Independent monitor unit verification for dynamic flattened beam plans on the Halcyon linac.

Independent monitor unit verification (MUV) methods for the dynamic beam-flattening (DBF) technique have not been established. The purpose of this study was to clarify whether MU values for the DBF technique can be calculated using in-air and in-water output ratios (Sc and Scp ). Sc and Scp were measured in the DBF mode, and the phantom scatter factor (Sp ) was calculated. The difference between calculated and planned MUs with square and rectangle fields and clinical plans for different treatment sites was also evaluated. Sc values for the 4 × 4 to 24 × 24 cm2 fields of the distal multi-leaf collimator (MLC) layer at 2-cm intervals were 0.887, 0.815, 0.715, 0.716, 0.611, 0.612, 0.511, 0.373, 0.374, 0.375, and 0.374, respectively. No collimator exchange effect was observed. Sc also depends slightly on the field size of the distal MLC layer. If the distal-MLC-layered field size was less than 20% of the corresponding MLC sequence size in the proximal MLC layer, Sc was affected by >1%, which was compensated using a correction factor (CF). Sp increased as the field sizes of the MLC sequence and distal MLC leaves increased. MUs calculated using measured Sc , Sp , and CF for square and rectangle fields agreed with planned MUs within ±1.2%. A larger difference (-1.5%) between calculated and planned MUs was observed for clinical plans, whereas differences in MUs were within 2 MU for most fields (56 out of 64 fields). MU calculation for the DBF technique can be performed with Sc , Sp , and CF for independent MUV.

Hemodynamic Characteristics After Fontan Procedure in Patients with Down's Syndrome.

Patients with Down's syndrome (DS) are generally regarded as not being good candidates for the Fontan procedure. However, detailed hemodynamic changes over time are not fully clarified. A retrospective chart review of all patients with DS who underwent the Fontan procedure and 5 times that number of Fontan patients without DS performed in Fukuoka Children's Hospital and Kyushu University Hospital. Seven Fontan patients with DS were identified, and 35 Fontan patients without DS were recruited. During the mean observational periods of 14.7 years and 15.0 years (DS and non-DS, respectively) after the Fontan procedure, only one DS patient died. Central venous pressure (CVP) and transpulmonary pressure gradient significantly increased, and arterial oxygen saturation significantly decreased over time in DS patients after the Fontan procedure compared with those without DS. CVP in DS patients after the Fontan procedure increased over time compared with non-DS patients. Better management including the efficacy of Pulmonary arterial hypertension-specific therapy should be clarified in further studies.

New strategy of a lung compensating technique with STR for total body irradiation.

PURPOSE: To determine the thickness of a soft variable shape tungsten rubber (STR) as a lung compensating filter in total body irradiation. METHODS: A tough water (TW) phantom and tough lung (TL) phantom were used as water and lung-equivalent phantoms. The TW with a thickness of 3 cm simulating the thoracic wall was used (upper layer). The TW or TL with a thickness from 1 to 15 cm (1 cm increments) was placed beneath the upper layer (middle layer). The TW with a thickness of 5 cm simulating the mediastinum was placed beneath the middle layer (lower layer), and a farmer ionization chamber was placed beneath this layer. The relative doses of a 10 MV X-rays were then measured. The TL was compensated in 1 mm increments from 1 to 11 mm of the STR, and the thickness of the STR at the same dose of TW (water equivalent) was obtained. RESULTS: The compensating ability of STR increased as the thickness of the TL increased, and an STR with a thickness of 1 mm reduced the dose by 2%-4%, depending on the thickness of lung. The STR thickness as an equivalent dose of TW per cm of TL was approximately linear, and the thickness was 0.62 mm/cm of TL. CONCLUSION: The STR can be used as a lung compensating filter for a water equivalent dose with 0.62 mm of STR per cm of lung.

An updating approach for knowledge-based planning models to improve plan quality and variability in volumetric-modulated arc therapy for prostate cancer.

PURPOSE: The purpose of this study was to compare the dose-volume parameters and regression scatter plots of the iteratively improved RapidPlan (RP) models, specific knowledge-based planning (KBP) models, in volumetric-modulated arc therapy (VMAT) for prostate cancer over three periods. METHODS: A RP1 model was created from 47 clinical intensity-modulated radiation therapy (IMRT)/VMAT plans. A RP2 model was created to exceed dosimetric goals which set as the mean values +1SD of the dose-volume parameters of RP1 (50 consecutive new clinical VMAT plans). A RP3 model was created with more strict dose constraints for organs at risks (OARs) than RP1 and RP2 models (50 consecutive anew clinical VMAT plans). Each RP model was validated against 30 validation plans (RP1, RP2, and RP3) that were not used for model configuration, and the dose-volume parameters were compared. The Cook's distances of regression scatterplots of each model were also evaluated. RESULTS: Significant differences (p < 0.05) between RP1 and RP2 were found in Dmean (101.5% vs. 101.9%), homogeneity index (3.90 vs. 4.44), 95% isodose conformity index (1.22 vs. 1.20) for the target, V40Gy (47.3% vs. 45.7%), V60Gy (27.9% vs. 27.1%), V70Gy (16.4% vs. 15.2%), and V78Gy (0.4% vs. 0.2%) for the rectal wall, and V40Gy (43.8% vs. 41.8%) and V70Gy (21.3% vs. 20.5%) for the bladder wall, whereas only V70Gy (15.2% vs. 15.8%) of the rectal wall differed significantly between RP2 and RP3. The proportions of cases with a Cook's distance of <1.0 (RP1, RP2, and RP3 models) were 55%, 78%, and 84% for the rectal wall, and 77%, 68%, and 76% for the bladder wall, respectively. CONCLUSIONS: The iteratively improved RP models, reflecting the clear dosimetric goals based on the RP feedback (dose-volume parameters) and more strict dose constraints for the OARs, generated superior dose-volume parameters and the regression scatterplots in the model converged. This approach could be used to standardize the inverse planning strategies.

Knowledge-based planning using pseudo-structures for volumetric modulated arc therapy (VMAT) of postoperative uterine cervical cancer: a multi-institutional study.

BACKGROUND: The aim of this study was to investigate the performance of the RapidPlan (RP ) using models registered pseudostructures, and to determine how many structures are required for automatic optimization of volumetric modulated arc therapy (VMAT) for postoperative uterine cervical cancer. MATERIALS AND METHODS: Pseudo-structures around the PTV were retrospectively contoured for patients who had completed treatment at five institutions. For 22 common patients, plans were generated with a single optimization for models with two (RP_2), four (RP_4), and five (RP_5) registered structures, and the dosimetric parameters of these models were compared with a clinical plan with several optimizations. RESULTS: Most dosimetric parameters showed no major differences between each RP model. In particular, the rectum Dmax, V50Gy, and V40Gy with RP_2, RP_4, and RP_5 were not significantly different, and were lower than those of the clinical plan. The average proportions of plans achieving acceptable criteria for dosimetric parameters were close to 100% for all models. Using RP_2, the average time for the VMAT planning was reduced by 88 minutes compared with the clinical plan. CONCLUSION: The RapidPlan model with two registered pseudo-structures could generate clinically acceptable plans while saving time.

[Dosimetric Characteristics of a Real Time Shapeable Tungsten Containing Rubber with Electron Beams].

PURPOSE: This study investigated the dosimetric characteristics of electron beams shaped with a real-time shapeable tungsten-containing rubber (STR) collimator. METHODS: Circular irradiation fields of 40 mm diameter were shaped using STR or low melting-point alloy (LMA) placed on the electron applicator. The STR heated with approximately 70-degree warm water was molded into the template bottom of the applicator. Percent depth doses (PDDs) and lateral dose profiles of 6 and 12 MeV electron beams were measured and compared between STR and LMA. For the PDDs, the depths of maximum dose (dmax), 90% dose (d90), and 80% dose (d80) were evaluated. For the lateral dose profiles, penumbra as the width of the off-axis distance from 80% to 20% doses and treatment diameter covering over 90% dose were evaluated at the surface, dmax, d90, and d80. The transmission of the STR was also investigated at thicknesses fit to electron applicator for 6 and 12 MeV electron beams. RESULTS: The STR was softened with 70-degree warm water. Therefore, it was easy to mold it and attach the applicator. The PDDs and penumbras at the surface, dmax, d90, and d80 for the STR were almost equal to those for the LMA with 6 and 12 MeV electron beams. The treatment diameters covering over 90% dose for the irradiation fields with 40 mm diameter at dmax (LMA vs. STR) were 20.9 vs. 21.1 mm and 19.2 vs. 18.4 mm for 6 and 12 MeV electron beams, respectively. The transmission of the STR was almost same as that of LMA. CONCLUSIONS: The dosimetric characteristics of the STR on the electron applicator were almost same as those of the LMA. The heated STR was shaped easily, flexibly, and immediately. The STR can be used as a substitute for LMA in electron radiotherapy.

[Comparison of O-ring and General Linacs for Treatment Planning of Volumetric Modulated Arc Therapy].

PURPOSE: Novel linac improvements in speed of gantry, collimator, leaf and dose rate may increase the time-efficiency of volumetric modulated arc therapy (VMAT) delivery, however remains to be investigated. In this study, a fast-rotating O-ring linac (Halcyon) with fast moving leaves is compared with a general linac (TrueBeam: TB) in terms of plan quality for VMAT of C-shape, prostate, multi target and, head and neck (H&N) cases from AAPM TG-119. MATERIALS AND METHODS: For the four test cases, VMAT planning was performed using single to four-arc VMAT on a Halcyon and using single to three-arc VMAT on a TrueBeam. Same conditions for optimization were used in each test case. Target coverage metrics and organ at risks (OAR) dose were compared. Monitor unit (MU) and irradiation time in each plan were also compared. RESULTS: In all cases, single-arc plans of Halcyon were inferior to TB plans on dose objectives. Conformity index (CI) to outer target of C-shape case was better for Halcyon (1-arc: 1.242, 2-arc: 1.202, 3-arc: 1.198, 4-arc: 1.181) than for TB (1-arc: 1.247, 2-arc: 1.211, 3-arc: 1.211) except to single arc. D5 (Gy) of core for C-shape case was better for halcyon (1-arc: 23.29, 2-arc: 21.01, 3-arc: 20.64, 4-arc: 20.47) than for TB (1-arc: 24.04, 2-arc: 22.94, 3-arc: 23.04). Calculated MU was smaller for Halcyon than for TB. In addition, Halcyon is more faster than TB because mechanical movements were improved. CONCLUSION: For VMAT plan in each case, Halcyon as well or better at the plan quality of two or three arcs on TB while reducing the delivery time.

[Study of Stability and Sensitivity of Three-dimensional Diode Array Detector].

PURPOSE: Intensity modulated radiation therapy (IMRT) has become a widely accepted and efficient treatment technique for many types of cancers. Patient's specific quality assurance (QA) should be performed with QA devices. Stability and sensitivity tests conducted on the ArcCHECK (AC) 3D diode array were performed. METHODS: Set-up error test with AC was performed. The set-up position moved to lateral (mm), longitudinal (mm) and rotational (°) were 0.5, 1.0, 2.0 and 3.0, respectively. Sensitivity change test of diode array with AC through 230 days was also performed. Same array calibration data was applied to all measurements of volumetric-modulated arc therapy benchmark test through 230 days. Gamma method (2 mm/2% criteria) was performed to analyze the result of all measurements. RESULTS: In the results of positional error, gamma pass rate become degenerate according to positional error became larger. With 0.5 mm or 0.5° positional error, decreasing rate of the pass rate of lateral, longitudinal and rotational were 1.0%, 2.5% and 4.2%, respectively. In the sensitivity change test, the gamma pass rate decreased 2.2%/100 days with same calibration data. CONCLUSION: AC has highly sensitivity against positional error. Sensitivity of AC has been changed and pass rate was decreased 2.2%/100 days through 230 days. Array calibration should be performed in consideration of change of sensitivity.

Volumetric Modulated Arc Therapy Planning for Craniospinal Irradiation With a New O-ring Linac.

This study aims to determine the feasibility of using a new O-ring linear accelerator (Halcyon, Varian Medical Systems, CA, USA) to perform treatment planning using volumetric modulated arc therapy (VMAT) for craniospinal irradiation (CSI). A 20-year-old male patient with leukemia was selected. The planning target volume (PTV) was contoured to include the entire contents of the brain and spinal canal. The PTV margin was 10 mm applied to the clinical target volume (CTV). VMAT (RapidArc, Varian Medical Systems, CA, USA) planning was performed using four isocenter with five arcs, two full rotation arcs to cover the brain and upper part of the spinal cord, and one full rotation arc for the lower part of the spinal cord. The plan was created using the auto-feathering photon optimizer calculation of the planning system. The conformity index (CI) and heterogeneity index (HI) as well as dose-volume histograms of organs at risk (OAR) were evaluated. The patient position of ±3.0 mm in the craniocaudal direction was moved in to simulate the effect of treatment inaccuracy. The total treatment time was also measured. The CI and HI were 1.09 and 8.44, respectively. The mean dose (PTV) was 105.5%, and the mean dose (OARs) was lower than the planning dose constraints. Simulations with a patient position shift of ±3.0 mm resulted in an error of less than ±10.0% of the planned dose to the spinal cord. The total treatment time was within 15 minutes. VMAT planning for CSI with Halcyon achieved high conformality, uniform dose distribution, low dose to the surrounding normal tissues, and reduced treatment time.

[Gantry QA Using Three-dimensional Diode Array Detector on O-ring Linear Accelerator].

PURPOSE: It is necessary to perform gantry quality assurance (QA) in high precision radiotherapy. However, the O-ring type linear accelerator (Halcyon) does not have a light field and laser as a reference of isocenter point. The aim of this study is to investigate the usefulness of a three-dimensional diode array detector for gantry angle QA, and an O-ring type linear accelerator. METHOD: The gantry angle and rotational center were verified using the ArcCHECK 3D diode array on the general linear accelerator (TrueBeam) as a reference and Halcyon. The gantry angles were measured at 0, 90, 180, and 270°. The accuracy of the gantry rotational center was evaluated using rotational irradiation in the clockwise and counterclockwise directions between 181° and 179°. RESULTS: The QA system with ArcCHECK was able to apply on the TrueBeam and Halcyon. As a result of the accuracy of the gantry angle, the maximum error of value calculated from ArcCHECK was 0.1° compared with the nominal gantry angle of Halcyon. As a result of the accuracy of the gantry rotation isocenter of Halcyon, the distance between the isocenter and the gantry rotation center was 0.45 mm and 0.41 mm in the clockwise and counterclockwise directions, respectively. CONCLUSION: The QA system with ArcCHECK was useful for the gantry angle and the rotation center accuracy on the Halcyon.

Difference in VMAT dose distribution for prostate cancer with/without rectal gas removal and/or adaptive replanning.

We investigated differences in the volumetric-modulated arc therapy (VMAT) dose distribution in prostate cancer patients treated by rectal gas removal and/or adaptive replanning. Cone-beam computed tomography (CBCT) scans were performed daily for 22 treatments in eight prostate cancer patients with excessive rectal gas, and the CBCT images were analyzed. Rectal gas removal was performed, and irradiation was delivered after prostate matching. We compared dose-volume histograms for the daily CBCT images before and after rectal gas removal. Plan A was the original plan on CBCT images before rectal gas removal. Plan B was a single reoptimized plan on CBCT images before rectal gas removal. Plan C was the original plan on CBCT images after rectal gas removal. Plan D was a single reoptimized plan on CBCT images after rectal gas removal. D95 of the planning target volume (PTV) minus the rectum of Plan C (94.7% ± 6.6%) was significantly higher than that of Plan A (88.5% ± 10.4%). All dosimetric parameters of Plan C were improved by rectal gas removal compared with Plan A, regardless of the initial rectal gas volume. Dosimetric parameters of PTV minus the rectum of Plan B were significantly improved compared with Plan C. Additionally, the V78 of the rectal wall of Plan B (0.2% ± 0.5%) was significantly improved compared with Plan C (3.9% ± 6.3%, p = 0.003). The dosimetric parameters of Plan D were not significantly different from Plan B. The dose distribution of prostate VMAT was improved by rectal gas removal and/or adaptive replanning. An adaptive replanning on daily CBCT images might be a better method than rectal gas removal for prostate cancer patients with excessive rectal gas.

Dosimetric Evaluation of Simplified Knowledge-Based Plan with an Extensive Stepping Validation Approach in Volumetric-Modulated Arc Therapy-Stereotactic Body Radiotherapy for Lung Cancer.

PURPOSE: We investigated the performance of the simplified knowledge-based plans (KBPs) in stereotactic body radiotherapy (SBRT) with volumetric-modulated arc therapy (VMAT) for lung cancer. MATERIALS AND METHODS: For 50 cases who underwent SBRT, only three structures were registered into knowledge-based model: total lung, spinal cord, and planning target volume. We performed single auto-optimization on VMAT plans in two steps: 19 cases used for the model training (closed-loop validation) and 16 new cases outside of training set (open-loop validation) for TrueBeam (TB) and Halcyon (Hal) linacs. The dosimetric parameters were compared between clinical plans (CLPs) and KBPs: CLPclosed, KBPclosed-TB and KBPclosed-Hal in closed-loop validation, CLPopen, KBPopen-TB and KBPopen-Hal in open-loop validation. RESULTS: All organs at risk were comparable between CLPs and KBPs except for contralateral lung: V5 of KBPs was approximately 3%-7% higher than that of CLPs. V20 of total lung for KBPs showed comparable to CLPs; CLPclosed vs. KBPclosed-TB and CLPclosed vs. KBPclosed-Hal: 4.36% ± 2.87% vs. 3.54% ± 1.95% and 4.36 ± 2.87% vs. 3.54% ± 1.94% (P = 0.54 and 0.54); CLPopen vs. KBPopen-TB and CLPopen vs. KBPopen-Hal: 4.18% ± 1.57% vs. 3.55% ± 1.27% and 4.18% ± 1.57% vs. 3.67% ± 1.26% (P = 0.19 and 0.27). CI95 of KBPs with both linacs was superior to that of the CLP in closed-loop validation: CLPclosed vs. KBPclosed-TB vs. KBPclosed-Hal: 1.32% ± 0.12% vs. 1.18% ± 0.09% vs. 1.17% ± 0.06% (P < 0.01); and open-loop validation: CLPopen vs. KBPopen-TB vs. KBPopen-Hal: 1.22% ± 0.09% vs. 1.14% ± 0.04% vs. 1.16% ± 0.05% (P ≤ 0.01). CONCLUSIONS: The simplified KBPs with limited number of structures and without planner intervention were clinically acceptable in the dosimetric parameters for lung VMAT-SBRT planning.

Plan Complexity and Delivery Accuracy of Knowledge-based Volumetric Modulated Arc Therapy Plans with Single Optimization for Oropharyngeal Cancer.

BACKGROUND/AIM: We investigated the plan complexity of volumetric modulated arc therapy (VMAT) with knowledge-based plan (KBP) for oropharyngeal cancer (OPC) with a single optimization and whether it could be used clinically. MATERIALS AND METHODS: KBP model was configured using 55 consecutive OPC and nasopharyngeal cancer plans. Plan complexity as a characteristic of multileaf collimator (MLC) motion and γ pass rate (2%/2 mm criterion) were compared between clinical manual plan (CMP) and KBP for other 10 plans. RESULTS: Plan complexity metrics that had significant differences (p<0.05) (CMP vs. KBP), were mean lateral displacement of MLC from central axis (15.82 mm vs. 18.90 mm), proportions of MLC aperture sizes of ≤5 mm (0.14 vs. 0.11), ≤10 mm (0.24 vs. 0.19), and ≤20 mm (0.41 vs. 0.34), and monitor units (578.68 vs. 505.04). The γ pass rate was 91.3% vs. 93.3%. CONCLUSION: Single optimized KBP for OPC had simple plan complexity features and comparable delivery accuracy to CMP, and could be clinically applied.

Feasibility study of volumetric modulated arc therapy with Halcyon™ linac for total body irradiation.

BACKGROUND: The use of total body irradiation (TBI) with linac-based volumetric modulated arc therapy (VMAT) has been steadily increasing. Helical tomotherapy has been applied in TBI and total marrow irradiation to reduce the dose to critical organs, especially the lungs. However, the methodology of TBI with Halcyon™ linac remains unclear. This study aimed to evaluate whether VMAT with Halcyon™ linac can be clinically used for TBI. METHODS: VMAT planning with Halcyon™ linac was conducted using a whole-body computed tomography data set. The planning target volume (PTV) included the body cropped 3 mm from the source. A dose of 12 Gy in six fractions was prescribed for 50% of the PTV. The organs at risk (OARs) included the lens, lungs, kidneys, and testes. RESULTS: The PTV D98%, D95%, D50%, and D2% were 8.9 (74.2%), 10.1 (84.2%), 12.6 (105%), and 14.2 Gy (118%), respectively. The homogeneity index was 0.42. For OARs, the Dmean of the lungs, kidneys, lens, and testes were 9.6, 8.5, 8.9, and 4.4 Gy, respectively. The V12Gy of the lungs and kidneys were 4.5% and 0%, respectively. The Dmax of the testes was 5.8 Gy. Contouring took 1-2 h. Dose calculation and optimization was performed for 3-4 h. Quality assurance (QA) took 2-3 h. The treatment duration was 23 min. CONCLUSIONS: A planning study of TBI with Halcyon™ to set up VMAT-TBI, dosimetric evaluation, and pretreatment QA, was established.

Plan complexity quantification of dual-layer multi-leaf collimator for volumetric modulated arc therapy with Halcyon linac.

This study characterized the plan complexity of the dual-layer multi-leaf collimator (MLC) performance for volumetric modulated arc therapy (VMAT) in Halcyon. Plan complexity metrics were computed independently for each MLC layer (proximal and distal): modulation complexity score (MCS), plan averaged beam area, irregularity, and modulation (PA, PI, and PM) were obtained. These were investigated when weighted by proportions of the effective proximal MLC and distal MLC (MCSw, PAw, PIw, and PMw). Conventional plan complexity metrics for the effective 5.0 mm resolution MLC (MCS5, PA5, PI5, PM5), small aperture score (SAS), effective distal MLC score (EDS), and MU were also evaluated. Forty-five consecutive VMAT plans were analyzed retrospectively for treatment sites of the prostate, head and neck (HN), and other parts. The mean values of the metrics for proximal MLC and distal MLC were 0.310 and 0.245 (MCS), 71.09 cm2 and 55.92 cm2 (PA), 6.24 and 10.19 (PI), and 0.593 and 0.645 (PM), respectively. The degree of plan complexity in the distal MLC was larger than in the proximal MLC. The percentage of the effective distal MLC was higher than that of the proximal MLC (mean EDS = 0.555). The MCSw, PAw, PIw, and PMw showed significant differences between the prostate and HN groups like the PA5, PI5, and PM5; however, the MCS5 did not. Additionally, the MCS5 correlated with MU and PM5, while the MCSw correlated with SAS and PA5. The plan complexity for each MLC layer and subsequently combining these complexities might be useful to better understand the dual-layer MLC performance.

Design of commissioning process for Halcyon™ linac with a new rigid board: A clinical experience.

PURPOSE: This study performed the accurate measurements of beam profiles with a new rigid board, which was consistent with the supplied reference beam profiles (RBPs) for clinical Halcyon model. METHODS: Percentage depth doses (PDDs), lateral and diagonal dose profiles were measured and compared with RBPs. A water tank was set on the rigid board bridged Halcyon bore without sagging and source-to-surface distance was 90.0 cm. Field sizes were from 2.0 to 28.0 cm squares and depths of lateral and diagonal dose profiles were 1.3, 5.0, 10.0, and 20.0 cm. For the PDD, the depth of maximum dose (dmax), PDD value at depth of 10.0 cm (PDD10), and absolute dose difference (DD) between RBP and measured beam profiles (MBP) were evaluated. For lateral and diagonal dose profiles, DDs for the whole and divided areas (central, shoulder, and extended areas) defined by third derivative, and distance-to-agreement (DTA) in the penumbra area were evaluated. RESULTS: For PDDs, the differences of dmax and PDD10 and DD beyond the dmax were within 1.0 mm, 0.3%, and 1.0%, respectively. For lateral and diagonal dose profiles, the DDs reached approximately 5.0% in the whole area because of penumbra area, while the DDs in the central, shoulder, and extended areas were within 1.0%, 2.0%, and 1.0%, respectively. The DTAs in the penumbra area were within 0.8 mm. CONCLUSIONS: The supplied RBPs can be used clinically owing to the good agreement with the accurate MBPs with rigid board.

Simplified sigmoidal curve fitting for a 6 MV FFF photon beam of the Halcyon to determine the field size for beam commissioning and quality assurance.

BACKGROUND: An O-ring gantry-type linear accelerator (LINAC) with a 6-MV flattening filter-free (FFF) photon beam, Halcyon, includes a reference beam that contains representative information such as the percent depth dose, profile and output factor for commissioning and quality assurance. However, because it does not provide information about the field size, we proposed a method to determine all field sizes according to all depths for radiation therapy using simplified sigmoidal curve fitting (SCF). METHODS: After mathematical definition of the SCF using four coefficients, the defined curves were fitted to both the reference data (RD) and the measured data (MD). For good agreement between the fitting curve and the profiles in each data set, the field sizes were determined by identifying the maximum point along the third derivative of the fitting curve. The curve fitting included the field sizes for beam profiles of 2 × 2, 4 × 4, 6 × 6, 8 × 8, 10 × 10, 20 × 20 and 28 × 28 cm2 as a function of depth (at 1.3, 5, 10 and 20 cm). The field size results from the RD were compared with the results from the MD using the same condition. RESULTS: All fitting curves show goodness of fit, R2, values that are greater than 0.99. The differences in field size between the RD and the MD were within the range of 0 to 0.2 cm. The smallest difference in the field sizes at a depth of 10 cm, which is a surface-to-axis distance, was reported. CONCLUSION: Application of the SCF method has been proven to accurately capture the field size of the preconfigured RD and the measured FFF photon beam data for the Halcyon system. The current work can be useful for beam commissioning as a countercheck methodology to determine the field size from RD in the treatment planning system of a newly installed Halcyon system and for routine quality assurance to ascertain the correctness of field sizes for clinical use of the Halcyon system.

Variations of the Dose Distribution Between CT- and CBCT-based Plans for Oropharyngeal Cancer.

BACKGROUND/AIM: The parotid glands in the head and neck are organs at risks (OARs) adjacent to high dose region and dose of OARs might be increased during the course of radiotherapy. The influence factors of the dose distribution for the parotid glands were investigated in terms of weight loss and mandibular rotation for head and neck cancers treated with volumetric modulated arc therapy (VMAT). PATIENTS AND METHODS: Ten oropharyngeal cancer patients (OPC) who underwent VMAT were enrolled. The dose volume histogram (DVH) parameters of the parotid glands and planning target volume (PTV) were compared between the planning computed tomography (CT) and the on board imager (OBI) at 1, 5, 10, 15, and 20 fractions. RESULTS: The variation of dose distribution in PTV was not observed in both factors. The relationship between the mandibular rotation and dose difference for the right and left parotid glands (linear regression, r2=0.1577 and -0.689) showed a slighty stronger correlation with dose difference than the weight loss (linear regression, r2=-0.079 and -0.547). CONCLUSION: The mandibular rotation tends to have a large influence on dose distribution of the parotid glands for head and neck cancers treated with VMAT.

Correction: Evaluation of lung toxicity risk with computed tomography ventilation image for thoracic cancer patients.

[This corrects the article DOI: 10.1371/journal.pone.0204721.].