Couch Top Research Articles

Accuracy Compensation Based on Optimized Neural Network for a Robotic Patient Positioning System with 6 Degrees of Freedom Used in Proton Therapy System

Proton therapy for tumor treatment is a typical application of nuclear technology. For proton therapy systems, robotic patient positioning systems (PPSs) are increasingly used because of their high flexibility and efficiency. Most robotic PPSs are developed based on industrial robots, which have good repeatability but low absolute position accuracy (1 to 3 mm) and do not satisfy the requirement of highly precise treatment. In this study, an optimized algorithm, named the Back Propagation Neural Network (BPNN) algorithm based on particle swarm optimization, is proposed to improve the performance of absolute positioning accuracy. A comparison of the training for the traditional BPNN and the optimized algorithm is presented. A series of experiments with different payload weights and tools is implemented to validate the performance of the proposed method. The training results show that the proposed method can improve the average predicted positioning error from 0.55 to 0.38 mm. The results of the experiment with a calibration tool show that the average position error is reduced from 4.10 to 0.32 mm. The results of the experiment with a carbon fiber couch top show that the average and maximal positioning errors are 0.35 and 0.77 mm, respectively. All the results verify the feasibility of the proposed method in this study in improving the position accuracy of the robotic PPS.

Improving Patient Compliance for More Efficient Deep Inspiration Breath Hold Treatment through Innovation and Education.

Visual guidance has been widely proven to improve the reproducibility and stability of Deep Inspiration Breath Hold (DIBH) treatments. Here, we present a quality improvement project by implementing the use of visual guidance in a multi-campus institution to improve clinical efficiency, including the design of a novel mount for the visual aid and the creation of a patient education video to better prepare lung and GI patients for their DIBH treatments. Prior to institution-wide implementation, the clinical feasibility of utilizing visual guidance for DIBH treatments was determined by a pilot study with 10 lung/GI patients. A commercial visual guidance device was used, which consisted of a tablet-like device, attached to a mount affixed to the treatment couch. The device is positioned over the patient's head, displaying the real-time vertical motion of a block with infrared markers on the patient's abdomen. The original mount for the device locks onto the superior end of the treatment couch, which occupies space used for immobilization devices and limits the number of patients eligible for visual guidance when the isocenter is inferior, as for GI patients. A novel in-house mount was designed to overcome this limitation. Also, a patient education video was created to introduce the concept of DIBH and the visual aid device. Visual guidance is offered at the first treatment; its use is optional, and patients may start or stop using it at any time. If the device is not used, the patient relies on audio coaching from the radiation therapists (RTTs), which is the standard practice in our clinic. All 10 patients in the pilot who were offered the visual aid used it, found it helpful and continued to use it throughout their treatment, with no issues reported. The median number of fractions for all 10 patients was 5(3-15). The in-house mount was manufactured from 3D printed and machined components. The treatment couch was 3D scanned, and the mount was designed to clamp laterally on the couch top and align with the indexing indentations. The in-house mount is easy to use and is more versatile than the commercial mount, enabling use for patients with inferior isocenters. Prior to treatment, patients were encouraged to view the <3 min long patient education video that consisted of a combination of animation and real-actor content. Verbal instructions for use were also given by RTTs at the first treatment. Upon successful completion of the pilot study, the use of visual guidance was implemented institution-wide across 5 campuses for all lung and GI DIBH patients, being utilized on an average of 14 patients/week. A quality improvement project was developed and successfully implemented to introduce the use of visual guidance for lung and GI DIBH patients at a large institution, improving the efficiency of DIBH treatment for both the patients and RTTs.

Scalp Volumetric Modulated Arc Therapy Using 3D Milled Bolus: Dosimetry, Toxicity and Outcome.

Radiotherapy for scalp lesions is frequently challenging due to the competing needs for target volume coverage, sparing of numerous critical nearby OARs, and reproducible bolus setup despite the irregular and highly convex scalp geometry. Here, we evaluate the use of 3D milled bolus in treating such tumors. A retrospective analysis of scalp patients (pts) treated between 2016-2022 using 3D milled bolus and VMAT. All pts were treated with 6 MV photon beams, IGRT via daily CBCT, and 6-degree of Freedom (6-DOF) couch tops. Pts demographics, tumor characteristics, DVHs, toxicities and outcomes were evaluated. Regional control (RC) defined as control of the disease in the rest of the scalp/neck and local control (LC) defined as disease control within the PTV. A total of 23 pts were identified. Median age 74 (46-85), ECOG performance status was 0-1 in 20 pts (87%) and 92% were males. The histopathologies were squamous cell carcinoma (SCC) (61%), angiosarcoma (AS) (35%) and melanoma (4%). 35% of pts were treated with definitive intent to gross disease; the remaining 65% were treated post-operatively, 22% with microscopically positive margins and 43% with negative margins. 21% had perineural invasion (PNI), and none had lymphovascular invasion (LVI). 78% had T3/T4 and 13% had N+ disease. Median radiation dose was 66 Gy (60-69.96Gy). 44% received concurrent systemic therapy (Paclitaxel 22%, Cemiplimab 12%, Temozolomide 5%, Nivolumab 5%). In 40% of pts, the whole scalp was treated; in the remaining 60%, the median ratio of PTV volume to scalp volume was 35% (25-90%). 22% of pts (n = 5) had neck irradiation for prophylactic (3pts)/ neck lymph node involvement (2 pts). The median brain Dmax was 65 Gy (52.5-71.9), median brain mean dose was 15.4Gy (1.4-38.4), median eye Dmax was 10.9Gy (0.23-49), median cochlea mean was 8.4Gy (0.1-20.4), median lacrimal gland mean dose was 6.8Gy (0.11-38), median hippocampus Dmin was 5.9Gy (0.1-14.9) and Dmax was 10Gy (0.3-23). Most common acute side effects were grade 1-2 fatigue (100%), grade 1-2 pain (78%), grade 1 dysgeusia (22%), grade 1-2 dermatitis (74%). The only grade > = 3 adverse event was grade 3 dermatitis in 26%. Regarding long-term side effects, one pt had grade 2 osteoradionecrosis and other one developed grade 2 skin ulcer. 17% developed grade 1 memory impairment. 17% had grade 1 eye dryness. None developed brain radionecrosis. At a median follow-up was 14.4 months (1-73.5), the 1-yr LC was 96%. The 1-yr RC was 75% overall, 100% in angiosarcoma vs 62% in SCC (p = 0.18). Of the 4 regional recurrences, 1 pt had marginal recurrence along V1, 2 had neck nodal recurrence, and 1 had recurrence elsewhere in the scalp. 1-yr DMFS was 62% overall. 1-yr OS was 78% overall, 100% for AS vs 64% for SCC (P = 0.097). VMAT planning with 3D milled bolus permits technically sound radiotherapy for scalp targets with an acceptable toxicity profile and relatively favorable clinical outcomes. This approach warrants further evaluation in a larger prospective study.

Dosimetric characterization of foam padding with posterior fields in palliative radiation therapy

Patients undergoing external beam radiation therapy for the palliative treatment of painful bony metastases may have difficulty maintaining a still position on a rigid uncovered couch top, both during CT simulation as well as during patient setup, image guidance, and treatment on the linear accelerator. For these patients, a thin foam pad or mattress is sometimes used to mitigate patient discomfort. It was desired to quantify the effect of the padding in cases in which the patient is to be treated supine with posterior beams when the majority of the beam weighting traverses both the couch and the pad. Ion chamber measurements in-phantom were acquired with 6 MV, 10 MV, and 15 MV photon beams. At depths of maximum dose, the pad resulted in a difference of signal collected ≤1%. At the phantom surface, the pad resulted in an increase in signal ranging from 1% to 6.5% for the measured beams. CT data of the pad, both with and without applied pressure, indicated that the pad had average HU values close to air.

Dosimetric effects and impacts caused by a carbon fiber table and its accessories in a linear accelerator

Objective : Most contemporary treatment planning systems (TPS) exclude the couch during treatment planning. The present investigation aimed to evaluate the impact of couches and accessories on radiation therapy planning by quantifying the degree of attenuation for two-photon beam energies, 6 MV and 10 MV, at two different field sizes, 5 cm x 5 cm and 10 cm x 10 cm. &#x0D; Methods: An x-ray radiation beam at two energies (6 MV and 10 MV) generated by a linear accelerator (Elekta Synergy). The output dose was measured using a digital parallel plate ionization chamber of the IBA cc13 type (IBA Dosimetry, Germany), and the readings were recorded with a 2944 Farmer Type chamber. Different commercially available couch tops and accessories evaluated in this study.&#x0D; Results: The results show that the highest attenuation values were observed at 1300 gantry angles for both energies and field sizes. The lowest attenuation values were recorded at 1000 gantry angles with 6 MV energy for 5cm x 5cm and 10cm x 10cm field sizes. The breast board with D level demonstrated the highest attenuation of 6.27% and 5.51% for 6 MV and 10 MV energies, respectively.&#x0D; Conclusion: the findings indicate that the degree of attenuation is not uniform across all angles and may exceed tolerable limits, indicating the need for careful consideration during treatment planning to ensure optimal treatment outcomes.

Dosimetric effect of six degrees of freedom couch top with rotational setup error corrections in proton therapy.

To investigate the dosimetric effect of six degrees of freedom (6DoF) couch top with rotational corrections in proton therapy (PT). The water equivalent thickness (WET) was measured using a proton beam with a 6DoF couch top and patient immobilization base plate (PIBP) placed in front of a motorized water phantom. The accuracy verification was performed with the beam axis set perpendicular to the 6DoF couch top and tilted in 10° steps from 10° to 30°. Up to 3° rotational correction may be added during the actual treatment to correct the rotational setup error on our system. The measured and calculated values using the treatment planning system were compared. Additionally, the effect of the 3° difference was evaluated using actual measurements concerning each angle on the proton beam range. The WET of the 6DoF couch top and PIBP were 8.5±0.1mm and 6.8±0.1mm, respectively. The calculation and the actual measurement at each angle agreed within 0.2mm at the maximum. A maximum difference of approximately 0.6mm was confirmed when tilted at 3° following 30° with the 6DoF couch top plus PIBP. The dosimetric effect of the 6DoF couch top with rotational corrections in PT differs depending on the incidence angle on the couch top, and it increased with the increased oblique angle of incidence. However, the effect on the range was as small as 0.6mm at the maximum. The amount of rotational correction, the angle of incidence of the beam, and the effect of rotational corrections on the proton beam range may differ depending on the structure of the couch top. Therefore, sufficient prior confirmation, and subsequent periodical quality assurance management are important.

Clinical Characterization of a Table Mounted Range Shifter Board for Synchrotron-Based Intensity Modulated Proton Therapy for Pediatric Craniospinal Irradiation.

Purpose: To report our design, manufacturing, commissioning and initial clinical experience with a table-mounted range shifter board (RSB) intended to replace the machine-mounted range shifter (MRS) in a synchrotron-based pencil beam scanning (PBS) system to reduce penumbra and normal tissue dose for image-guided pediatric craniospinal irradiation (CSI). Methods: A custom RSB was designed and manufactured from a 3.5 cm thick slab of polymethyl methacrylate (PMMA) to be placed directly under patients, on top of our existing couch top. The relative linear stopping power (RLSP) of the RSB was measured using a multi-layer ionization chamber, and output constancy was measured using an ion chamber. End-to-end tests were performed using the MRS and RSB approaches using an anthropomorphic phantom and radiochromic film measurements. Cone beam CT (CBCT) and 2D planar kV X-ray image quality were compared with and without the RSB present using image quality phantoms. CSI plans were produced using MRS and RSB approaches for two retrospective pediatric patients, and the resultant normal tissue doses were compared. Results: The RLSP of the RSB was found to be 1.163 and provided computed penumbra of 6.9 mm in the phantom compared to 11.8 mm using the MRS. Phantom measurements using the RSB demonstrated errors in output constancy, range, and penumbra of 0.3%, -0.8%, and 0.6 mm, respectively. The RSB reduced mean kidney and lung dose compared to the MRS by 57.7% and 46.3%, respectively. The RSB decreased mean CBCT image intensities by 86.8 HU but did not significantly impact CBCT or kV spatial resolution providing acceptable image quality for patient setup. Conclusions: A custom RSB for pediatric proton CSI was designed, manufactured, modeled in our TPS, and found to significantly reduce lateral proton beam penumbra compared to a standard MRS while maintaining CBCT and kV image-quality and is in routine use at our center.

Quality assurance and temporal stability of a 1.5 T MRI scanner for MR-guided Photon and Particle Therapy

PurposeTo describe performance measurements, adaptations and time stability over 20 months of a diagnostic MR scanner for integration into MR-guided photon and particle radiotherapy. Material and methodsFor realization of MR-guided photon and particle therapy (MRgRT/MRgPT), a 1.5 T MR scanner was installed at the Heidelberg Ion Beam Therapy Center. To integrate MRI into the treatment process, a flat tabletop and dedicated coil holders for flex coils were used, which prevent deformation of the patient external contour and allow for the use of immobilization tools for reproducible positioning. The signal-to-noise ratio (SNR) was compared for the diagnostic and therapy-specific setup using the flat couch top and flexible coils for the a) head & neck and b) abdominal region as well as for different bandwidths and clinical pulse sequences. Additionally, a quality assurance (QA) protocol with monthly measurements of the ACR phantom and measurement of geometric distortions for a large field-of-view (FOV) was implemented to assess the imaging quality parameters of the device over the course of 20 months. ResultsThe SNR measurements showed a decreased SNR for the RT-specific as compared to the diagnostic setup of (a) 26% to 34% and (b) 11% to 33%. No significant bandwidth dependency for this ratio was found. The longitudinal assessment of the image quality parameters with the ACR and distortion phantom confirmed the long-term stability of the MRI device. ConclusionA diagnostic MRI was commissioned for use in MR-guided particle therapy. Using a radiotherapy specific setup, a high geometric accuracy and signal homogeneity was obtained after some adaptions and the measured parameters were shown to be stable over a period of 20 months.

Verification of Uniformity of Dosimetric Characteristics of Couch Tops in Proton Radiation Therapy

In proton therapy the couches used for patient positioning and alignment can affect the uncertainty in the dose distribution from the applied proton beam. In order to limit the additional uncertainty of the proton range, it is necessary to know the determined water equivalent thickness (WET) of the couch top and carefully reflect it in the calculations of the dosimetry planning system. The method presented in this article is based on the difference in the measured depth of the proton beam detected by the multilayer ionization chamber with the beam passing through the couch top and without passing through it.&#x0D; A range shifter (proton beam modifier) installed in the nozzle with the known WET for evaluating couch tops before commissioning was used in this research work. A nozzle is a proton beam delivery system consisting of quadrupole magnets, ionization chambers and an X-ray tube. CT scans were also acquired to evaluate the internal homogeneity of the test couch tops.&#x0D; Several kVue couch inserts Proton one (Qfix Systems, Avondale, PA) were evaluated in this study. The results show that the inserts are largely interchangeable for this type of design and the use of the range shifter during treatment, using gantry angles with the proton beam passing through the couch top, does not affect its WET, adding a deflection of approximately 0.1 mm. It was also determined that the reproducibility of the WET determination method has a deviation less than ±0.03 mm. In conclusion, the described above method of couch top testing before commissioning in proton therapy can also be applied to other patient immobilization systems.

Dosimetric Study of the Effect of Calypso-Compatible Couch Top for Spine Stereotactic Body Radiation Therapy

Purpose: To study the effect of the Qfix kVue Calypso-compatible couch top on the dosimetry of Spine Stereotactic Body Radiation Therapy (SBRT). Methods and Materials: The computed tomography (CT) data set for Qfix kVue Calypso-compatible couch top with rails were imported into the treatment planning system (TPS). Nine patients who underwent spine SBRT were selected for this study. The inclusion criteria included patients who were treated on a stereotactic linear accelerator with 5 fractions or less from 2016 to 2017 without the couch model. Seven patients were treated with static intensity-modulated radiation therapy (IMRT) fields and two patients were treated using volumetric modulated arc therapy (VMAT) technique. The dose was recalculated for 1) couch top and rails setup (CR) 2) couch-top no rails setup (CNR), and then compared to 3) no couch no-rails setup (NCNR). Dose to 100% of the target volume (D100%), Dose to cover 99% of the target volume (D99%), Dose to cover 95% of the target volume (D95%), Dose to cover 90% of the target volume (D90%), volume receiving 100% of the prescription dose (V100%), conformity index (CI), dose gradient index (DGI), and spinal cord threshold and maximum dose were compared to the plan with NCNR. Results: The average D100% was 77.89% ± 11.78%, 74.51% ± 12.24%, and 75.83 ± 12.67% for NCNR, CR, CNR (р = 0.84), respectively. The average D99% was 91.64% ± 9.57%, 89.93% ± 9.48%, and 91.15% ± 9.55% for NCNR, CR, CNR (р = 0.98), respectively. The average D95% was 99.14% ± 9.96%, 95.23% ± 9.76, and 96.78% ± 9.84% for NCNR, CR, CNR (р = 0.047), respectively. The average D90% was 101.3% ± 0.65%, 97.11% ± 2.48%, and 98.75% ± 2.12% for NCNR, CR, CNR (р = 0.0004), respectively. The maximum dose to the spinal cord was 1750.79 ± 41.84, 1672.90 ± 40.90, and 1709.91 ± 41.35 (cGy) for NCNR, CR, CNR (р = 0.97), respectively. In all cases, the spinal cord threshold dose was far below the tolerances and the differences were insignificant. Average CI was 1.18 ± 0.16, 0.53 ± 0.39, and 0.86 ± 0.24 for NCNR, CR, CNR (р = 0.0002), respectively. Conclusions: The study investigated the dosimetric impact of Qfix kVue Calypso-compatible couch top on the quality of the spinal SBRT treatment using static IMRT or VMAT techniques. IMRT plans showed more sensitivity to the couch being in the plan than the VMAT plans.

Real-time motion management for robotic treatment couch in proton therapy system

In proton therapy, robotic couch was used for patient positioning to achieve high precision in cancer treatment. However, no studies have focused on the real-time motion management system (MMS) for the robotic couch. This study was to develop the MMS and verify its feasibility. A real-time MMS was set up with a robotic couch, an NDI Vega optical tracking system, a Special Measurement Tool (SMT) and the developed software. A Leica laser tracker was adopted to verify the accuracy of the tests in this study. The stability and accuracy of the SMT in NDI Vega system were verified to guarantee the reliability of the results. The coordinate transformation was used to unify the measurement. 83 points inside the treatment volume were applied to verify the accuracy of the MMS, with the comparison of results measured by the laser tracker. The synchronism of the SMT in the MMS and Patient Positioning System (PPS) based on the reciprocating motion along axis z was tested to obtain the maximum time delay. Trajectory tracking was performed with different payloads on the couch top. A series of tests were conducted and the results demonstrated the feasibility of potential applications based on real-time motion capturing and monitoring.

Assessing the impact of magnetic resonance treatment simulation (MRSIM) on target volume delineation and dose to organs at risk for oropharyngeal radiotherapy.

IntroductionAssessing the use of a radiation therapy (RT) planning MRI performed in the treatment position (pMRI) on target volume delineation and effect on organ at risk dose for oropharyngeal cancer patients planned with diagnostic MRI (dMRI) and CT scan.MethodsDiagnostic MRI scans were acquired for 26 patients in a neutral patient position using a 3T scanner (dMRI). Subsequent pMRI scans were acquired on the same scanner with a flat couch top and the patient in their immobilisation mask. Each series was rigidly registered to the patients planning CT scan and volumes were first completed with the CT/dMRI. The pMRI was then made available for volume modification. For the group with revised volumes, two IMRT plans were developed to demonstrate the impact of the modification. Image and registration quality was also evaluated.ResultsThe pMRI registration led to the modification of target volumes for 19 of 26 participants. The pMRI target volumes were larger in absolute volume resulting in reduced capacity for organ sparing. Predominantly, modifications occurred for the primary gross tumour volume (GTVp) with a mean Dice Similarity Coefficient (DSC) of 0.7 and the resulting high risk planning target volume, a mean DSC of 0.89. Both MRIs scored similarly for image quality, with the pMRI demonstrating improved registration quality and efficiency.ConclusionsA pMRI provides improvement in registration efficiency, quality and a higher degree of oncologist confidence in target delineation. These results have led to a practice change within our department, where a pMRI is acquired for all eligible oropharyngeal cancer patients.

Applications of various range shifters for proton pencil beam scanning radiotherapy

BackgroundA range pull-back device, such as a machine-related range shifter (MRS) or a universal patient-related range shifter (UPRS), is needed in pencil beam scanning technique to treat shallow tumors.MethodsThree UPRS made by QFix (Avondale, PA, USA) allow treating targets across the body: U-shaped bolus (UB), anterior lateral bolus (ALB), and couch top bolus. Head-and-neck (HN) patients who used the UPRS were tested. The in-air spot sizes were measured and compared in this study at air gaps: 6 cm, 16 cm, and 26 cm. Measurements were performed in a solid water phantom using a single-field optimization pencil beam scanning field with the ALB placed at 0, 10, and 20 cm air gaps. The two-dimensional dose maps at the middle of the spread-out Bragg peak were measured using ion chamber array MatriXX PT (IBA-Dosimetry, Schwarzenbruck, Germany) located at isocenter and compared with the treatment planning system.ResultsA UPRS can be consistently placed close to the patient and maintains a relatively small spot size resulting in improved dose distributions. However, when a UPRS is non-removable (e.g. thick couch top), the quality of volumetric imaging is degraded due to their high Z material construction, hindering the value of Image-Guided Radiation Therapy (IGRT). Limitations of using UPRS with small air gaps include reduced couch weight limit, potential collision with patient or immobilization devices, and challenges using non-coplanar fields with certain UPRS. Our experience showed the combination of a U-shaped bolus exclusively for an HN target and an MRS as the complimentary device for head-and-neck targets as well as for all other treatment sites may be ideal to preserve the dosimetric advantages of pencil beam scanning proton treatments across the body.ConclusionWe have described how to implement UPRS and MRS for various clinical indications using the PBS technique, and comprehensively reviewed the advantage and disadvantages of UPRS and MRS. We recommend the removable UB only to be employed for the brain and HN treatments while an automated MRS is used for all proton beams that require RS but not convenient or feasible to use UB.

Improved Design and Acceptance Test of Localization Couch for New Type of CT Simulator

Accurate CT simulation is the key link of precision radiotherapy, and the performance of the localization couch of CT simulator directly affects the accuracy of radiotherapy. With the rapid development of precision radiotherapy, conventional large aperture radiotherapy special CT simulator is difficult to meet the needs of precision radiotherapy localization, so most radiotherapy centers choose high-end diagnostic CT machines equipped with a flat tabletop for radiotherapy localization. In clinical work, the performance testing of the CT simulator localization couch is easy to be ignored. In addition, there are some problems such as insufficient precision in transforming the cradle-shaped couch top of diagnostic CT into a special flat couch top for radiotherapy. This paper provided an in-depth description of the improved design and performance test of the localization couch of the first special GE Revolution CT simulator for radiotherapy introduced by West China Hospital of Sichuan University. After the improvement, all the acceptance tests of the localization couch are in line with the standard, and the performance meets the high-precision radiotherapy localization needs of patients with different body weight in the center.

Assessment of dosimetric leaf gap correction factor in Mobius3D commissioning affected by couch top.

This study assesses the dosimetric leaf gap (DLG) correction factor in Mobius3D commissioning affected by a couch top platform and calculates the optimal DLG value according to the point dose difference function. DLG optimizations were performed for 3 LINAC machines and a total of 30 patient volumetric modulated arc therapy plans (i.e., 10 plans per each LINAC). Point dose calculations were performed using an automatic dose calculation system in Mobius3D as well as Mobis3D calculation using a Mobius Verification Phantom (MVP)-based quality assurance plan with a carbon fiber couch top. Subsequently, the results were compared with measurement data. The averaged point dose measured for the MVP with a couch top decreased by approximately 2% relative to that without the couch top. The average of the optimal DLG factors increased by 1.153mm due to the couch top effect for a dose decrease of 2% at the measured point. In the procedure of Mobius beam commissioning, users should adjust the DLG correction factor using a specific phantom (including MVP) with a couch top structure. If the DLG optimization were performed by using MVP automatic dose calculation system, the factor should be increased by approximately 1.2mm per 2% dose difference considering user's couch top effect.

Prediction of conical collimator collision for stereotactic radiosurgery.

The purpose of this study is to predict the collision clearance distance of stereotactic cones with treatment setup devices in cone‐based stereotactic radiosurgery (SRS). The BrainLAB radiosurgery system with a Frameless Radiosurgery Positioning Array and dedicated couch top was targeted in this study. The positioning array and couch top were scanned with CT simulators, and their outer contours of were detected. The minimum clearance distance was estimated by calculating the Euclidian distances between the surface of the SRS cones and the nearest surface of the outer contours. The coordinate transformation of the outer contour was performed by incorporating the Beam's Eye View at a planned arc range and couch angle. From the minimum clearance distance, the collision‐free gantry ranges for each couch angle were sequentially determined. An in‐house software was developed to calculate the clearance distance between the cone surface and the outer contours, and thus determine the occurrence of a collision. The software was extensively tested for various combinations of couch and arc angles at multiple isocenter locations for two combinations of cone‐couch systems. A total of 50 arcs were used to validate the calculation accuracies of the software for each system. The calculated minimum distances and collision‐free angles from the software were verified by physical measurements. The calculated minimum distances were found to agree with the measurements to within 0.3 ± 0.9 mm. The collision‐free arc angles from the software also agreed with the measurements to within 1.1 ± 1.1° with a 5‐mm safety margin for 20 arcs. In conclusion, the in‐house software was able to calculate the minimum clearance distance with <1.0 mm accuracy and to determine the collision‐free arc range for the cone‐based BrainLab SRS system.

A collision prediction framework for noncoplanar radiotherapy planning and delivery.

PurposeNoncoplanar radiotherapy can provide significant dosimetric benefits. However, clinical implementation of such techniques is not fully realized, partially due to the absence of a collision prediction tool integrated into the clinical workflow. In this work, the feasibility of developing a collision prediction system (CPS) suitable for integration into clinical practice has been investigated.MethodsThe CPS is based on a geometric model of the Linear Accelerator (Linac), and patient morphology acquired at the simulator using a combination of the planning CT scan and 3‐D vision camera (Microsoft, Kinect) data. Physical dimensions of Linac components were taken to construct a geometric model. The Linac components include the treatment couch, gantry, and imaging devices. The treatment couch coordinates were determined based on a correspondence among the CT couch top, Linac couch, and the treatment isocenter location. A collision is predicted based on dot products between vectors denoting points in Linac components and patient morphology. Collision test cases were simulated with the CPS and experimentally verified using ArcCheck and Rando phantoms to simulate a patient.ResultsFor 111 collision test cases, the sensitivity and specificity of the CPS model were calculated to be 0.95 and 1.00, respectively. The CPS predicted collision states that left conservative margins, as designed, relative to actual collision locations. The average difference between the predicted and measured collision states was 2.3 cm for lateral couch movements. The predicted couch rotational position for a collision between the gantry and a patient analog differed from actual values on average by 3.8°. The magnitude of these differences is sufficient to account for interfractional patient positioning variations during treatment.ConclusionThe feasibility of developing a CPS using geometric models and standard vector algebra has been investigated. This study outlines a framework for potential clinical implementation of a CPS for noncoplanar radiotherapy.

Implications of applicators orientations on bladder, and rectal doses in gynecological mHDR brachytherapy: A retrospective analysis of reproducibility in multi-fractionated regimen.

Variation of doses due to positional uncertainties of applicators based on orthogonal radiograph has been evaluated by several researchers. The present study has analyzed the various possibilities of special alterations of applicator placements in intracavitary brachytherapy (ICBT) and its impact on the dose to target volume, rectum, and bladder. An innovative approach has been suggested to quantify and utilize the special coordinates for reproducibility of applicator based on the pelvic bone landmark. A total of 27 fractions of 9 (n = 9) cancer cervix patients treated with external beam radiotherapy followed by ICBT have been evaluated retrospectively. The first fraction of each patient was planned as per the International Commission on Radiation Units and Measurements report 38 guidelines and was considered reference for consecutive fractions regarding dwell positions, dose to target volume, bladder, and rectum points. For each fraction, positioning of applicators regarding their spatial orientations with respect to pelvic bone landmarks and their correlation with dose to bladder and rectum was recorded and analyzed. It was found that mean angulations between (1) the two applicator points with respect to tip of central tandem (α), (2) the two applicator points with junction point situated on the sacrum bone (β) and (3) the angle between the line joining applicator points and a point defined on pelvic bone (γ) is 62.20° ± 5.74°, 37.13° ± 5.64° and 105.51° ± 6.58°, respectively. Bladder dose increased with increment in α but decreased with increase of β and γ. Dose to rectum remained unaffected for γ. Mean distance from couch top to tip of central tandem and two ovoids is, respectively, 11.17 cm and 8.70 cm. Optimal plans even with orthogonal radiographs may be generated by verifying the application based on the parameters mentioned in the present study and computed tomography-based plans could be utilized more effectively instead of doing automatic or manual optimization. Whereever orthogonal radiograph based or template based ICBT practices is performed this study may create a dataset to have an optimal treatment plan even without three-dimensional images.

18 Dosimetric study of the couch top influence in VMAT

Introduction During these last years, treatment planning system (TPS) algorithms that compute the absorbed dose have improved. A high precision between delivered absorbed dose and prescribed dose is required, limiting the uncertainties up to 5% between delivered dose and prescribed dose according to the ICRU recommendations [1] . One of the parameters contributing to these uncertainties is the couch top, which attenuates the beam from 2% to 15% when it passes through [2] . Last generation’s carbon fiber couch tops act as a bolus material, increasing skin dose while decreasing depth-dose. Therefore, the consideration of the couch top by the TPS is important for avoiding bad tumor control, overdose of organs at risk and skin radiotoxicity effects [2] . Methods The reference technique for the dose distribution calculations is based on Monte Carlo method (MC) [3] . Among the most well-known codes, BEAMnrc/EGSnrc allows to model the accelerator treatment head and to use CT images (DICOM format) in order to simulate the dose distribution to voxelized phantoms. Furthermore, the CT data have to be converted to densities and to the corresponding materials, taking into account the couch top [4] . A software developed by the team using a conversion ramp between Hounsfield Units (HU) and densities/materials, specific to the couch top, will be used to create the voxelized phantoms. Results MC method takes into consideration not only voxel densities, but also materials, if a wrong material assignment is made, it could lead to dosimetric errors in the dose distribution calculations. Different treatment sites will be considered in Volumetric Modulated Arc Therapy (VMAT). Phantoms with the couch top will be created and dose distributions will be compared to TPS dose distributions. Conclusions The results of this study contributed to evaluate different methods of taking into account the couch top, using MC.

Abstract ID: 188 Monte Carlo simulation of “out-of-field” distributions in a pediatric phantom in external radiotherapy – Couch top impact

Introduction The scattered radiation from external devices in radiotherapy may cause altered dose distribution among other complications. Although some dose corrections are made for blocking trays, ion chambers, etc. the dose perturbation by the treatment couch top scatter is often overlooked. This out-of-field scattered radiation may induce secondary cancer, especially in pediatric patients, due to biological and geometric factors. Since the out-of-field dosimetric accuracy of treatment planning systems (TPS) is known to decrease, and many of them do not provide the capability of implementing the actual treatment couch top, a PENELOPE based MC code named PRIMO was used to study the couch impact on the out-of-field dose estimation in pediatric irradiation. Material and methods A treatment plan was designed within PRIMO for a hypothetical brain tumor, using two lateral and one cranio-caudal 6MV beam configuration. The plan was created on a CT dataset of 5-year anthropomorphic pediatric phantom. The original dataset was modified with MatlabTM to: keep the original HU values inside the body contour; introduce a real treatment couch HU values and; define the pixel values outside the body and couch as air. Two simulations were performed: with and without the presence of the treatment couch. The PRIMO output data for both simulations were reshaped, manipulated and written as two single multi-frames DICOM files. The dose distribution outside the body contour in both plans was eliminated, and both dose distributions inside the body along the axial plan were compared using 3D gamma function (2%, 2 mm) on PTW-Verisoft. Results Within the treatment field, the results showed good agreement between the two dose distributions (over than 99% of points passed 3D gamma test), indicating that no significant attenuation occurred to the lateral beam crossing the treatment couch. The agreement decreased gradually far away from the isocenter to reach 93% at 20 cm away from isocenter, showing a cranio-caudal back-scattered contribution to dose from the treatment couch. Conclusion For out-of-field scattered dose estimation it is advisable to include the treatment couch in dose calculations.