Modeling of couch transmission in the RayStation treatment planning system

Commissioning of a RayStation structure template for the iBEAM evo Couchtop

Accurate radiotherapy treatment planning requires attenuation through the treatment couch to be accounted for in dose calculation. This is commonly performed by using contouring tools to add a virtual structure in the shape of the treatment couch and assigning the preferred absorption properties. The RayStation treatment planning system (TPS) allows users to assign a material that comprises both an elemental structure and a physical density. The selection of such parameters should be made so that modelled attenuation through the couch closely matches measured data. When these measurements involve the use of plastic phantoms and rotational beams, the validity of the data is dependent upon aspects of TPS and linear accelerator performance that can be difficult to quantify. A fundamental measure of couch attenuation using an ionisation chamber in water and perpendicular beam geometry that required no gantry movement was implemented to eliminate the identified uncertainties. This data was used to determine the combination of elemental composition and density assigned to a modelled couch structure that provided the most accurate representation of beam attenuation in this simple geometry. The preferred material was then validated using a cylindrical phantom and rotational beams. The findings were equivalent between the static gantry with water phantom and rotating gantry with cylindrical phantom. Of the elemental compositions investigated, it was possible to achieve suitable agreement with the measured data for each option provided the density was optimised. Choice of the elemental composition was not observed to be an important factor in achieving a good model.

Development of a device that remotely removes a mask in the head and neck immobilization system: a prototype and demonstration experiment

In this study, a prototype device was developed to quickly remove the mask used to immobilize the head and neck by remotely releasing the quick fasteners. As a first step in investigating the usefulness of this prototype, we performed repeated removal tests and examined the accuracy of dose calculation. The results showed that the quick-release fasteners of a Type-S system (CIVCO Medical Solutions, Iowa, USA) could be removed remotely and accurately (success rate: 100%). Additionally, the dose errors in treatment planning were negligible (< 1.0%), and the gamma pass rate was equivalent (99.9%). Therefore, this prototype device with a remote system would help manage patient safety in emergencies, such as a disaster or a sudden change in the patient's condition. However, age-related deterioration with long-term clinical use or its ability to link with beam-off still requires further exploration.

Dose estimation for cone-beam computed tomography in image-guided radiation therapy using mesh-type reference computational phantoms and assuming head and neck cancer

This study aimed to estimate the additional dose the cone-beam computed tomography (CBCT) system integrated into the Varian TrueBeam linear accelerator delivers to a patient with head and neck cancer using mesh-type International Commission on Radiological Protection reference computational phantoms. In the first part, for use as a benchmark for the accuracy of the Monte Carlo geometry of CBCT, Particle and Heavy Ion Transport code System (PHITS) calculations were confirmed against measured lateral and depth dose profiles using a computed tomography dose profiler. After obtaining good agreement, organ dose calculations were performed by PHITS using mesh-type reference computational phantom (MRCP) and irradiating the neck region; the effective dose was calculated utilising absorbed organ doses and tissue weighting factors for male and female MRCP. Substantially, it has been found that the effective doses for male and female MRCP are 0.81 and 1.06 mSv, respectively. As this study aimed to assess the imaging dose from the CBCT system used in image-guided radiation therapy, it is required to take into account this dose in terms of both the target organ and surrounding tissues. Although the absorbed organ dose values and effective dose values obtained for both MRCP males and females were small, attention should be paid to the additional dose resulting from CBCT. This study can create awareness on the importance of doses arising from imaging techniques, especially CBCT.

The root cause analysis on failed patient-specific measurements of pencil beam scanning protons using a 2D detection array with finite size ionization chambers

The aim of this report is to present the root cause analysis on failed patient‐specific quality assurance (QA) measurements of pencil beam scanning (PBS) protons; referred to as PBS‐QA measurement. A criterion to fail a PBS‐QA measurement is having a <95% passing rate in a 3.0%‐3.0 mm gamma index analysis. Clinically, we use a two‐dimensional (2D) gamma index analysis to obtain the passing rate. The IBA MatriXX PT 2D detection array with finite size ionization chamber was utilized. A total of 2488 measurements performed in our PBS beamline were cataloged. The percentage of measurements for the sites of head/neck, breast, prostate, and other are 53.3%, 22.7%, 10.5%, and 13.5%, respectively. The measurements with a passing rate of 100 to >94%, 94 to >88%, and <88% were 93.6%, 5.6%, and 0.8%, respectively. The percentage of failed measurements with a <95% passing rate was 10.9%. After removed the user errors of either re‐measurement or re‐analysis, 8.1% became acceptable. We observed a feature of >3% per mm dose gradient with respect to depth on the failed measurements. We utilized a 2D/three‐dimensional (3D) gamma index analysis toolkit to investigate the effect of depth dose gradient. By utilizing this 3D toolkit, 43.1% of the failed measurements were improved. A feature among measurements that remained sub‐optimal after re‐analysis was a sharp >3% per mm lateral dose gradient that may not be well handled using the detector size of 5.0 mm in‐diameter. An analysis of the sampling of finite size detectors using one‐dimensional (1D) error function showed a large dose deviation at locations of low‐dose areas between two high‐dose plateaus. User error, large depth dose gradient, and the effect of detector size are identified as root causes. With the mitigation of the root causes, the goals of patient‐specific QA, specifically detecting actual deviation of beam delivery or identifying limitations of the dose calculation algorithm of the treatment planning system, can be directly related to failure of the PBS‐QA measurements.

Dosimetric Evaluation of the QFix kVueTM Calypso Couch Top

PURPOSE

To evaluate the dosimetric accuracy of the default couch model of the QFix kVueTM Calypso couch top in the treatment planning system.

METHODS

With the gantry 180°, field size 20 × 20 cm, 6 MV, we measured the depth dose, off-axis dose, and dose plane of different depths in the phantom with the couch rails in and out, respectively. Isocenter doses at different angles were also obtained. The results were compared to the doses calculated using the default couch top model and the real scanned couch top model. Then we revised the default model according to the measured results.

RESULTS

With "Rails In," the depth dose, off-axis dose, and dose plane of the default couch top model had a big difference with the dose of the real scanned couch top model and the measured result. The dose of the real scanned couch top model was much closer to the measured result, but in the region of the rail edge, the difference was still significant. With "Rails Out," there was a minor difference between the measured result, the dose of the default couch top model and the real scanned couch top model. The difference between the measurement and the default couch top model became very small after being revised.

CONCLUSIONS

It is better to avoid the beam angle passing through the couch rails in treatment plans, or you should revise the parameter of the QFix kVueTM Calypso couch top model based on the measured results, and verify the treatment plan before clinical practice.

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 1.153 mm 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 1.2 mm per 2% dose difference considering user's couch top effect.

[Development of an In-house Couch Model to Improve Dose Attenuation Correction Accuracy for a Couch with Different Thickness in the Superior-inferior Direction]

As the couch used in external radiation therapy attenuate radiation by interaction, it is necessary to correct attenuation of radiation by inserting a couch model in the treatment planning systems. For a couch whose thickness is different in the superior-inferior direction, it is possible to perform dose calculations with an error within ±1% by using separate different couch models provided by vendors. However, it is difficult to correct attenuation correction accurately with a single couch model. In this study, we created an in-house couch model which can set couch shape and physical density in detail by acquiring CT images of actual couch. When we performed dose calculation by optimizing the physical densities of in-house and vendor couch, it was found that the difference between the measured and the calculated values can be significantly reduced by using in-house couch model. Additionally, by using in-house couch model, it is found that the dose attenuation can be corrected within ±1% for a couch whose thickness is different in the superior-inferior direction.

Improvement of VMAT plan quality for head and neck cancer with high resolution fluences generated by couch shift between arcs

PURPOSE

To investigate the changes in quality of the volumetric modulated arc therapy (VMAT) plans with couch-shift between arcs by half of a multi-leaf collimator (MLC) leaf width.

METHODS

A total of 22 patients with head-and-neck cancer were retrospectively selected. Since the smallest MLC leaf width was 5 mm in this study, the couch was shifted by 2.5 mm in the longitudinal-direction between arcs to increase the resolution of fluence map. A total of three types of VMAT plans were generated for each patient; the three types of plans were a two-full-arc plan without couch-shift (NS plan), a two-half-arc-pair plan with couch-shift (HAS plan), and a two-full-arc pair plan with couch-shift (FAS plan). Changes in the dose-volumetric parameters were investigated.

RESULTS

The FAS plan showed the best plan quality for the target volumes and organs at risk compared to the NS and HAS plans. However, the magnitudes of differences among the three types of plans were minimal, and every plan was clinically acceptable. The average integral doses of the NS, HAS, and FAS plans were 160,549 ± 37,600 Gy-cc, 147,828 ± 33,343 Gy-cc, and 156,030 ± 36,263 Gy-cc, respectively. The average monitor unit of the NS, HAS, and FAS plans were 717 ± 120 MU, 648 ± 100 MU, and 763 ± 158 MU, respectively.

CONCLUSIONS

The HAS plan was better than the others in terms of normal tissue sparing and plan efficiency. By shifting the couch by half of the MLC leaf width in the longitudinal direction between arcs, the VMAT plan quality could be improved.

Quantification and comparison the dosimetric impact of two treatment couch model in VMAT

The use of Monte Carlo treatment planning systems (TPS) in radiation therapy has increased the dosimetric accuracy of VMAT treatment sequences. However, this accuracy is compromised by not including the treatment couch into the treatment planning process. Therefore, the impact of the treatment couch on radiation delivery output was determined, and two different couch models (uniform couch model A vs two components model B) were included and tested in the Monaco TPS to investigate which model can better quantify the couch influence on radiation dose. Relative attenuation measurements were performed following procedures outlined by TG‐176 with three phantom positions for A–B direction: on the left half (L), in the center (C) and on the right half (R) of the couch. As well as absolute dose comparison of static fields of 10 × 10 cm² that were delivered through the couch tops with that calculated in the TPS with the couch model at 2 mm and 5 mm computing grid size respectively. The most severe percentage deviation was 4.60% for the phantom positioned at the left half of the couch with 5 mm grid size at gantry angle 120°. The couch model was included in the TPS with a uniform ED of 0.26 g/cm³ or a two component model with a fiber 0.52 g/cm³ and foam core 0.1 g/cm³. After including the treatment couch, the maximum mean dose attenuation was reduced from 3.68% without couch included to (0.60, 0.83, 0.72, and 1.02) % for model A and model B at 2 and 5 mm voxel grid size. The results obtained showed that Model A performed better than the model B, demonstrating lower deviations from measurements and better robustness against dose grid resolution changes. Considering the results of this study, we propose the systematic introduction of the couch Model A in clinical routine. All the reported findings are valid for the Elekta iBEAM^® evo Extension 415 couch and these methods can also be used for other couch model.