 T magnetic field. Potential dependencies of kB,Q values with respect to field size, depth, and phantom material were investigated by performing additional simulations. Changes to the detectors' readings due to the presence of small asymmetrical air gaps (0.1 up to 1 mm) around the chambers' sensitive volume in an RW3 solid phantom were quantified for three small-cavity chambers and two orientations. RESULTS: For both parallel (to the magnetic field) orientations, kB,Q values were found close to unity. The maximum correction needed was 1.1%. For each detector studied, the kB,Q values calculated for the two parallel orientations agreed within uncertainties. Larger corrections (up to 5%) were calculated when the detectors were oriented perpendicularly to the magnetic field. Results were compared with corresponding ones found in the literature, wherever available. No considerable dependence of kB,Q with respect to field size (down to 3 × 3 cm2 ), depth, or phantom material was noticed, for the detectors investigated. As compared to the perpendicular one, in the parallel to the magnetic field orientation, the air gap effect is minimized but is still considerable even for the smallest air gap considered (0.1 mm). CONCLUSION: For the 10 × 10 cm2 field, magnetic field correction factors for 12 ionization chambers and four orientations were determined. For each detector, the kB,Q value may be also applied for dosimetry procedures under different irradiation parameters provided that the orientation is taken into account. Moreover, if solid phantoms are used, even the smallest asymmetrical air gap may still bias small-cavity chamber response. This work substantially expands the availability and applicability of kB,Q correction factors that are detector- and orientation-specific, enabling more options in MR-Linac dosimetry checks, end-to-end tests, and quality assurance protocols.

Monte Carlo-Based Radiobiological Investigation of the Most Optimal Ion Beam Forming SOBP for Particle Therapy.

Proton (p) and carbon (C) ion beams are in clinical use for cancer treatment, although other particles such as He, Be, and B ions have more recently gained attention. Identification of the most optimal ion beam for radiotherapy is a challenging task involving, among others, radiobiological characterization of a beam, which is depth-, energy-, and cell type- dependent. This study uses the FLUKA and MCDS Monte Carlo codes in order to estimate the relative biological effectiveness (RBE) for several ions of potential clinical interest such as p, 4He, 7Li, 10Be, 10B, and 12C forming a spread-out Bragg peak (SOBP). More specifically, an energy spectrum of the projectiles corresponding to a 5-cm SOBP at a depth of 8 cm was used. All secondary particles produced by the projectiles were considered and RBE was determined based on radiation-induced Double Strand Breaks (DSBs), as calculated by MCDS. In an attempt to identify the most optimal ion beam, using the latter data, biological optimization was performed and the obtained depth-dose distributions were inter-compared. The results showed that 12C ions are more effective inside the SOBP region, which comes at the expense of higher dose values at the tail (i.e., after the SOBP). In contrast, p beams exhibit a higher DSOPB/DEntrance ratio, if physical doses are considered. By performing a biological optimization in order to obtain a homogeneous biological dose (i.e., dose × RBE) in the SOBP, the corresponding advantages of p and 12C ions are moderated. 7Li ions conveniently combine a considerably lower dose tail and a DSOPB/DEntrance ratio similar to 12C. This work contributes towards identification of the most optimal ion beam for cancer therapy. The overall results of this work suggest that 7Li ions are of potential interest, although more studies are needed to demonstrate the relevant advantages. Future work will focus on studying more complex beam configurations.

Geometric distortion assessment in 3T MR images used for treatment planning in cranial Stereotactic Radiosurgery and Radiotherapy.

Magnetic Resonance images (MRIs) are employed in brain Stereotactic Radiosurgery and Radiotherapy (SRS/SRT) for target and/or critical organ localization and delineation. However, MRIs are inherently distorted, which also impacts the accuracy of the Magnetic Resonance Imaging/Computed Tomography (MRI/CT) co-registration process. In this phantom-based study, geometric distortion is assessed in 3T T2-weighted images (T2WIs), while the efficacy of an MRI distortion correction technique is also evaluated. A homogeneous polymer gel-filled phantom was CT-imaged before being irradiated with 26 4-mm Gamma Knife shots at predefined locations (reference control points). The irradiated phantom was MRI-scanned at 3T, implementing a T2-weighted protocol suitable for SRS/SRT treatment planning. The centers of mass of all shots were identified in the 3D image space by implementing an iterative localization algorithm and served as the evaluated control points for MRI distortion detection. MRIs and CT images were spatially co-registered using a mutual information algorithm. The inverse transformation matrix was applied to the reference control points and compared with the corresponding MRI-identified ones to evaluate the overall spatial accuracy of the MRI/CT dataset. The mean image distortion correction technique was implemented, and resulting MRI-corrected control points were compared against the corresponding reference ones. For the scanning parameters used, increased MRI distortion (>1mm) was detected at areas distant from the MRI isocenter (>5cm), while median radial distortion was 0.76mm. Detected offsets were slightly higher for the MRI/CT dataset (0.92mm median distortion). The mean image distortion correction improves geometric accuracy, but residual distortion cannot be considered negligible (0.51mm median distortion). For all three datasets studied, a statistically significant positive correlation between detected spatial offsets and their distance from the MRI isocenter was revealed. This work contributes towards the wider adoption of 3T imaging in SRS/SRT treatment planning. The presented methodology can be employed in commissioning and quality assurance programmes of corresponding treatment workflows.

Assessment of Radiation-Induced Bladder and Bowel Cancer Risks after Conventionally and Hypo-Fractionated Radiotherapy for the Preoperative Management of Rectal Carcinoma.

Preoperative management of rectal carcinoma can be performed by employing either conventionally or hypo-fractionated Radiotherapy (CFRT or HFRT, respectively), delivered by Intensity Modulated Radiotherapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT) plans, employing 6 MV or 10 MV photon beams. This study aims to dosimetrically and radiobiologically compare all available approaches, with emphasis on the risk of radiation-induced second cancer to the bladder and bowel. Computed Tomography (CT) scans and relevant radiotherapy contours from 16 patients were anonymized and analyzed retrospectively. For each case, CFRT of 25 × 2 Gy and HFRT of 5 × 5 Gy were both considered. IMRT and VMAT plans using 6 MV and 10 MV photons were prepared. Plan optimization was performed, considering all clinically used plan quality indices and dose-volume constraints for the critical organs. Resulting dose distributions were analyzed and compared. Moreover, the Lifetime Attributable Risk (LAR) for developing radiation-induced bladder and bowel malignancies were assessed using a non-linear mechanistic model, assuming patient ages at treatment of 45, 50, 55 and 60 years. All 128 plans created were clinically acceptable. Risk of second bladder cancer reached 0.26% for HFRT (5 × 5 Gy) and 0.19% for CFRT (25 × 2 Gy) at the age of 45. Systematically higher risks were calculated for HFRT (5 × 5 Gy) as compared to CFRT (25 × 2 Gy), with 6 MV photons resulting in slightly increased LAR, as well. Similar or equal bowel cancer risks were calculated for all techniques and patient ages investigated (range 0.05-0.14%). This work contributes towards radiotherapy treatment protocol selection criteria for the preoperative irradiation of rectal carcinoma. However, more studies are needed to establish the associated radiation-induced risk of each RT protocol.

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.

On the use of a novel Ferrous Xylenol-orange gelatin dosimeter for HDR brachytherapy commissioning and quality assurance testing.

PURPOSE: To evaluate a commercially available Ferrous-Xylenol Orange-Gel (FXG) dosimeter (TrueView™) coupled with Optical-Computed Tomography (OCT) read out, for 3D dose verification in an Ir-192 superficial brachytherapy application. METHODS: Two identical polyethylene containers filled with gel from the same batch were used. One was irradiated with an 18 MeV electron field to examine the dose-response linearity and obtain a calibration curve. A flap surface applicator was attached to the other to simulate treatment of a skin lesion. The dose distribution in the experimental set up was calculated with the TG-43 and the model based dose calculation (MBCA) algorithms of a commercial treatment planning system (TPS), as well as Monte Carlo (MC) simulation using the MCNP code. Measured and calculated dose distributions were spatially registered and compared. RESULTS: Apart from a region close to the container's neck, where gel measurements exhibited an over-response relative to MC calculations (probably due to stray light perturbation), an excellent agreement was observed between measurements and simulations. More than 97% of points within the 10% isodose line (80 cGy) met the gamma index criteria established from uncertainty analysis (5%/2 mm). The corresponding passing rates for the comparison of experiment to calculations using the TG-43 and MBDCA options of the TPS were 57% and 92%, respectively. CONCLUSION: TrueView™ is suitable for the quality assurance of demanding radiotherapy applications. Experimental results of this work confirm the advantage of the studied MBDCA over TG-43, expected from the improved account of scatter radiation in the treatment geometry.

Assessment of sequence dependent geometric distortion in contrast-enhanced MR images employed in stereotactic radiosurgery treatment planning.

This work focuses on MR-related sequence dependent geometric distortions, which are associated with B 0 inhomogeneity and patient-induced distortion (susceptibility differences and chemical shift effects), in MR images used in stereotactic radiosurgery (SRS) applications. Emphasis is put on characterizing distortion at target brain areas identified by gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) paramagnetic contrast agent uptake. A custom-made phantom for distortion detection was modified to accommodate two small cylindrical inserts, simulating small brain targets. The inserts were filled with Gd-DTPA solutions of various concentrations (0-20 mM). The phantom was scanned at 1.5 T unit using both the reversed read gradient polarity (to determine the overall distortion as reflected by the inserts centroid offset) and the field mapping (to determine B 0 inhomogeneity related distortion in the vicinity of the inserts) techniques. Post-Gd patient images involving a total of 10 brain metastases/targets were also studied using a similar methodology. For the specific imaging conditions, contrast agent presence was found to evidently affect phantom insert position, with centroid offset extending up to 0.068 mm mM-1 (0.208 ppm mM-1). The Gd-DTPA induced distortion in patient images was of the order of 0.5 mm for the MRI protocol used, in agreement with the phantom results. Total localization uncertainty of metastases-targets in patient images ranged from 0.35 mm to 0.87 mm, depending on target location, with an average value of 0.54 mm (2.24 ppm). This relative wide range of target localization uncertainty results from the fact that the B 0 inhomogeneity distortion vector in a specific location may add to or partly counterbalance Gd-DTPA induced distortion, thus increasing or decreasing, respectively, the total sequence dependent distortion. Although relatively small, the sequence dependent distortion in Gd-DTPA enhanced brain images can be easily taken into account for SRS treatment planning and target definition purposes by carefully inspecting both the forward and reversed polarity series.

Time resolved dose rate distributions in brachytherapy.

PURPOSE: To investigate the biological significance of introducing time-resolved dose rate distributions (TR-DRD) in brachytherapy. MATERIALS AND METHODS: The treatment plan of a head and neck patient treated with pulsed-dose-rate (PDR) brachytherapy was considered. The TR-DRD was calculated on the basis of a Monte Carlo generated single source dose rate matrix taking into account the dose rate per source dwell position. Biologically Effective Dose (BED) was obtained considering either the mean dose rate per pulse (analytical method) or the TR-DRD (numerical method). Corresponding Tumor Control Probabilities (TCP) were calculated and compared for various PDR schemes and repair half-times from the literature. The dose of the biologically equivalent high-dose-rate (HDR) treatment schedule was also evaluated. RESULTS: The analytical method presents an overall BED underestimation (up to 2%) relative to TR-DRD results. This is associated with an analytical-based TCP underestimation which increases with dose/pulse, pulse duration and period time and decreases with total dose. The half-time of repair seems to have the largest impact on the TCP calculations, with significant differences (up to 39.1%) corresponding to the shorter repair half-times. Regarding the equivalent HDR treatment schedule, the analytical method resulted to a HDR isoeffective dose underestimation lower than 2.2% and thus does not warrant any change in the derivation of the equivalent HDR scheme. CONCLUSION: TR-DRD data should be taken into account for PDR biological effectiveness estimations, especially for short tissue repair half-times. This does not appear however to influence dose prescription of the equivalent HDR treatment schedule for mobile tongue carcinoma.

On the experimental validation of model-based dose calculation algorithms for 192Ir HDR brachytherapy treatment planning.

There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in 192Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1 × 1 × 1 mm3 microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an 192Ir high dose rate (HDR) source. Dose to medium in medium, [Formula: see text], was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k = 1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4-6.4, 2.5-6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between 192Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min-1). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for 192Ir and the reduction of experimental uncertainties.

MRI-Related Geometric Distortions in Stereotactic Radiotherapy Treatment Planning: Evaluation and Dosimetric Impact.

In view of their superior soft tissue contrast compared to computed tomography, magnetic resonance images are commonly involved in stereotactic radiosurgery/radiotherapy applications for target delineation purposes. It is known, however, that magnetic resonance images are geometrically distorted, thus deteriorating dose delivery accuracy. The present work focuses on the assessment of geometric distortion inherent in magnetic resonance images used in stereotactic radiosurgery/radiotherapy treatment planning and attempts to quantitively evaluate the consequent impact on dose delivery. The geometric distortions for 3 clinical magnetic resonance protocols (at both 1.5 and 3.0 T) used for stereotactic radiosurgery/radiotherapy treatment planning were evaluated using a recently proposed phantom and methodology. Areas of increased distortion were identified at the edges of the imaged volume which was comparable to a brain scan. Although mean absolute distortion did not exceed 0.5 mm on any spatial axis, maximum detected control point disposition reached 2 mm. In an effort to establish what could be considered as acceptable geometric uncertainty, highly conformal plans were utilized to irradiate targets of different diameters (5-50 mm). The targets were mispositioned by 0.5 up to 3 mm, and dose-volume histograms and plan quality indices clinically used for plan evaluation and acceptance were derived and used to investigate the effect of geometrical uncertainty (distortion) on dose delivery accuracy and plan quality. The latter was found to be strongly dependent on target size. For targets less than 20 mm in diameter, a spatial disposition of the order of 1 mm could significantly affect (>5%) plan acceptance/quality indices. For targets with diameter greater than 2 cm, the corresponding disposition was found greater than 1.5 mm. Overall results of this work suggest that efficacy of stereotactic radiosurgery/radiotherapy applications could be compromised in case of very small targets lying distant from the scanner's isocenter (eg, the periphery of the brain).