Collapsed Cone Convolution Superposition Research Articles

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.

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.

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.

A preliminary study of a photon dose calculation algorithm using a convolutional neural network

The aim of dose calculation algorithm research is to improve the calculation accuracy while maximizing the calculation efficiency. In this study, the three-dimensional distribution of total energy release per unit mass (TERMA) and the electron density (ED) distribution are considered inputs in a method for calculating the three-dimensional dose distribution based on a convolutional neural network (CNN). Attempts are made to improve the efficiency of the collapsed cone convolution/superposition (CCCS) algorithm while providing an approach to improve the efficiency of other traditional dose calculation algorithms. Twelve sets of computed tomography (CT) images were employed for training. Data sets were generated by the CCCS algorithm with a random beam configuration. For each monoenergetic photon model, 7500 samples were generated for the training set, and 1500 samples were generated for the validation set. Training occurred for 0.5 MeV, 1 MeV, 2 MeV, 3 MeV, 4 MeV, 5 MeV, and 6 MeV monoenergetic photon models. To evaluate the usability under linac conditions, a comparison between CCCS and CNN-Dose was performed for the Mohan 6-MV spectrum for 12 additional new sets of CT images with different anatomies. A total of 1512 test samples were generated. For all anatomies, the mean value, 95% lower confidence limit (LCL) and 95% upper confidence limit (UCL) were 99.56%, 99.51% and 99.61%, respectively, at the 3%/2 mm criteria. The mean value, 95% LCL and 95% UCL were 98.57%, 98.46% and 98.67%, respectively, at the 2%/2 mm criteria. The results meet the relevant clinical requirements. In the proposed methods, the dose distribution of clinical energy can be obtained by TERMA, and the electronic density can be obtained with a CNN. This method can also be used for other traditional dose algorithms and displays potential in treatment planning, adaptive radiation therapy, and in vivo verification.

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.

Effect of beam configuration with inaccurate or incomplete small field output factors on the accuracy of treatment planning dose calculation.

To evaluate the effect of beam configuration with inaccurate or incomplete small field output factors on the accuracy of dose calculations in treatment planning systems. Output factors were measured using various detectors and for a range of field sizes. Three types of treatment machines were configured in two treatment planning systems. In the first (corrected) machine, the Exradin W1 scintillator was used to determine output factors. In the second (uncorrected) machine, the measured output factors by the A1SL ion chamber without considering output correction factors for small field sizes were utilized. In the third (clinical) machine, measured output factors by the Exradin W1 were used but not for field sizes smaller than 2×2cm2 . The dose computed by the anisotropic analytical algorithm (AAA), Acuros XB (AXB) and collapsed cone convolution/superposition (CCC) algorithms in the three machines were delivered using static (jaw-, MLC-, and jaw/MLC-defined), and composite [intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT)] fields. The differences between measured and calculated dose values were analyzed. For static fields, the percentage differences between measured and calculated doses by the three algorithms in three configured machines were <2% for field sizes larger than 2×2cm2 . In jaw- and jaw/MLC-defined fields smaller than 2×2cm2 , the corrected machine presented better agreement with measurement. Considering output correction factors in MLC-defined fields, among the three configured machines, the accuracy of calculation improved to within ±0.5%. For MLC-defined field size of 1×1cm2 , AXB showed the smallest percentage difference (1%). In IMRT and VMAT plans, the percentage differences between measured and calculated doses at the isocenter, as well as the gamma analysis of different plans, which include field sizes larger than 3×3cm2 , did not vary noticeably. For smaller field sizes, using the corrected machine influences dose calculation accuracy. Configuration with corrected output factors improves accuracy of dose calculation for static field sizes smaller than 2×2cm2 . For very small fields, the robustness of the dose calculation algorithm affects the accuracy of dose as well. In IMRT and VMAT plans, which include small subfields, the size of the jaw-defined field is an important factor and using corrected output factors increases dose calculation accuracy.

Impact of choice of dose calculation algorithm on PTV and OAR doses in lung SBRT

Five dose calculation algorithms commonly used in lung SBRT planning are evaluated for their dosimetric impact on planning target volume (PTV) and organ-at-risk (OAR) doses. Treatment plans for thirty lung SBRT patients were included in this multi-institutional planning study. Lung SBRT plans were initially generated using BrainLab Pencil Beam (PB) convolution algorithm to deliver 50 Gy in 5 fractions to PTV (PB_OP plans). Plans were recalculated using BrainLab Monte Carlo (MC) algorithm with the same beam parameters, field settings, and MUs (MC_NO plans) to investigate accuracy limits of PB plans. Next, these plans were re-optimized via MU scaling in MC algorithm while keeping original beam parameters and settings (MC_OP). Further, with these new MUs, all patient plans were recalculated with Pinnacle collapsed cone convolution superposition (CCC), Eclipse anisotropic analytic algorithm (AAA), and Eclipse Acuros XB (AXB) algorithms, and are referred to as AP_NO, AAA_NO, and AXB_NO plans, respectively. DVH of PTV and OARs were used to calculate dosimetric parameters for comparison with MC_OP plans. Patient plans were compared based on PTV size as well as the location in the lung: island targets, adjacent to the ribs/chest wall, or mixed. Lung SBRT plans using PB dose algorithm overestimated target doses by 10–20% of prescribed dose and these differences were highlighted mainly in the target periphery/dose-buildup region as seen in Dmin and D90. Compared with MC, both Pinnacle and AAA plans overestimated PTV dose in the penumbra region, whereas Acuros plans were in good agreement with MC plans. PTV dose differences ranged 3–5% among Pinnacle, AAA, and Acuros. In general, Acuros AXB performed well for adjacent and mixed targets near the ribs and chest wall, whereas Pinnacle CCC was favored for island targets. OAR Dmax and lung V20 were better reproduced in Pinnacle, whereas Dmean were comparable among all TPS. Knowing the strengths and limitations of clinical treatment planning system allows more consistent and accurate dose comparison for patients enrolled in protocol studies.

VMAT Optimization and Dose Calculation in the Presence of Metallic Hip Prostheses.

Introduction:This research quantifies and compares the effect of hip prostheses on dose distributions calculated using collapsed cone convolution superposition and Monte Carlo (with and without correcting for the density of the implant and surrounding tissues). The use of full volumetric modulated arc therapy arcs versus volumetric modulated arc therapy arcs avoiding the hip implants (skip arcs) was also studied.Materials and Methods:Six prostate patients with hip prostheses were included in this study. The hip prostheses and the streaking artifacts on the computed tomography images were contoured by a single physician, and full volumetric modulated arc therapy arcs were created in the Pinnacle3 TPS. Copies of each plan were made, and the doses were recalculated with the densities of the prostheses and surrounding tissues overridden. The plans were then exported to Monaco and recalculated using a Monte Carlo dose calculation algorithm, with and without densities of the prosthesis and surrounding tissues overridden.Results:With density overrides, Pinnacle3 had a 4.4% error for ion chamber measurements. Monaco was within 0.2% of ion chamber measurement when density overrides were used. On average, when density overrides were used in Pinnacle3 for patient dose calculations, the planning target volume D95 value dropped from 99.3% to 82.7%. Monaco also showed decreased planning target volume coverage when plans were recalculated with correct density information. Full arc plans (with density overrides) for the patient with a bilateral prosthesis provided significant bladder sparing and some rectal sparing compared to skip arc plans.Conclusion:When planning for prostate patients with hip prostheses, correct density information for implants and surrounding tissues should be used to optimize the plan and ensure optimal accuracy. If available, a Monte Carlo algorithm should be used as a second check. Full arcs could be used to spare dose to organs at risk, while maintaining adequate planning target volume coverage, when using a Monte Carlo dose calculation algorithm.

173. Dosimetrical evaluation of interplay effect for lung cancer treatments with Vero system: Comparison between three different techniques

PurposeTo evaluate interplay effect on delivered dose distributions for lung cancer treatments using treatment techniques (DynamicWaveArc [1,2], DynamicConformalArc and DynamicTumorTracking) performed with VERO system. MethodsFor each patient with lung nodules selected for stereotactic treatments a 4DCT with RPM Varian System was acquired. Internal Target Volume (ITV) was created on maximum intensity projection reconstructed from a dataset of ten phases and enlarged to PTV (5 mm margins). For each plan an extreme hypo-IGRT schedule was applied (36 Gy/3fz) with a prescription of D95% PTV = 100%. DWA planning was performed with Collapsed Cone Convolution Superposition on Raystation (v.6.1.1.2) (2 full arcs, GenericX1 template), while DCA and DTT with Monte Carlo algorithm on iPlannet RT Dose (v.4.5.4) (1 full arc and 5 conformal beams, respectively). Plans were recalculated as pre-treatment QA and delivered on BrainLab Respiratory Gating Phantom, applying real patients’ breathing patterns. Dose maps were acquired with EBT3 Gafchromic, applying a single scan protocol for absolute dose measurements [3,4]. All films were read with an EPSON XL10000 scanner in transmission mode (48-bit RGB and 72 dpi resolution) and analyzed with FilmQAPro® 2016 (Ashland). For each technique, absolute dose γ analysis was performed between static and moving delivery, applying 2%-2 mm criteria and no dose threshold. Evaluations were performed considering single and 3-fractions schedule and a mean γ passing rate for 5 patients. ResultsResults obtained from analysis of all treatment techniques and both schedules are reported in Table 1. ConclusionsFrom preliminary studies, the interplay effect is more pronounced in DWA treatments than in other cases, as expected. However as the number of fractions is increased, the γ passing rates become almost acceptable also for DWA. We also started to investigate the interplay effect varying patients’ breathing patterns [5]. Results suggest using this technique for moving targets could lead to better OARs sparing. [Display omitted]

121. Automated planning for Hodgkin lymphoma radiotherapy

PurposePurpose of the present study was to report on the evaluation of Pinnacle3Auto-Planning (AP) algorithm for Hodgkin lymphoma (HL) radiation therapy (RT). MethodsPlanning CT-scans of 10 female patients with supradiaphragmatic HL were considered. Involved site clinical target volume (CTV) included the upper mediastinal and supraclavear nodes. Planning Target Volume (PTV) was obtained by CTV uniform 10-mm expansion. A total dose of 30 Gy was prescribed in 20 fractions.A “butterfly” (BF) volumetric modulated arc therapy was planned with Pinnacle3 v. 9.10 using SmartArc module and Collapsed Cone Convolution Superposition algorithm dose calculation. Human-driven (Manual-BF) and AP-BF optimization plans were generated using published priority and constraints on the OARs. In addition to BF technique, the AP engine was applied to a 2 coplanar disjointed arches (AP-ARC) technique. A single AP optimization list of PTV/OAR clinical goals was created for each patient. For plan comparison, DVHs of PTVs and OARs were extracted; homogeneity and conformity indices (HI and CI) computed and OARs dose-volume constraints and NTCP models for RP, HT, and CD evaluated. Non-parametric Friedman and Dunn tests were used to identify significant differences between groups. ResultsAP (AP-BF or AP-ARC) offers comparable coverage of the PTV as the manual plan and a consistently better sparing of OARs. In particular, the heart and thyroid mean doses and lung V20 were significantly reduced using AP engine. Accordingly, AP provides similar, if not lower, complication risks. The median number of monitor units was slightly reduced by AP (3.5%). Hands-on planning time decreased on average by a factor of 3 by AP. ConclusionsThe AP module was able to limit heart and thyroid complication risks, producing clinically acceptable HL plans with stable quality without additional user input. Overall AP-ARC provided better results in terms of OAR sparing when compared with AP-BF.

Treatment planning dose accuracy improvement in the presence of dental implants

Streaking artifacts in computed tomography (CT) scans caused by metallic dental implants (MDIs) can lead to inaccuracies in dose calculations. This study quantifies and compares the effect of MDIs on dose distributions using the collapsed cone convolution superposition (CCCS) and Monte Carlo (MC) algorithms, with and without correcting for the density of the MDIs. Ion chamber measurements were taken to test the ability of the algorithms in Pinnacle3 and Monaco to calculate dose near high-Z materials. Nine previously treated patients with head and neck cancer were included in this study. The MDI and the streaking artifacts on the CT images were carefully contoured. For each patient, a plan was optimized and calculated using the Pinnacle3 treatment planning system (TPS). Two dose calculations were performed for each patient: one with overridden densities of the MDI and CT artifacts and one without overridden densities of the MDI and CT artifacts. The plans were then exported to the Monaco TPS and recalculated for the same number of monitor units (MUs) using its MC dose calculation algorithm. The changes in dose to the planning target volume (PTV) and surrounding healthy tissues were examined between all the plans using VelocityAI. For the ion chamber measurements, when correct density information was used, Monaco was within 3% of the measured values, whereas the doses calculated in Pinnacle3 varied up to 7%. The CCCS algorithm in Pinnacle3 calculated only a significant decrease in PTV coverage for 1 patient when the densities were overridden. The MC algorithm in Monaco was able to calculate a significant change in PTV coverage for five of the patients when the density was overridden. Additionally, when healthy tissues affected by streaking artifacts were assigned the correct density, cumulative (from all the fractions) point doses increased up to 46.2 Gy. Not properly accounting for MDIs can impact both the high-dose regions (PTVs) and surrounding healthy tissues. This study demonstrates that if MDIs and the artifacts are not appropriately accounted for by contouring and assigning to them the correct density, there is a potential risk of compromising the quality of the plan regarding PTV coverage and dose to healthy tissues.

Auto- versus human-driven plan in mediastinal Hodgkin lymphoma radiation treatment

BackgroundTechnological advances in Hodgkin lymphoma (HL) radiation therapy (RT) by high conformal treatments potentially increase control over organs-at-risk (OARs) dose distribution. However, plan optimization remains a time-consuming task with great operator dependent variability. Purpose of the present study was to devise a fully automated pipeline based on the Pinnacle3 Auto-Planning (AP) algorithm for treating female supradiaphragmatic HL (SHL) patients.MethodsCT-scans of 10 female patients with SHL were considered. A “butterfly” (BF) volumetric modulated arc therapy was optimized using SmartArc module integrated in Pinnacle3 v. 9.10 using Collapsed Cone Convolution Superposition algorithm (30 Gy in 20 fractions). Human-driven (Manual-BF) and AP-BF optimization plans were generated. For AP, an optimization objective list of Planning Target Volume (PTV)/OAR clinical goals was first implemented, starting from a subset of 5 patients used for algorithm training. This list was then tested on the remaining 5 patients (validation set). In addition to the BF technique, the AP engine was applied to a 2 coplanar disjointed arc (AP-ARC) technique using the same objective list. For plan evaluation, dose-volume-histograms of PTVs and OARs were extracted; homogeneity and conformity indices (HI and CI), OARs dose-volume metrics and odds for different toxicity endpoints were computed. Non-parametric Friedman and Dunn tests were used to identify significant differences between groups.ResultsA single AP objective list for SHL was obtained. Compared to the manual plan, both AP-plans offer comparable CIs while AP-ARC also achieved comparable HIs. All plans fulfilled the clinical dose criteria set for OARs: both AP solutions performed at least as good as Manual-BF plan. In particular, AP-ARC outperformed AP-BF in terms of heart sparing involving a lower risk of coronary events and radiation-induced lung fibrosis. Hands-on planning time decreased by a factor of 10 using AP on average.ConclusionsDespite the high interpatient PTV (size and position) variability, it was possible to set a standard SHL AP optimization list with a high level of generalizability. Using the implemented list, the AP module was able to limit OAR doses, producing clinically acceptable plans with stable quality without additional user input. Overall, the AP engine associated to the arc technique represents the best option for SHL.

OA073] A prospective study of dose–response relationship and NTCP of conductive hearing loss in patients undergoing head and neck radiotherapy

Purpose Head-and-neck (H&N) radiotherapy may cause hearing loss. Despite its negative effects on patients’ quality of life, little has been published on the dose–response relationship and normal tissue complication probability (NTCP) of the conductive subtype of hearing loss. The goals of this study were: (i) to observe the incidence of hearing loss in patients undergoing non-intensity-modulated H&N radiotherapy, (ii) to obtain the relationship between dose and conductive hearing loss, (iii) to test the current parameters of the Lyman-Kutcher-Burman (LKB) radiobiological model for estimating its NTCP, and (iv) to assess the need for considering the dose to middle and external ear as organ-at-risk when optimizing treatment plans. Methods In this prospective study, the dose–response relationship in the auditory system of 35 patients (70 ears) undergoing 3D conformal H&N radiotherapy was studied using the physical parameter of mean dose as well as the LKB NTCP model (with parameters TD50 = 40 Gy, M = 0.15, n = 0.10) calculated by BioSuite software. In order to include a wider dose range, the study was conducted at a clinic where, at the time, more advanced treatment delivery techniques were unavailable. The patients underwent routine external-beam megavoltage X-ray treatments following 3D treatment planning involving dose calculation using collapsed-cone convolution superposition. Prescribed doses were in the range 30–72 Gy, delivered 1.8–2.0 Gy per fraction. Hearing status was evaluated by pure tone audiometry one day before the start and one and three months after the end of the radiotherapy course. Results Nineteen ears (27%) suffered hearing loss. Sixteen ears (23%) had conductive hearing loss and 3 ears (4%) the sensorineural subtype (p 0.05). An approximately 40 Gy mean dose to the middle-ear planning organ-at-risk volume was predicted by the LKB NTCP model to cause a 50% risk of conductive hearing loss, which was also observed in the studied patients. Conclusions Incidence of conductive hearing loss in typical 3D conformal H&N radiotherapy can be significant. This suggests that the auditory system should also be considered in treatment plan optimization. The LKB model parameters provided a reasonably accurate NTCP. However, a study is underway to optimize its parameters.

P249] Influence of gamma index comparison on differences in dose calculation algorithm deposition method

Purpose Phantomless patient specific QA saves time and is able to detect errors by comparing the TPS dose distribution with a secondary independent dose calculation. TG 218 recommends that gamma analysis should be performed on metrics such as, mean gamma, γ > 1 and γ > 1.5, per structure. The aim of this study is to investigate how gamma index comparison is affected by differences on dose calculation algorithms when treating bone metastases. Methods 10 patients treated for bone metastases in different sites were chosen and 3D and VMAT plans were created using Elekta Monaco TPS in a Versa linac and 10 MV photons. Both 3D and VMAT dose distributions were calculated with Monaco XVMC Monte Carlo (MC) calculation algorithm using dose to medium in medium MC(Dm,m) and dose to water in medium MC(Dw,m) dose deposition modes. All plans were also calculated and evaluated using IBA Compass incorporating a Collapsed Cone convolution superposition algorithm calculating dose to water in medium CC(Dw,m). 3D dose distribution evaluation was performed in terms of gamma metrics such as mean gamma values and % PTV with γ > 1 and γ > 1.5 using 3%, 3 mm criteria for the following combinations: (a) MC(Dw,m) vs CC(Dw,m) and (b) MC(Dm,m) vs CC(Dw,m). Results For the 3D plans, average mean gamma values for MC(Dw,m) vs CC(Dw,m) and MC(Dm,m) vs CC(Dw,m) were found 0.37 ± 0.1 and 0.52 ± 0.12 respectively. The average % PTV points with γ > 1 were found 6.34 ± 5.7% and 12.6 ± 7.5% and the average % PTV points with γ > 1.5 1.45 ± 1.87% and 2.21 ± 2.28%, respectively. Corresponding values for the VMAT plans were 0.46 ± 0.12 and 0.68 ± 0.09 (mean gamma values), 9.48 ± 7.26% and 21.5 ± 7.3% (% PTV points with γ > 1) and 2.37 ± 2.8% and 7.95 ± 7.4% (% PTV points with γ > 1.5). Conclusions Gamma index comparison revealed that both MC and CC calculation algorithms are in fair agreement in dose calculation determination providing that the same dose deposition mode (i.e. Dw,m or Dm,m) is used. The use of different dose deposition mode, results in increased differences in dose calculation determination in structures/areas where materials different than water, such as bone, is included. The observed differences are also increased as the complexity of the plan increases (e.g. VMAT vs 3D).

Refinement of MLC modeling improves commercial QA dosimetry system for SRS and SBRT patient-specific QA.

Mobius 3D (M3D) provides a volumetric dose verification of the treatment planning system's calculated dose using an independent beam model and a collapsed cone convolution superposition algorithm. However, there is a lack of investigation into M3D's accuracy and effectiveness for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT) quality assurance (QA). Here, we collaborated with the vendor to develop a revised M3D beam model for SRS/SBRT cases treated with a 6X flattening filter-free (FFF) beam and high-definition multiple leaf collimator (HDMLC) on an Edge linear accelerator. Eighty SRS/SBRT cases, planned with AAA dose algorithm and validated with Gafchromic film, were compared to M3D dose calculations using 3D gamma analysis with 2%/2 mm gamma criteria and a 10% threshold. A revised beam model was developed by refining the HD-MLC model in M3D to improve small field dose calculation accuracy and beam profile agreement. All cases were reanalyzed using the revised beam model. The impact of heterogeneity corrections for lung cases was investigated by applying lung density overrides to five cases. For the standard and revised beam models, respectively, the mean gamma passing rates were 94.6% [standard deviation (SD): 6.1%] and 98.0% [SD: 1.7%] (for the overall patient), 88.2% [SD: 17.3%] and 93.8% [SD: 6.8%] (for the brain PTV), 71.4% [SD: 18.4%] and 81.5% [SD: 14.3%] (for the lung PTV), 83.3% [SD: 16.7%] and 67.9% [SD: 23.0%] (for the spine PTV), and 78.6% [SD: 14.0%] and 86.8% [SD: 12.5%] (for the PTV of all other sites). The lung PTV mean gamma passing rates improved from 74.1% [SD: 7.5%] to 89.3% [SD: 7.2%] with the lung density overridden. The revised beam model achieved an output factor within 3% of plastic scintillator measurements for 2 × 2 cm2 MLC field size, but larger discrepancies are still seen for smaller field sizes which necessitate further improvement of the beam model. Special attention needs to be paid to small field dosimetry, MLC modeling, and inhomogeneity corrections in the beam model for SRS/SBRT QA. The improvements noted in this study, and further collaborations between clinical physicists and the vendor to refine the M3D beam model could enable M3D to become a premier SRS/SBRT QA tool.

Assessment of Imprecise Small Photon Beam Modeling by Two Treatment Planning System Algorithms

Dosimetric accuracy in intensity-modulated radiation therapy (IMRT) is the main part of quality assurance program. Improper beam modeling of small fields by treatment planning system (TPS) can lead to inaccuracy in treatment delivery. This study aimed to evaluate of the dose delivery accuracy at small segments of IMRT technique using two-dimensional (2D) array as well as evaluate the capability of two TPSs algorithm in modeling of small fields. Irradiation were performed using 6 MV photon beam of Siemens Artiste linear accelerator. Dosimetric behaviors of two dose calculation algorithms, namely, collapsed cone convolution/superposition (CCCS) and full scatter convolution (FSC) in small segments of IMRT plans were analyzed using a 2D diode array and gamma evaluation. Comparisons of measurements against TPSs calculations showed that percentage difference of output factors of small fields were 2% and 15% for CCCS and FSC algorithm, respectively. Gamma analysis of calculated dose distributions by TPSs against those measured by 2D array showed that in passing criteria of 3 mm/3%, the mean pass rate for all segment sizes is higher than 95% except for segment sizes below 3 cm × 3 cm optimized by TiGRT TPS. High pass rate of gamma index (95%) achieved in planned small segments by Prowess relative to results obtained with TiGRT. This study showed that the accuracy of small field modeling differs between two dose calculation algorithms.

Single fraction computed tomography-guided high-dose-rate brachytherapy or stereotactic body radiotherapy for primary and metastatic lung tumors?

PurposeTo provide a pilot dosimetric study of computed tomography (CT)-guided high-dose-rate (HDR) brachytherapy (BRT) and stereotactic body radiotherapy (SBRT) for primary and metastatic lung lesions.Material and methodsFor nine lung primary and metastasis patients, 3D image-based BRT plan using a single virtual catheter was planned for 34 Gy in single fraction to the gross tumor volume (GTV) + 3 mm margin to account for tumor deformation. These plans were compared to margin-based (MB-) and robustness optimized (RO-) SBRT, assuming the same tumor deformation under real-time tumor tracking. Consistent dose calculation was ensured for both BRT and SBRT plans using the same class of collapsed cone convolution superposition algorithm. Plan quality metrics were compared by Friedman tests and Wilcoxon t-tests.Results and ConclusionsBrachytherapy plans showed significant higher GTV mean dose compared to MB- and RO-SBRT (122.2 Gy vs. 50.4 and 44.7 Gy, p < 0.05), and better dose gradient index (R50) = 2.9 vs. 4.3 and 8.4 for MB- and RO-SBRT, respectively. Dose constraints per the RTOG 0915 protocol were achieved for all critical organs except chest wall in BRT. All other dose-volume histograms (DVH) metrics are comparable between BRT and SBRT. Treatment delivery time of BRT and SBRT plans significantly increased and decreased with increasing GTV size, respectively. SBRT using advanced MLC tracking technique and non-coplanar VMAT can achieve comparable dosimetric quality to HDR BRT. Whether or not, the significantly higher GTV dose can increase killing of radioresistant tumor cells and offset the effect of tumor reoxygenation in single fraction BRT, requires further clinical investigation.

Abstract ID: 220 Evaluating the effect of dental implants in the treatment of head and neck cancer using Monte Carlo dose calculation

Purpose To quantify and compare the effect of metallic dental implants (MDI) on dose distributions calculated using Collapsed Cone Convolution Superposition (CCCS) algorithm or a Monte Carlo algorithm (with and without correcting for the density of the MDI). Methods Seven previously treated patients to the head and neck region were included in this study. The MDI and the streaking artifacts on the CT images were carefully contoured. For each patient a plan was optimized and calculated using the Pinnacle treatment planning system (TPS). For each patient two dose calculations were performed, a) with the densities of the MDI and CT artifacts overridden (12 g/cc and 1 g/cc respectively) and b) without density overrides. The plans were then exported to the Monaco TPS and recalculated using Monte Carlo dose calculation algorithm. The changes in dose to PTVs and surrounding Regions of Interest (ROIs) were examined between all plans. Results The Monte Carlo dose calculation indicated that PTVs received 6% lower dose than the CCCS algorithm predicted. In some cases, the Monte Carlo algorithm indicated that surrounding ROIs received higher dose (up to a factor of 2). Conclusion Not properly accounting for dental implants can impact both the high dose regions (PTV) and the low dose regions (OAR). This study implies that if MDI and the artifacts are not appropriately contoured and given the correct density, there is potential significant impact on PTV coverage and OAR maximum doses.

Independent 3D dose calculation for IMRT based on simplification of beam model and collapsed-cone convolution/superposition algorithm

Objective To realize independent 3D dose calculation for intensity-modulated radiotherapy (IMRT) by building a two-source beam model of medical linear accelerator combined with a collapsed-cone convolution/superposition (CCCS) algorithm. Methods Two-source beam models of medical linear accelerators (Varian and Elekta) were built to calculate the 3D dose distributions using the CCCS algorithm. Scp, percent depth dose, and off-axis dose distribution were compared with the scanning data of ion chamber to confirm the calculation model. Twelve intensity-modulated treatment plans from each accelerator (a total of 24 plans) were selected for comparison. The calculation results of treatment planning system (TPS) were independently validated, and further compared with the measurement results of detector matrix. Results The dose deviations at the center of rectangle fields were lower than 1%, the deviation between doses at the same position in the field was not higher than 1%, and the positional deviation in the penumbra region was not higher than 1 mm. Gamma analysis based on 3%/3 mm standard was used to compare the results calculated by detectors and TPS.The pass rates were higher than 90%. Conclusions The independent 3D dose calculation for IMRT based on two-source beam model combined with CCCS algorithm has been successfully set up. The comparison between regular field and IMRT plan indicates that this method and calculation model can be used for independent 3D dose calculation of clinical plan. Key words: Clinical linear accelerators beam model; Independent dose calculation; Quality assurance