M Meltsner

AAPM and GEC-ESTRO guidelines for image-guided robotic brachytherapy: Report of Task Group 192

In the last decade, there have been significant developments into integration of robots and automation tools with brachytherapy delivery systems. These systems aim to improve the current paradigm by executing higher precision and accuracy in seed placement, improving calculation of optimal seed locations, minimizing surgical trauma, and reducing radiation exposure to medical staff. Most of the applications of this technology have been in the implantation of seeds in patients with early-stage prostate cancer. Nevertheless, the techniques apply to any clinical site where interstitial brachytherapy is appropriate. In consideration of the rapid developments in this area, the American Association of Physicists in Medicine (AAPM) commissioned Task Group 192 to review the state-of-the-art in the field of robotic interstitial brachytherapy. This is a joint Task Group with the Groupe Européen de Curiethérapie-European Society for Radiotherapy & Oncology (GEC-ESTRO). All developed and reported robotic brachytherapy systems were reviewed. Commissioning and quality assurance procedures for the safe and consistent use of these systems are also provided. Manual seed placement techniques with a rigid template have an estimated in vivo accuracy of 3-6 mm. In addition to the placement accuracy, factors such as tissue deformation, needle deviation, and edema may result in a delivered dose distribution that differs from the preimplant or intraoperative plan. However, real-time needle tracking and seed identification for dynamic updating of dosimetry may improve the quality of seed implantation. The AAPM and GEC-ESTRO recommend that robotic systems should demonstrate a spatial accuracy of seed placement ≤1.0 mm in a phantom. This recommendation is based on the current performance of existing robotic brachytherapy systems and propagation of uncertainties. During clinical commissioning, tests should be conducted to ensure that this level of accuracy is achieved. These tests should mimic the real operating procedure as closely as possible. Additional recommendations on robotic brachytherapy systems include display of the operational state; capability of manual override; documented policies for independent check and data verification; intuitive interface displaying the implantation plan and visualization of needle positions and seed locations relative to the target anatomy; needle insertion in a sequential order; robot-clinician and robot-patient interactions robustness, reliability, and safety while delivering the correct dose at the correct site for the correct patient; avoidance of excessive force on radioactive sources; delivery confirmation of the required number or position of seeds; incorporation of a collision avoidance system; system cleaning, decontamination, and sterilization procedures. These recommendations are applicable to end users and manufacturers of robotic brachytherapy systems.

Comparison of bulk electron density and voxel-based electron density treatment planning

The use of magnetic resonance imaging (MRI) alone for radiation planning is limited by the lack of electron density for dose calculations. The purpose of this work is to evaluate the dosimetric accuracy of using bulk electron density as a substitute for computed tomography (CT)‐derived electron density in intensity‐modulated radiation therapy (IMRT) treatment planning of head and neck (HN) cancers. Ten clinically‐approved, CT‐based IMRT treatment plans of HN cancer were used for this study. Three dose distributions were calculated and compared for each treatment plan. The first calculation used CT‐derived density and was assumed to be the most accurate. The second calculation used a homogeneous patient density of 1 g/cm3. For the third dose calculation, bone and air cavities were contoured and assigned a uniform density of 1.5 g/cm3 and 0 g/cm3, respectively. The remaining tissues were assigned a density of 1 g/cm3. The use of homogeneous anatomy resulted in up to 4%–5% deviations in dose distribution as compared to CT‐derived electron density calculations. Assigning bulk density to bone and air cavities significantly improved the accuracy of the dose calculations. All parameters used to describe planning target volume coverage were within 2% of calculations based on CT‐derived density. For organs at risk, most of the parameters were within 2%, with the few exceptions located in low‐dose regions. The data presented here show that if bone and air cavities are overridden with the proper density, it is feasible to use a bulk electron density approach for accurate dose calculation in IMRT treatment planning of HN cancers. This may overcome the problem of the lack of electron density information should MRI‐only simulation be performed. PACS number: 87.55.D‐

Motion-compensated estimation of delivered dose during external beam radiation therapy: Implementation in Philips’ Pinnacle3 treatment planning system

PURPOSE Recent research efforts investigating dose escalation techniques for three-dimensional conformal radiation therapy (3D CRT) and intensity modulated radiation therapy (IMRT) have demonstrated great benefit when high-dose hypofractionated treatment schemes are implemented. The use of these paradigms emphasizes the importance of smaller treatment margins to avoid high dose to surrounding normal tissue or organs at risk (OARs). However, tighter margins may lead to underdosage of the target due to the presence of organ motion. It is important to characterize organ motion and possibly account for it during treatment delivery. The need for real-time localization of dynamic targets has encouraged the use and development of more continuous motion monitoring systems such as kilo-voltage/fluoroscopic imaging, electromagnetic tracking, and optical monitoring systems. METHODS This paper presents the implementation of an algorithm to quantify translational and rotational interfractional and intrafractional prostate motion and compute the dosimetric effects of these motion patterns. The estimated delivered dose is compared with the static plan dose to evaluate the success of delivering the plan in the presence of prostate motion. The method is implemented on a commercial treatment planning system (Pinnacle(3), Philips Radiation Oncology Systems, Philips Healthcare) and is termed delivered dose investigational tool (DiDIT). The DiDIT implementation in Pinnacle(3) is validated by comparisons with previously published results. Finally, different workflows are discussed with respect to the potential use of this tool in clinical treatment planning. RESULTS The DiDIT dose estimation process took approximately 5-20 min (depending on the number of fractions analyzed) on a Pinnacle(3) 9.100 research version running on a Dell M90 system (Dell, Inc., Round Rock, TX, USA) equipped with an Intel Core 2 Duo processor (Intel Corporation, Santa Clara, CA, USA). The DiDIT implementation in Pinnacle(3) was found to be in agreement with previously published results, on the basis of the percent dose difference (PDD). This metric was also utilized to compare plan dose versus delivered dose, for prostate targets in three clinically acceptable treatment plans. CONCLUSIONS This paper presents results from the implementation of an algorithm on a commercially available treatment planning system that quantifies the dosimetric effects of interfractional and intrafractional motion in external beam radiation therapy (EBRT) of prostate cancer. The implementation of this algorithm within a commercial treatment planning system such as Pinnacle(3) enables easy deployment in the existing clinical workflow. The results of the PDD tests validate the implementation of the DiDIT algorithm in Pinnacle(3), in comparison with previously published results.

Angular disparity in ETACT scintimammography.

Emission tuned aperture computed tomography (ETACT) has been previously shown to have the potential for the detection of small tumors (<1 cm) in scintimammography. However, the optimal approach to the application of ETACT in the clinic has yet to be determined. Therefore, we sought to determine the effect of the angular disparity between the ETACT projections on image quality through the use of a computer simulation. A small, spherical tumor of variable size (5, 7.5 or 10 mm) was placed at the center of a hemispherical breast (15 cm diameter). The tumor to nontumor ratio was either 5:1 or 10:1. The detector was modeled to be a gamma camera fitted with a 4-mm-diam pinhole collimator. The pinhole-to-detector and the pinhole-to-tumor distances were 25 and 15 cm, respectively. A ray tracing technique was used to generate three sets of projections (10 degrees, 15 degrees, and 20 degrees, angular disparity). These data were blurred to a resolution consistent with the 4 mm pinhole. The TACT reconstruction method was used to reconstruct these three image sets. The tumor contrast and the axial spatial resolution was measured. Smaller angular disparity led to an improvement in image contrast but at a cost of degraded axial spatial resolution. The improvement in contrast is due to a slight improvement in the in-plane spatial resolution. Since improved contrast should lead to better tumor detectability, smaller angular disparity should be used. However, the difference in contrast between 10 degrees and 15 degrees was very slight and therefore a reasonable clinical choice for angular disparity is 15 degrees.

WE‐A‐BRB‐10: Validation of the AAPM/ESTRO TG‐192 Protocol for Robotic Implantation of Brachytherapy Seeds: Spatial Positioning Assessment

Purpose: To date, 13 roboticbrachytherapy systems have been developed incorporating a variety of imaging, control, and delivery techniques. The joint AAPM/ESTRO TG‐192 report presents a new protocol for the validation of brachytherapyrobotic systems and a review of the various brachytherapy systems. This study examines spatial positioning accuracy of implanted seeds. Methods: A recommendation is that a uniform test protocol should be followed for evaluating performance of any robotic systems that would be used for brachytherapy, especially for radioactive source implantation. Parameters needed to be evaluated are needle tip positioning accuracy, needle tip positioning repeatability, positioning accuracy of the delivered sources, robot‐to‐imager calibration accuracy, and qualitative assessment of tissue damage if needle rotation is used. To accomplish these goals, a phantom (polyvinylchloride) mimicking soft tissue is recommended. For the validation study, phantoms were prepared at Thomas Jefferson University and then sent to participating institutes to ensure phantom consistency and quality. A treatment plan was developed with needle and seed coordinates. Participating institutes would deposit 100 dummy seeds in the phantom using their robotic system. Five seeds would be deposited along each needle at inter‐seed spacing of 10mm; needles would be arranged in a specified order in a 10mm×10mm grid. Upon completion of seed deposition, the phantoms would be imaged using CT and fluoroscopy and/or digital photography. An example test run and observed accuracies are presented. Results: Observations so far from two roboticbrachytherapy systems indicate that the protocol is feasible and easy to implement. Robotic systems following this protocol can deposit seeds within 1mm (3D) of the intended location. Conclusions: The TG‐192 protocol will be useful for commissioning any roboticbrachytherapy seed‐implantation system. Use of this standardized method will allow quantitative comparisons of seed deposition positioning accuracies obtained from any roboticbrachytherapy system.

MO‐D‐351‐02: An Efficient Approach To Volumetric Modulated Arc Therapy Optimization and Sequencing

Purpose: To develop and evaluate an efficient algorithm for inverse treatment planning of volumetric arc therapy (VMAT). Method and Materials: VMAT plans were generated by first applying a direct machine parameter optimization algorithm from a research version of a commercial treatment planning system (Pinnacle 8.1v) to 18 beams spaced equidistantly at 20 degrees. At each beam angle, the number of segments was variable (typically 3 to 8) to match the intensity modulation fluence map. Low weight or small area segments were eliminated during optimization. The resulting segments were redistributed around the arc such that each individual segment was at a unique arc angle. Segments were sorted according to shape similarity in order to reduce leaf travel in between arc angle positions. To ensure accuracy of the final dose calculation, additional interpolated segments were generated, followed by a segment weight optimization to identify beam‐off segments that would otherwise conflict with plan objectives. The approach was evaluated for six cases (three head‐and‐necks, two prostate, and one brain) with respect to treatment delivery time, and overall plan quality in comparison to step‐and‐shoot IMRT using a conventional number of static beams and identical objectives and constraints as in the VMAT case. Results: The dynamic arc plans consistently demonstrated target coverage and critical structure sparing comparable to step‐and‐shoot plans. For the head‐and‐neck cases, sparing of the brainstem and the spinal cord could be improved substantially. Treatment parameter optimization and dose calculation required 15 to 20 minutes on standard hardware. Estimated delivery times were between 1.5 and 4 minutes, which is mainly determined by the leaf trajectories, gantry speed and dose rate. Conclusion: A method for inverse planning for VMAT delivery was developed. The method produces treatment plans with high dosimetric quality and efficient delivery time with clinically feasible user time and effort.

SU‐E‐J‐107: Feasibility of Complete Brain Simulation Using Single MRI Acquisition

PURPOSE To evaluate the information available from a single MRI acquisition for a complete MRI-based simulation in brain. METHOD AND MATERIALS A 3.0T MRI scanner (Achieva TX, Philips Healthcare) was used to acquire a complete brain simulation data set in a single acquisition on four consenting volunteers. The acquisition consisted of collecting the FID signal (TE1 = 100μs) followed by two additional gradient echoes (TE2/TE3= 1.4ms/2.5ms) using a 3D volumetric excitation and radial read-out toachieve 1.3mm isotropic voxels. The data from this single acquisition was used to reconstruct five volumetric data sets: Bone-enhanced, fat-only, water-only, in-phase and out-of-phase. The resulting image sets were assessed for image quality sufficient for organ delineation and used to generate digitally-reconstructed radiographs (DRRs) (Pinnacle Workstation, Philips Healthcare). RESULTS Each volunteer study took less than 10 minutes to complete, and the single acquisition required less than 3 minutes. Images from all four volunteers had excellent image quality sufficient for organ delineation and complete cortical bone segmentation. In addition, the DDRs for all four volunteers were sufficient for 2D patient matching. CONCLUSION This study confirms the feasibility of using a single acquisition MRI as a sole imaging modality for treatment planning simulation in the brain. To validate this method, we plan to use this imaging protocol in a group of patients and compare the DRRs and dose plans with those acquired during CT simulation. Both authors are employees of Philips Healthcare.

WE‐A‐BRB‐05: AAPM Guidelines for Image‐Guided Robotic Brachytherapy: Progress Report from Task Group 192

Purpose: To report progress made by AAPM Task Group 192 which was charged to review the state‐of‐the‐art systems for robotic interstitial brachytherapy and to recommend commissioning and quality assurance procedures for the safe and consistent clinical use of these systems. Methods: In the last decade, there have been significant developments in medical robots and automation tools, which have been integrated into brachytherapy systems. These developments have led to higher precision and reproducibility in source placement, optimization of source locations, improvement in consistency, elimination of clinicians fatigue as well as further reduction of radiation exposure to medical staff. Most of the applications of these technologies have been in the implantation of seeds in patients with early stage prostate cancer. Nevertheless, the techniques apply to any clinical sites where interstitial brachytherapy is appropriate. The TG‐192 has reviewed all the available pertinent robotic systems, and is testing a procedure for commissioning and quality assurance for the safe and reliable clinical use of these systems. The committee addressed both the characteristics of robotic behaviors, and the interactions between the robots and the clinicians in an operational radiotherapy environment. Results: Existing roboticbrachytherapy systems are capable of achieving a spatial accuracy of about 1 mm for source placement in a phantom. Considering that manual source placement with a rigid template has an estimated accuracy of 2–3 mm and source placement may vary from ideal within a patient due to multiple factors such as tissue deformation, source displacement, and edema. This task group recommends that robotic systems should have a spatial accuracy of source placement in phantom of <1 mm. Conclusions: Preliminary recommendations are that during clinical commissioning, specified tests should be conducted to ensure that this level of accuracy is maintained. The recommended tests mimic the real operating procedure as closely as possible.

Poster — Thur Eve — 39: Dosimetric Evaluation of Bulk Electron Density Based Treatment Planning in IMRT Head and Neck Patients: Can It Be Used for MRI‐Based Planning?

One limitation of using MRI alone for radiation planning is the lack of electron density for dose calculation. We evaluated the dosimetric accuracy of using bulk density overrides as a substitute for CT-derived densities in IMRT treatment planning for head and neck cancer. Ten clinically-approved, CT-based treatment plans were used for this study. Three dose distributions were calculated for each treatment plan. The first calculation used CT-derived density as a basis for heterogeneity correction. The second calculation assumed a homogeneous patient density of 1 g/cm3. For the third dose calculation we contoured bones and air cavities and assigned them a uniform density of 1.5 g/cm3 and 0 g/cm3, respectively. The remaining tissue was assigned a density of 1 g/cm3. All three calculations utilized identical beam parameters (angles, segments and MUs). Actual MR images were not used for contouring to avoid effects of gradient distortion and volumetric uncertainties associated with them. All calculations were done using the Collapsed Cone Superposition algorithm in the Pinnacle3 treatment planning system. Our results show that the assignment of bulk density to bones and air cavities was a feasible approach to IMRT treatment planning for head and neck patients. In almost all cases, the dosimetric results were within 2% of the treatment plans based on CT-derived density. This method may overcome the lack of electron density information in MR-based planning. The use of homogeneous geometry, while simpler and less time consuming, resulted in unacceptably high errors in the dose distribution compared to the nominal plan. This research project is supported by Philips Medical Systems.

Engineering Radioactive Stents for the Prevention of Restenosis

Radiation has become an accepted treatment for the prevention of restenosis (re-blockage) of coronary arteries following angioplasty. Radioactive stents could be the easiest method of delivery for the radiation, although clinical trials were disappointing. One likely reason was the choice of P-32 as the radionuclide, which fails to match the biological needs of the problem. What radionuclide would perform best remains unknown. This project established the physical infrastructure necessary for a rational investigation to determine the optimum radiological characteristics for radioactive stents in the prevention of restenosis following angioplasty. The project investigated methods to activate coronary stents with radionuclides that spanned a range of energies and radiation types that could provide a mapping of the biological response. The project also provided calibration methods to determine the strength of the stents, an a process to calculate the dose distribution actually delivered to the patients artery--quantities necessary for any future scientific study to improve the effectiveness of radioactive stents. Such studies could benefit the thousands of patients who receive angioplasty each year.