Convolution Superposition Algorithm Research Articles

Evaluation of Dosimetric and Radiobiological Parameters for Different TPS Dose Calculation Algorithms and Plans for Lung Cancer Radiotherapy

Lung cancer presents a major public health concern in our country and around the globe. Radiotherapy is one of the main treatment modalities for lung cancer management for several years. This study aims to evaluate differences in the dosimetric and radiobiological parameters and in the dose distributions of Planning Target Volume (PTV) and organs at risk (OAR) in patients with lung tumors using different Treatment Planning System (TPS) algorithms and Volumetric Modulated Arc Therapy (VMAT) technique. This study was accomplished in a group of 19 patients with lung tumors who were treated in our clinic. In the treatment planning of the patients; Elekta-Monaco with Monte Carlo (XVMC), Pencil Beam algorithm; Varian-Eclipse with Anisotropic Analytical Algorithm (AAA), Acuros XB (AXB)algorithms and Tomo-plan Treatment Planning System with Convolution Superposition algorithm of Tomotherapy device were used. In these treatment planning systems, plans were done by 6 MV photon energy using Volumetric Arc Therapy (VMAT) techniques. The prescribed dose to the PTV was 60 Gy in 30 fractions. Statistical analysis was performed using SPSS Statistics v.29.0.2.0 programme. Statistically, significant differences were found in Dmean, Dmax, D2 and D98 values for PTV between the algorithms, while small differences were found in Dmax values of the contralateral lung, total lung and esophagus in critical organs. It is concluded that the difference between algorithms for PTV increases especially as the volume of the target tumor decreases. TPS with C/S algorithm gave closer results with XVMC. Algorithms were found to have an impact on radiobiological parameters.

Evaluation of monte carlo to support commissioning of the treatment planning system of new pencil beam scanning proton therapy facilities

ABSTRACT Objective. To demonstrate the potential of Monte Carlo (MC) to support the resource-intensive measurements that comprise the commissioning of the treatment planning system (TPS) of new proton therapy facilities. Approach. Beam models of a pencil beam scanning system (Varian ProBeam) were developed in GATE (v8.2), Eclipse proton convolution superposition algorithm (v16.1, Varian Medical Systems) and RayStation MC (v12.0.100.0, RaySearch Laboratories), using the beam commissioning data. All models were first benchmarked against the same commissioning data and validated on seven spread-out Bragg peak (SOBP) plans. Then, we explored the use of MC to optimise dose calculation parameters, fully understand the performance and limitations of TPS in homogeneous fields and support the development of patient-specific quality assurance (PSQA) processes. We compared the dose calculations of the TPSs against measurements (DDTPSvs.Meas.) or GATE (DDTPSvs.GATE) for an extensive set of plans of varying complexity. This included homogeneous plans with varying field-size, range, width, and range-shifters (RSs) (n = 46) and PSQA plans for different anatomical sites (n = 11). Main results. The three beam models showed good agreement against the commissioning data, and dose differences of 3.5% and 5% were found for SOBP plans without and with RSs, respectively. DDTPSvs.Meas. and DDTPSvs.GATE were correlated in most scenarios. In homogeneous fields the Pearson’s correlation coefficient was 0.92 and 0.68 for Eclipse and RayStation, respectively. The standard deviation of the differences between GATE and measurements (±0.5% for homogeneous and ±0.8% for PSQA plans) was applied as tolerance when comparing TPSs with GATE. 72% and 60% of the plans were within the GATE predicted dose difference for both TPSs, for homogeneous and PSQA cases, respectively. Significance. Developing and validating a MC beam model early on into the commissioning of new proton therapy facilities can support the validation of the TPS and facilitate comprehensive investigation of its capabilities and limitations.

Evolution of 3D X-Ray Dose Computation Algorithms

Radiation treatment planning of individual cancer patients relies on the accurate computation of dose distributions in irradiated tissue. Inaccurate dose maps have the potential to mislead clinical decision-making and compromise the balance between effective tumour control and side effects in surrounding normal tissue. In the context of this conference, 3D dosimetry is important for the experimental validation of computed dose distributions. Dose computation methods for external beams of high energy x rays have evolved over the past decade with computer simulation models more closely aligned with the fundamental physics of x-ray scattering and absorption in heterogeneous tissue. In this article, we first present a historical review from a Canadian perspective, followed by an introductory intuitive description of contemporary algorithms used in clinical treatment planning: (1) Convolution-superposition algorithm fundamentally based on the Green’s function method; (2) Stochastic Monte Carlo simulation of x-ray interactions in tissue, and (3) Deterministic numerical solution of a system of Boltzmann transport equations. In principle, all these methods solve the same problem of predicting x-ray scattering and absorption in heterogeneous tissue. However, the mathematical tools differ in their approach and approximations to achieve sufficient speed for routine clinical application. In the conclusion of this article, the evolution of 3D x-ray dose computation is summarized, in terms of accuracy and computational speed.

From Protons to Picasso: Recreating Famous Paintings Using Proton Beams as a “Paintbrush”

To many, medicine is just as much an artform as it is a science. From the colorful stains used in diagnostic pathology, to the tactile sculpting of surgery, to the dose painting in radiation oncology, there is beauty and art to be found across all disciplines. In pencil-beam-scanning proton therapy, the dose is painted layer-by-layer, spot-by-spot, allowing for significantly more control compared to conventional radiation. This work intends to showcase the immense capability of intensity-modulated proton therapy (IMPT) through the recreation of some of the most renowned artworks throughout history, and has potential applications in patient education, medical/physics education, and quality assurance. Five (5) well-recognized paintings spanning from the Golden Age to the modern era were recreated as treatment plans in the Eclipse (version 16.1) External Beam Planning environment. For each "painting" (treatment plan), clinical protons within the range of 70-250 MeV were used to deposit "paint" (radiation dose) onto a "canvas" (water phantom) to a prescription of 100 Gy. The individual elements of each artwork were first delineated as separate structures. Colors were then assigned to appropriate isodose lines to mimic the corresponding palettes found in the original artworks. For each treatment plan, a single field was used along with inverse optimization to deposit the different doses at a depth of 10 centimeters. The resulting dose distribution was calculated with the proton convolution-superposition algorithm used in clinical practice. The dose distributions of the "paintings" demonstrated a marked semblance to the original artworks with sufficient resolution to elucidate fine details. The "paintings" varied in dimension from 20-30 cm at isocenter and on average took between 5-6 hours to complete. The number of structures used for planning ranged from 6-65 depending on the style and complexity of the original artwork. For a fractional dose of 200 cGy, the number of monitor units ranged from 73,145-269,970. With the advent of IMPT and modern planning techniques, we can now realize increasingly complex treatment plans, allowing for the creation of dose distributions that are highly targeted and specific. This is demonstrated by the capability to create recognizable works of art using proton beams as a "paintbrush".

A convolution-superposition fluence model for the Siemens HD120 multi leaf collimator with application to a 3D VMAT dose engine

Purpose. To construct a fast-calculating fluence modelfor the Siemens HD120 multi leaf collimator (MLC) using convolution-superposition techniques, and to develop a 3D VMAT dose engine using this fluence model. This work offers analternative to time-consuming open-source Monte Carlo simulations for thosedeveloping in-house dose-calculating software for research or clinical needs.Methods. EPID-acquired images of sweeping-window and sweeping-checker field profiles were used to commission transmission, 2 Dinterleaf leakage, and tongue-and-groove maps specific to the HD120 MLC. These maps, along with a 2D head-scattermodel were incorporated into a convolution-superposition algorithm to provide a fluence model for the HD120 MLC. This fluence model was used to develop a 3D VMAT dose engine, where 3D pre-computed 6MV dose kernels (EGSnrc) and a 3D fluence curvature-correction map were incorporatedto calculate 3D VMAT doses in a 22 cm diameter cylindrical phantom. Four VMAT patient plans witha large range of PTV sizes (36 cc to 604 cc) were chosen to test the fluence model and dose engine. Results. Excellent agreement was observed between the simulated commissioning fields and measured EPID-responses. 2D 2%/2 mm gamma analysis yielded a 98.9% pass rate for 1 cm, 2 cm, and 4 cm sweeping-window fields. 2D 2%/2mm gamma analysis for outer/inner MLC leaves yielded 89.1%/77.0% and 95.2%/91.1% pass rates from 1 cm and 2 cm sweeping-checker fields. Mean 3%/3 mm gamma analysis showed excellent agreement between our dose engine and Eclipse (Acuros) regardless of PTV size: 98.7% pass rate, with 95.1% pass rate in the high-dose volume. Fluence calculation times were13.6 seconds per dynamic MLC field and 1.4 minutes/arc for 3D VMAT dose on a standard PC. Conclusions. A fast-calculating convolution-superposition fluence model has been commissioned for the Siemens HD120 MLC and incorporatedinto a 3D VMAT dose engine. This work can be used to facilitate the development of fast in-house dose-calculating software.

Accuracy Evaluation of Collapsed Cone Convolution Superposition Algorithms for the Nasopharynx Interface in the Early Stage of Nasopharyngeal Carcinoma.

This study combined the use of radiation dosimeteric measurements and a custom-made anthropomorphic phantom in order to evaluate the accuracy of therapeutic dose calculations at the nasopharyngeal air-tissue interface. The doses at the nasopharyngeal air-tissue interface obtained utilizing the Pinnacle and TomoTherapy TPS, which are based on collapsed cone convolution superposition (CCCS) algorithms, were evaluated and measured under single 10 × 10 cm2, 2 × 2 cm2, two parallel opposed 2 × 2 cm2 and clinical fields for early stage of nasopharyngeal carcinoma by using EBT3, GR-200F, and TLD 100. At the air-tissue interface under a 10 × 10 cm2 field, the TPS dose calculation values were in good agreement with the dosimeter measurement with all differences within 3.5%. When measured the single field 2 × 2 cm2, the differences between the average dose were measured at the distal interface for EBT3, GR-200F, and TLD-100 and the calculation values were -15.8%, -16.4%, and -4.9%, respectively. When using the clinical techniques such as IMRT, VMAT, and tomotherapy, the measurement results at the interface for all three techniques did not imply under dose. Small-field sizes will lead to dose overestimation at the nasopharyngeal air-tissue interface due to electronic disequilibrium when using CCCS algorithms. However, under clinical applications of multiangle irradiation, the dose errors caused by this effect were not significant.

PO-1642 Validation of photon convolution-superposition algorithm applied to daily in vivo QC in radiotherapy

PO-1642 Validation of photon convolution-superposition algorithm applied to daily in vivo QC in radiotherapy

Extending in aqua portal dosimetry with dose inhomogeneity conversion maps for accurate patient dose reconstruction in external beam radiotherapy.

Background and purposeIn aqua dosimetry with electronic portal imaging devices (EPIDs) allows for dosimetric treatment verification in external beam radiotherapy by comparing EPID-reconstructed dose distributions (EPID_IA) with dose distributions calculated with the treatment planning system in water-equivalent geometries. The main drawback of the method is the inability to estimate the dose delivered to the patient. In this study, an extension to the method is presented to allow for patient dose reconstruction in the presence of inhomogeneities.Materials and methodsEPID_IA dose distributions were converted into patient dose distributions (EPID_IA_MC) by applying a 3D dose inhomogeneity conversion, defined as the ratio between patient and water-filled patient dose distributions computed using Monte Carlo calculations. EPID_IA_MC was evaluated against dose distributions calculated with a collapsed cone convolution superposition (CCCS) algorithm and with a GPU‐based Monte Carlo dose calculation platform (GPUMCD) using non-transit EPID measurements of 25 plans. In vivo EPID measurements of 20 plans were also analyzed.ResultsIn the evaluation of EPID_IA_MC, the average γ-mean values (2% local/2mm, 50% isodose volume) were 0.70 ± 0.14 (1SD) and 0.66 ± 0.10 (1SD) against CCCS and GPUMCD, respectively. Percentage differences in median dose to the planning target volume were within 3.9% and 2.7%, respectively. The number of in vivo dosimetric alerts with EPID_IA_MC was comparable to EPID_IA.ConclusionsEPID_IA_MC accommodates accurate patient dose reconstruction for treatment disease sites with significant tissue inhomogeneities within a simple EPID-based direct dose back-projection algorithm, and helps to improve the clinical interpretation of both pre-treatment and in vivo dosimetry results.

The Effect of Various Dose Normalization Strategies When Implementing Linear Boltzmann Transport Equation Dose Calculation for Lung Stereotactic Body Radiation Therapy Planning

PurposeTo explore implications of various plan normalizations when implementing a linear Boltzmann transport equation solver dose calculation algorithm (LBTE) for lung stereotactic body radiation therapy (SBRT). Methods and MaterialsEighty-seven plans originally calculated with a convolution-superposition algorithm (CS) were recalculated with LBTE and normalized in 3 ways: prescription covering 95% of planning target volume (PTV), 99% of internal target volume (ITV), and keeping the original planned PTV coverage. Effect on delivered dose after implementing the new algorithm was quantified using change in total monitor units for each renormalization strategy. Treatment planning system–reported changes in PTV, ITV, and organ at risk (OAR) doses were also quantified, along with the feasibility of LBTE plans to meet institutional OAR planning objectives. ResultsLBTE renormalization resulted in monitor unit increases of 7.0 ± 8.8%, 0.31 ± 5.8%, and 7.9 ± 8.6% when normalizing to the PTV D95%, ITV D99%, and planned coverage, respectively. When normalizing to PTV D95%, the LBTE reported increased PTV and ITV D1% (Gy) relative to CS (median, 3.4% and 3.2%, respectively), and normalizing to ITV D99% showed a median 1.9% decrease. For LBTE plans, reported OAR doses were increased when normalizing to PTV D95% or planned coverage (median chest wall V30 Gy [cc] increase of 0.85 and 1.7 cc, respectively) and normalizing to ITV D99% resulted in decreased dose (median chest wall V30 Gy [cc] decrease of 1.8 cc). LBTE plans normalized to PTV D95% showed inferior ability to meet the OAR objectives, but reoptimizing kept the objectives manageable while maintaining PTV coverage. ConclusionsWhen transitioning from CS to LBTE dose calculation for lung SBRT, maintaining a PTV coverage-based normalization generally results in increased dose delivered relative to CS and increased reported target and OAR dose. In cases where PTV normalization results in unacceptably high doses to targets or OARs, normalizing based on ITV coverage can be considered to maintain similar target dose as CS.

Dosimetric Impact of Systematic and Random Errors With Multi-Leaf Collimator Leaf Positioning in Intracranial Stereotactic Radiosurgery

Modern medical Linac equipped with multileaf collimators (MLC) are widely used for intracranial stereotactic radiosurgery (SRS). This study aimed to evaluate dosimetric impact of systematic and random positioning errors with MLC leaves and to establish practical limits on MLC leaf positioning uncertainties in SRS treatments.A single institution retrospective review identified 10 clinical single-target conformal arc SRS plans and 7 single- or multiple-target VMAT SRS plans treating a total of 15 PTVs. Each conformal arc SRS plan was prescribed 20 Gy in one fraction to a PTV using 2-5 arc fields with PTV volume ranging from 0.10 to 2.48 cc, and each volumetric modulated arc therapy (VMAT) SRS plan was prescribed 20 - 27 Gy in 1 - 5 fractions to 1 - 4 PTVs, with PTV volume ranging from 0.11 to 7.73 cc. 6-MV flattening-filter free photon fields were used in all the plans. A 120-leaf MLC was used for beam modulation with a leaf width of 2.5 mm for the central 32 leaf-pairs and 5.0 mm for the peripheral 28 leaf-pairs. Systematic or random leaf positioning errors ranging from 0.1 to 1.0 mm at 0.1-mm intervals were introduced to each SRS plan. Plan dose was re-calculated with original field MUs with a 1.0 mm calculation resolution using a linear Boltzmann transport algorithm and was verified using another dose calculation engine with a convolution superposition algorithm. Variations in PTV dosimetric parameters including mean dose and D(95%) due to systematic and random leaf positioning errors were analyzed. PTV dose variation was also verified with measurement using an ion chamber in water equivalent phantom.Both the PTV mean dose and D(95%) decreased linearly with increasing systematic and random positioning errors in all the SRS plans. In systematic leaf positioning errors with a 1-mm shift towards the isocenter for all MLC leaves, the PTV mean dose and D(95%) decreased by an average of 8.7% (range: 5.1 - 12.4%) and 15.2% (range: 10.4 - 20.1%) respectively in the conformal arc plans; the PTV mean dose and D(95%) decreased by an average of 18.2% (range: 11.6 - 25.0%) and 21.8% (range: 15.1 - 27.3%) respectively in the VMAT plans. The PTV volume positively correlated with the declining rate of mean PTV dose (correlation coefficient: 0.80, P-value = 0.01) with the conformal arc SRS plans. In random leaf positioning errors, with a standard deviation of up to 1 mm, the decrease of PTV mean dose and D(95%) was up to 2.4% and 3.3% respectively in the conformal arc plans, and decreased up to 2.7% and 2.8% respectively in the VMAT plans.Manufacturers' specifications for MLC leaf positioning accuracy are not adequate for accurate intracranial SRS dose delivery. To ensure that both the PTV mean dose and D(95%) are within 5% from plan dose, the systematic MLC leaf positioning error needs to be < 0.3 mm for conformal arc SRS plans and < 0.2 mm for VMAT SRS plans. On the other hand, random leaf positioning errors have smaller impact on dose delivery accuracy within the limits specified by manufacturers.C. Han: None. J.R. Liu: None. D. Du: None. J. Liang: None. K. Qing: None. W.T. Watkins: None. S. Zhang: None. A. Liu: None.

Dosimetric Comparison in Malignant Glioma Patients Clinically Treated on Hybrid Magnetic Resonance Imaging (MRI)-Linac (MRL) vs. Conventional Linac

The Magnetic Resonance Imaging (MRI)-Linac (MRL) is a hybrid machine integrating a high field strength 1.5T MRI with a linear accelerator, providing superior soft tissue visualization of tumors and organs-at-risk (OARs) during treatment delivery. A special consideration for MRL radiotherapy is accounting for interactions of secondary electrons generated within the magnetic field, which can alter dose deposition at air-tissue interfaces. We evaluated dosimetric outcomes in clinically treated malignant glioma patients who received at least one fraction of radiotherapy on both the MRL and a conventional Linac.Thirty-seven glioma patients treated on both the MRL and a conventional Linac for adjuvant chemoradiotherapy between July 2019 and February 2021 were analyzed. Planning was completed on treatment planning systems (TPS) using a Monte-Carlo algorithm that accounts for magnetic field effects (Monaco v5.40) for the MRL, and a convolution-superposition algorithm (Pinnacle v9.8) for the conventional Linac. Dosimetric parameters of interest from the target, OARs, and air-tissue interface volumes for each patients' clinical treatment plans were extracted and compared. For 10 representative patients, in vivo skin doses during a single fraction of MRL and conventional Linac treatment were obtained using an Optically Stimulated Luminescent Dosimeter (OSLD) placed in a defined location on the patient's skin near the Planning Target Volume (PTV). Student's t-test and Wilcoxon signed-rank test were used to compare parameters between Monaco and Pinnacle. Spearman's correlation was used to assess the relationship between in vivo OSLD measurements and TPS skin dose. Threshold for statistical significance was P < 0.05.Most patients were treated for high grade glioma (76% Grade III or IV, 24% Grade II), and median PTV was 257.4 cm3 (range, 37.1-570.3 cm3). MRL and conventional Linac had similar V100, V95, D98, and D95 for PTV, and D3cc for optic chiasm, optic nerves, and each cochlea (P = NS). However, clinically delivered Monaco plans had significantly greater doses within air cavities (mean Dmean higher by 1.3 Gy, P < 0.0001) and skin (mean Dmean higher by 1.9 Gy, P < 0.0001; mean D2cc higher by 8.1 Gy, P < 0.0001; mean V20 Gy higher by 7.2 cm3, P < 0.0001), compared to clinically delivered Pinnacle plans. In vivo OSLD skin readings were 14.5% greater for treatments delivered on the MRL (P = 0.0027), and were more accurately predicted by Monaco (r = 0.95, P < 0.0001) vs. Pinnacle (r = 0.80, P = 0.0096).In this prospective study of clinically treated glioma patients on both MRL and conventional Linac, the dosimetric impact of the magnetic field was minimal for the target and standard OARs. However, higher doses to skin and air cavities were observed. In vivo correlation of dose to skin was more accurately predicted with Monaco. Future MRL planning processes are being designed to account for skin dosimetry and treatment delivery.

Dosimetric Comparison of Single-Isocenter and Multiple-Isocenter Techniques for Two-Lesion Lung SBRT Using the RefleXion High-Speed Ring-Gantry System

Due to the technical challenges that accompany stereotactic body radiotherapy (SBRT) to multiple lesions, a single-isocenter treatment plan technique for multi-target scenarios may optimize the final, aggregate dose distribution. The RefleXion™ X1 is a new ring-gantry based system with several features that may optimize dosimetry in a multi-target SBRT scenario, including high speed rotation (60 revolutions per minute), a high-speed multi-leaf collimator (100 Hz), and a variable pitch couch. A comprehensive dosimetric comparison for multiple targets that contrasts multiple isocenters versus a single-isocenter technique has not been performed. Therefore, this investigation performed such a comparison in two-lesion lung SBRT cases.Three patients with two synchronous lung lesions with maximum dimension of 5 cm underwent single-isocenter and two-isocenter planning with the RefleXion treatment planning system using a Collapsed Cone Convolution Superposition algorithm. Treatment doses were in accordance with AAPM TG101 criteria (50Gy in 5 fractions) and planned such that 95% of the planning target volume (PTV) would receive at least 95% of the prescription dose. In the single-isocenter plans, the isocenter was placed between the two-lesions, whereas the two-isocenter plans localized the isocenters on each target with a limitation on the IEC-X direction to avoid couch-bore collisions. Conformity index (CI), homogenous index (HI), R50%, and D2cm were calculated. The volume of the normal lung receiving 5 Gy (V5), V10, V15, V20, mean lung dose, and dose to other OARs were evaluated. In addition, the treatment time to deliver each plan was recorded.The mean isocenter to the tumor distance in XYZ-space was 8.53 cm (range: 7.66-10.62 cm). The mean combined PTV was 42.47cc (range: 9.7- 97.8 cc). There was no significant difference in target coverage, maximum target dose, CI, HI, R50%, V20, V15, and V10 between single-isocenter and two-isocenters for lung SBRT plans. On average, single-isocenter plans contributed an increase in OAR dose, especially to the carina and esophagus, by 5.17% (range: 2.91-6.52%) and 15.62% (range: 6.44-22.97%), respectively. With the RefleXion X1 set at 850MU/min, the average difference in total beam-on time was approximately 33 seconds between the two approaches. However, set-up time for the second isocenter in the two-isocenter plan would likely prolong the total treatment delivery time.This study shows that a single-isocenter technique for the scenario of two lung lesions is dosimetrically feasible and clinically efficient with the RefleXion X1 in SBRT mode. Single-isocenter treatment delivery techniques may be a desirable option for efficient radio-ablative therapies directed at multiple metastatic targets.

Investigation of Buildup Region and Surface Dose: Comparison of Parallel Plane Ion Chamber, Treatment Planning System, and MC Simulation

In these days, Monte Carlo (MC) simulation is a method that can calculate the radiation dose that occurs in an environment in the most accurate way. The correct measurement of the dose occurring on the patient’s surface is of great importance to estimate the reactions that may occur on the patient’s skin. This importance encouraged us to do this study. The aim of this study is to determine buildup region and surface doses using MC simulation and to compare them with results of the parallel plane ion chamber and Treatment Planning System (TPS) measurements for 6-MV photon beams. Surface doses normalized to the maximum dose for the parallel plane ion chamber, MC simulation, fast photon (FP) algorithm, and collapsed cone convolution superposition (CC) algorithm are 13.6%, 30.28%, 0%, and 27.33%, respectively. The CC algorithm and parallel plane ion chamber measurements are compatible with MC simulation but the FP algorithm has calculated the dose less to a depth of 0.8 cm. Measuring the surface dose and the doses in the buildup region is of great importance in terms of accurately predicting the complications that may occur in the patient’s skin and taking precautions early. Using some methods and correction factors, the surface dose and the doses that may occur in the buildup region can be accurately calculated. It is recommended not to use the FP algorithm for stereotactic body radiation therapy and intensity-modulated radiation therapy treatments, as it cannot calculate doses correctly in the buildup region and surface.

Comparison of Skin Dose for Brain Cancer Patients Between a 1.5 T MR-Linac and a Conventional Linac

The MR-Linac (MRL) is a new technology, and due to the presence of the electron return effect (ERE) elicited by the strong magnetic field, there is concern about there being elevated surface dose due to the ERE. The hypothesis is that the skin doses for the MRL is greater than for the conventional Linac. Six brain cancer patients being treated on our MRL were included in the present study and were planned with an IMRT technique using the MRL’s Monte Carlo based treatment planning system (TPS-MRL). Each patient had a back-up IMRT 6 MV plan for a conventional (no magnetic field) Linac generated on our standard TPS, which used a collapsed cone-based dose engine (TPS-conventional). These back-up plans were clinically delivered during an MRL preventative maintenance day, which provided an opportunity to compare skin dose measurements between MRL and conventional Linac. For one fraction on each Linac, an optically stimulated luminescence dosimeter (OSLD) was placed at the same location on the skin proximal to the target. Within TPS-MRL and TPS-conventional, a dose point was created at the OSLD location and at a skin depth equivalent to the water equivalent thickness of the OSLD (0.6 mm). Five of six patients had higher OSLD readings on the MRL than for the conventional Linac, ranging from 11.5%-24.8% higher (the one exception had the MRL 7.5% less than the conventional Linac). The OSLD measurement for the MRL ranged from 11.8-45.7 Gy; the OSLD doses for the conventional Linac ranged from 10.0-39.4 Gy. The root mean square error between measurement and TPS was 7.3% and 17.4% for MRL and conventional, respectively. This may be explained because the conventional Linac TPS uses a superposition-convolution algorithm, and the MRL TPS uses a theoretically more accurate Monte Carlo algorithm. Skin dose, as determined in the present study with in vivo OSLD measurements, were higher on the MRL than a conventional Linac for brain patients. Furthermore, the skin dose as estimated by the TPS-MRL is more accurately modeled compared to the TPS-conventional. Although the exact cause(s) of the apparent skin dose enhancement can only be speculated, the potential implications of the measurements in the present study include enhanced adverse skin reactions on the MRL compared to conventional Linac. However, the fact that the MRL-TPS models the surface dose with a Monte Carlo dose engine can enable clinicians to incorporate strategies for reducing surface dose at the planning stage.

Effects of gas-filled temporary breast tissue expanders on radiation dose from modulated rotational photon beams

Gas-filled temporary tissue expanders (TTEs), implanted to assist in post mastectomy breast reconstructions, are expected to produce increased dosimetric uncertainty in breast radiotherapy treatments, due to their containing both a substantial metallic component and a comparatively large volume of gas. This study therefore builds on previous investigations of the dosimetric effects of gas-filled TTEs in static photon and electron beams, by examining the effects of these implants on dose distributions from common modulated rotational treatment techniques; volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT). Radiochromic film measurements were used to evaluate the accuracy of VMAT and HT dose calculations, for a humanoid phantom augmented with a sample Aeroform CO2-filled TTE (AirXpanders Inc, San Jose, USA) as well as purpose-designed and 3D printed “breast tissue.” Results showed that the TomoTherapy Hi-Art VoLO convolution-superposition algorithm (Accuray Inc, Sunnyvale, USA) produced comparatively accurate calculations of treatment dose within this complex phantom, including immediately anterior and posterior to the TTE. The Varian Eclipse Acuros (AXB) algorithm generally showed better agreement with the film measurement than the Varian Eclipse AAA algorithm (Varian Medical Systems, Palo Alto, USA), although the film measurements showed regions of 5% to 10% disagreement with both AAA and AXB in the dosimetrically-challenging region on the anterior side of the implant. Although the Aeroform CO2-filled TTE has substantial and obvious effects on the downstream dose from a static photon beam, the results of this study showed how inverse-planning of modulated rotational radiotherapy treatments can produce modulated fluence distributions that compensate for the dramatic density heterogeneities in the implant. Despite some disagreements with the planned dose, all film measurements showed that the use of inverse-planned modulated rotational photon beams resulted in comparatively homogeneous coverage of the radiotherapy target, in the complex patient-like phantom with a gas-filled TTE. Due to the importance of matching each planned fluence distribution to the density distribution within each TTE, careful use of available 3D imaging techniques is advisable, when modulated rotational radiotherapy treatments are delivered to patients with gas-filled TTEs.

Trajectory-based VMAT for cranial targets with delivery at shortened SAD.

Trajectory-based volumetric modulated arc therapy (tr-VMAT) treatment plans enable the option for noncoplanar delivery yielding steeper dose gradients and increased sparing of critical structures compared to conventional treatment plans. The addition of translational couch motion to shorten the effective source-to-axis distance (SAD) may result in improved delivery precision and an increased effective dose rate. In this work, tr-VMAT treatment plans using a noncoplanar "baseball stitch" trajectory were implemented, applied to patients presented with cranial targets, and compared to the clinical treatment plans. A treatment planning workflow was implemented: (a) beamlet doses were calculated for control points defined along a baseball stitch trajectory using a collapsed-cone convolution-superposition algorithm; (b) VMAT treatment plans were optimized using the column generation approach; (c) a final dose distribution was calculated in Varian Eclipse using the analytical anisotropic algorithm by importing the optimized treatment plan parameters. Tr-VMAT plans were optimized for ten patients presented with cranial targets at both standard and shortened SAD, and compared to the clinical treatment plans through isodose distributions, dose-volume histograms, and dosimetric indices. The control point specifications of the optimized tr-VMAT plans were used to estimate the delivery time. The optimized tr-VMAT plans with both shortened and standard SAD delivery yielded a comparable plan quality to the clinical treatment plans. A statistically significant benefit was observed for dose gradient index and monitor unit efficiency for shortened SAD tr-VMAT plans, while improved target volume conformity was observed for the clinical treatment plan (P≤0.05). A clear dosimetric benefit was not demonstrated between tr-VMAT delivery at shortened SAD compared to standard SAD, but shortened SAD delivery yielded a fraction size-dependent reduction in the estimated delivery time. The implementation of "baseball stitch" tr-VMAT treatment plans to patients presented with cranial targets demonstrated comparable plan quality to clinical treatment plans. The delivery at shortened SAD produced a fraction size-dependent decrease in estimated delivery time.

Dose Calculation Comparisons between Three Modern Treatment Planning Systems.

Purpose:Monaco treatment planning system (TPS) version 5.1 uses a Monte-Carlo (MC)-based dose calculation engine. The aim of this study is to verify and compare the Monaco-based dose calculations with both Pinnacle3 collapsed cone convolution superposition (CCCS) and Eclipse anisotropic analytical algorithm (AAA) calculations.Materials and Methods:For this study, 18 previously treated lung and head-and-neck (HN) cancer patients were chosen to compare the dose calculations between Pinnacle, Monaco, and Eclipse. Plans were chosen from those that had been treated using the Elekta VersaHD or a Novalis Tx linac. All of the treated volumetric-modulated arc therapy plans used 6 MV or 10 MV photon beams. The original plans calculated with CCCS or AAA along with the recalculated ones using MC from the three TPS were exported into Velocity software for intercomparison.Results:To compare the dose calculations, Planning target volume (PTV) heterogeneity indexes and conformity indexes were calculated from the dose volume histograms (DVH) of all plans. While mean lung dose (MLD), lung V5 and V20 values were recorded for lung plans, the computed dose to parotids, brainstem, and mandible were documented for HN plans. In plan evaluation, percent differences of the above dosimetric values in Monaco computation were compared against each of the other TPS computations.Conclusion:It could be concluded through this research that there can be differences in the calculation of dose across different TPSs. Although relatively small, these differences could become apparent when compared using DVH. These differences most likely arise from the different dose calculation algorithms used in each TPS. Monaco employs the MC allowing it to have much more detailed calculations that result in it being seen as the most accurate and the gold standard.

Calculation of clinical dose distributions in proton therapy from microdosimetry.

To introduce a new algorithm-MicroCalc-for dose calculation by modeling microdosimetric energy depositions and the spectral fluence at each point of a particle beam. Proton beams are considered as a particular case of the general methodology. By comparing the results obtained against Monte Carlo computations, we aim to validate the microdosimetric formalism presented here and in previous works. In previous works, we developed a function on the energy for the average energy imparted to a microdosimetric site per event and a model to compute the energetic spectrum at each point of the patient image. The number of events in a voxel is estimated assuming a model in which the voxel is completely filled by microdosimetric sites. Then, dose at every voxel is computed by integrating the average energy imparted per event multiplied by the number of events per energy beam of the spectral distribution within the voxel. Our method is compared with the proton convolution superposition (PCS) algorithm implemented in Eclipse™ and the fast Monte Carlo code MCsquare, which is here considered the benchmark, for in-water calculations, using in both cases clinically validated beam data. Two clinical cases are considered: a brain and a prostate case. For a SOBP beam in water, the mean difference at the central axis found for MicroCalc is of 0.86% against 1.03% for PCS. Three-dimensional gamma analyses in the PTVs compared with MCsquare for criterion (3%, 3mm) provide gamma index of 95.07% with MicroCalc vs 94.50% with PCS for the brain case and 99.90% vs 100.00%, respectively, for the prostate case. For selected organs at risk in each case (brainstem and rectum), mean and maximum difference with respect to MCsquare dose are analyzed. In the brainstem, mean differences are 0.25Gy (MicroCalc) vs 0.56Gy (PCS), whereas for the rectum, these values are 0.05Gy (MicroCalc) vs 0.07Gy (PCS). The accuracy of MicroCalc seems to be, at least, not inferior to PCS, showing similar or better agreement with MCsquare in the considered cases. Additionally, the algorithm enables simultaneous computation of other quantities of interest. These results seem to validate the microdosimetric methodology in which the algorithm is based on.

Dosimetric Analysis of a Longitudinal Clinical Experience Using Robotic Radiosurgery and LINAC-based SBRT with VMAT for Early-Stage Non-Small Cell Lung Cancer (ES-NSCLC)

SBRT is standard of care for medically inoperable patients with ES-NSCLC and can be delivered using robotic radiosurgery or standard linear accelerator (LINAC) platforms. CK uses hundreds of anterior oblique non-coplanar beams while LINAC SBRT commonly uses volumetric modulated arc therapy (VMAT) with multiple coplanar full or partial arc fields. SBRT plans are designed to cover the PTV in prescription dose while achieving steep dose fall-off in adjacent tissue which causes increased dose heterogeneity “i.e., hotspots” in the target volume. We aimed to compare the dosimetric characteristics of lung SBRT plans delivered at one institution based on CK vs LINAC VMAT technique. Patients were included who received CK-based (2012-16) or LINAC-based SBRT from (2016-17) for T1-2N0M0 NSCLC. SBRT plans of 198 patients who met inclusion criteria were analyzed (CK, n=149 and LINAC, n=59). CK plans were computed using Monte Carlo algorithm, while the LINAC-based VMAT plans were computed in collapsed cone convolution-superposition algorithm. All clinically delivered plans were reviewed to extract dosimetric parameters including target volume coverage and OARs dose distribution. Differences in mean dosimetric values by technique were estimated via Wilcoxon Rank-Sum Test. Multivariate analyses were run using logistical regression to test for confounding variables. LINAC SBRT was prescribed to a higher mean isodose line (IDL) (89% vs 80%, p<.001) yet had increased mean ratios of GTVmax (131% vs 125%, p<.001), GTVmean (120% vs 116%, p<.001), PTVmax (132% vs 126%, p<.001), and D90GTV (114% vs 111%, p<.001) doses to prescription dose compared to CK. LINAC-based SBRT had lower mean conformality index (1.10 vs. 1.21, p <.001) than CK, but there was no difference in mean R50 values (5.06 vs. 4.88, p=.67). Mean ratio of PTV to total lung volume did not differ significantly for LINAC SBRT vs. CK (1.17% vs. 1.54%, p=.08), but LINAC SBRT was associated with lower average lung V5 (18% vs. 25%, p<.001), V10 (11% vs. 14%, p=.02), mean lung dose (3.7 Gy vs. 5.1 Gy, p<.001) and lung volume (cc) receiving <12.5 Gy (3980 cc vs. 3300 cc, p=.002) than CK. LINAC SBRT was also associated with lower heart mean doses compared to CK (1.8 Gy vs. 2.5 Gy, p=.004) and heart V5 (12.7% vs. 19.5%, p=.009). There was no difference in max dose to the proximal bronchial tree. On multivariate analysis controlling for total dose, lung volume, PTV size, and target lobe the differences in lung V5 (p <.001) and mean lung dose (p <.001) by technique persisted but differences in lung cc receiving <12.5 Gy, lung V10, heart V5 and heart mean dose were no longer statistically significant. Despite being prescribed to higher mean IDLs, LINAC SBRT with VMAT technique delivered higher doses to the GTV and PTV as compared to CK and was associated with lower lung V5 and mean lung dose but not lower lung V10 or V20.

A Monte Carlo evaluation of dose distribution of commercial treatment planning systems in heterogeneous media.

Calculations from a treatment planning system (TPS) in heterogeneous regions may present significant inaccuracies due to loss of electronic equilibrium. The purpose of this study is to evaluate and quantify the differences of dose distributions computed by some of the newest dose calculation algorithms, including collapsed cone convolution (CCC), fast Fourier transform (FFT) convolution, and superposition convolution, in heterogeneity of the lung. A 6-MV Siemens Primus linear accelerator was simulated by MCNPX Monte Carlo (MC) code, and the results of percentage depth dose (PDD) and dose profile values were compared with measured data. The ISOgray TPS was used and PDDs of CCC, FFT, and superposition convolution algorithms were compared with the results obtained by MCNPX code. CT2MCNP software was used to convert the computed tomography images of the lung tissue to MC input files, and dose distributions from the three algorithms were compared to MC method. For PDD curves in buildup region, the maximum underdosage of ISOgray TPS was at the surface (19%) and comes in closer agreement when depth increases (average 7.08%). Dose differences (DD) between different algorithms and MC were typically 4.81% (range: 1.95% to 7.30%), -1.55% (range: -5.14% to 5.26%) and 4.96% (range: 2.00% to 7.4%) in the lung for the CCC, FFT, and superposition algorithms, respectively. The difference between monitor units and maximum dose calculated using the three algorithms were 0.5% and 1.61%, respectively. The maximum DD of 7% was observed between MC and TPS results. Significant differences were found when the calculation algorithms were compared with MC method in lung tissue, and this difference is not negligible. It is recommended to use of MC-based TPS for the treatment fields including lung tissue.