Mean Dose Difference Research Articles

P163] Dosimetric effects from high density markers for prostate cancer treatments

Purpose Implanted high density gold markers are used in radiotherapy of prostate cancer for a more accurate delivery of planned target dose. However, the presence of high density objects inside the prostate causes artefacts in the computed tomography (CT) images from which the dose calculations are based on. The interpretation of the tissues made by the different dose calculation algorithms becomes uncertain and this uncertainty may propagate to the final dose distribution. The aim of this work was to quantify the dosimetric effects from high density markers on dose distributions for prostate cancer treatments using three different algorithms. Methods About 200 patients treated with volumetric modulated arc therapy (VMAT) for prostate cancer were included. Dose calculations (dose to water) were executed using the treatment planning system (TPS) algorithms, anisotropic analytical algorithm (AAA), Acuros XB (AXB) and by an automated Monte Carlo (MC) system based on the EGSnrc code package with modifications. In order to quantify the effect of high density markers, dose distributions for prostate plans (post-operative and with gold markers) were compared. Dose volume histogram (DVH) estimates such as the mean dose to the clinical target volume (CTV), the planning target volume (PTV), D 98 % PTV and D 2 % PTV (dose to xx% of the PTV) were evaluated. Further analysis was performed by modifying the artefact areas in the CT images by setting the material in the PTV to water. Results The mean difference between AAA, AXB and MC estimations of the mean dose to the CTV and PTV was within 0.5% for all cases. Also, mean dose deviations up to 2% were observed for individual plans, especially for plans with gold markers. Dose deviations up to 5%, related to the shape of the DVH, were detected for D 98 % PTV and D 2 % PTV. The size of the CTV showed no impact on dose deviations for prostate plans with gold markers. Conclusions The presence of gold markers increases the variation between CTV and PTV dosimetry parameters obtained by different algorithms. However, no clinically relevant dosimetric effects arise from artefacts caused by high density markers.

OA009] Motion inclusive variations in bladder dose surface maps during the course of high-precision radiotherapy for prostate cancer

Purpose Modern radiotherapy (RT) protocols for prostate cancer often involve the use of narrow margins and image-based monitoring of rectum/bladder fill status. These protocols have allowed safe prostate dose escalation, maintaining acceptable levels for organs at risk (bladder and rectum). However, daily cone-beam CT (CBCT) image-guided RT has demonstrated considerable bladder volume variation throughout treatment, which suggests dose delivered may vary. By using CBCT-based parametrized 2D dose surface maps (DSM) of the inferior bladder, this study aims to evaluate bladder volume impact on bladder DSMs during high-precision RT for prostate cancer. Methods Seven prostate cancer patients treated using daily CBCT-based image-guided VMAT/IMRT were included in this study (81.0 Gy prescription dose). Planning CT and RT delivery adhered to a full bladder/empty rectum protocol, where daily CBCTs were used for patient realignment and to assess bladder/rectum filling status. Fourteen CBCTs per patient were rigidly registered to the planning CT using recorded treatment shifts, and the bladder was manually contoured on each CBCT. Contours were validated by the responsible radiation oncologist. For the planning CT and each CBCT, bladder wall dose was digitally projected onto 1024x1024 DSMs using orthographic ray-traced surface dose sampling. Bladder wall volumetric meshes were generated from contours using restricted Delaunay triangulation. Surface dose was sampled at ray-wall intersections, sub-pixel sampling was used to improve image quality, and rays were oriented parallel to the prostate-bladder center-of-mass connecting line. DSM dose distributions were compared between planned and delivered. Correlation with bladder volume variations was evaluated. Results Bladder volumes varied considerably during RT (15–42%), with slightly larger volumes at planning compared to treatment (p = 0.16). Differences at the central part of the DSM ranged between 1% and 7%. Overall, delivered doses were lower at the central part of the DSM compared to planned (range: −5.4 to 0.9 Gy). Plan-vs-delivery mean dose differences were slightly correlated with bladder volume differences (Rs = 0.55, p = 0.3). Conclusions No significant variations were observed in delivered doses at the interior part of the bladder although considerable bladder volume changes occurred during the RT course. The generally smaller treatment bladder volume possibly explains lower dose delivered to the inferior bladder sector.

OA036] Clinical implementation and evaluation of the Mobius3D system for independent dose calculation

Purpose The complexity of dose planning and treatment delivery systems has increased and necessitates a more comprehensive physics quality assurance (QA). These QA checks can, however, be quite time consuming. Therefore, there is a need for automation of QA which usually rely on empirically established tolerance levels. Mobius3D is a commercial fully automated QA system that uses a collapsed cone convolution/superposition algorithm for verification of treatment plans. The purpose of this study was to investigate the level of deviation between this system and the clinical commissioned treatment planning system (Eclipse v.13.5) using the Anisotropic Analytical Algorithm when the MLC modeling was optimized. Methods The Mobius3D system comes with golden beam data and calculates the dose on the patient CT data set. For MLC optimization, 8 IMRT/VMAT plans from various anatomical regions were included and recalculated in a cylindrical plastic phantom containing two planar diode arrays (Delta4 phantom, ScandiDos). The plans were then verified experimentally at the accelerator and all passed a local 3%/2 mm gamma criteria (passing rate > 90%). The dosimetric leaf gap (DLG) used to model the MLC in Mobius3D was optimized to keep the PTV mean dose of the 8 plans within 2% in the Delta4 geometry resulting in a +0.6 mm adjustment of the DLG from the default value. To establish the level of dose deviation and establish acceptance criteria, 34 treatment plans in different geometries, such us lung (10), head and neck (HN, 10), and pelvis (14) were evaluated for both IMRT and VMAT. Results The absolute mean and max PTV dose deviation for (lung/HN/pelvis) were: 0.8/2.2/2.1% and 1.4/2.9/2.8%. All max deviations were positive, i.e. a higher dose estimate by Mobius3D. 5 HN IMRT plans before DLG adjustment all showed negative deviations of −3.12/−3.8% (PTV mean/max). Conclusions A random sample of treatment plans were within 2.5% in absolute mean dose of the PTV between Mobius3D and the TPS. MLC parameter optimization plays an important role in establishing an acceptance criterion.

P125] First PDIP validation for stereotatic treatment control using FFF beams and EPID AS1000 on clinac

Objective Varian Medical System® proposes Portal dosimetry as an integrated solution to control modulated beam treatment. Among all Varian Electronic Portal Image Detectors (EPID), only the Portal Vision aS1200 allows verification of Flattening Filter-Free (FFF) treatment plans with PDIP algorithm. Many centers with a previous EPID, such as aS1000, are forced to use other methods for treatment verification. The purpose of this study is to verify the feasibility of setting up these controls by PDIP concerning the as1000 EPID model and modulated FFF beams performed on Varian Clinac. Materials/methods The EPID aS1000 response was studied for X6 FFF beams by analyzing SDD, linearity, field size, remanence and backscattering arm effects. PDIP algorithm was configured for pre-treatment verification of FFF beams at 1400 MU/min dose rate by placing the detector at SDD 150 cm to avoid saturation. The use of the Varian Portal Dosimetry software was validated using test plans provided by constructor. A retrospective study was carried out on 34 stereotactic treatment plans in VMAT X6 FFF corresponding to 140 beams. The plans were irradiated with the Varian Clinac 2300iX. Gamma analysis was performed on a Sun Nuclear ArcCheck® (Sun Nuclear, FL) plan control phantom. The gamma criteria of 2% 2 mm and 2.5% 2.5 mm for both global and local methods analysis were used. Results Dose linearity was within 0.99%. The backscattering arm effect results in asymmetries on the vertical dose profiles of 0.88 ± 0.36%. Integrated dose images had showed a remanence dose resulting on an overstatement of the dose of 0.38% at the maximum. The repeatability of the measurement showed mean dose deviations of 0.06 ± 0.17%. Conclusion The use of AS1000 and Clinac to measure FFF beams is a success. The locks due to the generation of the system have been lifted. The very complex and high dose field checks in VMAT type stereo allowed to validate the modeling. This solution saves measurement time, facilitates use, and improves spatial resolution which can be better than that in available 3D phantoms.

Validation of the Acuros XB dose calculation algorithm versus Monte Carlo for clinical treatment plans.

The two distinct dose computation paradigms of Boltzmann equation solvers and Monte Carlo simulation both promise in principle maximum accuracy. In practice, clinically acceptable calculation times demand approximations and numerical short-cuts on one hand, and modeling the beam characteristics of a real linear accelerator to the required accuracy on the other. A thorough benchmark of both algorithm types therefore needs to start with beam modeling, and needs to include a number of clinically challenging treatment plans. The Acuros XB (v 13.7, Varian Medical Systems) and SciMoCa (v 1.0, Scientific RT) algorithms were commissioned for the same Varian Clinac accelerator for beam qualities 6 and 15 MV. Beam models were established with water phantom measurements and MLC calibration protocols. In total, 25 patients of five case classes (lung/three-dimensional (3D) conformal, lung/IMRT, head and neck/VMAT, cervix/IMRT, and rectum/VMAT) were randomly selected from the clinical database and computed with both algorithms. Statistics of 3D gamma analysis for various dose/distance-to-agreement (DTA) criteria and differences in selected DVH parameters were analyzed. The percentage of points fulfilling a gamma evaluation was scored as the gamma agreement index (GAI), denoted as G(ΔD, DTA). G(3,3), G(2,2), and G(1,1) were evaluated for the full body, PTV, and selected organs at risk (OARs). For all patients, G(3,3) ≥ 99.9% and G(2,2) > 97% for the body. G(1,1) varied among the patients. However, for all patients, G(1,1) > 70% and G(1,1) > 80% for 68% of the patients. For each patient, the mean dose deviation was ΔD < 1% for the body, PTV, and all evaluated OARs, respectively. In dense bone and at off-axis distance > 10 cm, the Acuros algorithm yielded slightly higher doses. In the first layer of voxels of the patient surface, the calculated doses deviated between the algorithms. However, at the second voxel, good agreement was observed. The differences in D(98%PTV) were <1.9% between the two algorithms and for 76% of the patients, deviations were below 1%. Overall, an outstanding agreement was found between the Boltzmann equation solver and Monte Carlo. High-accuracy dose computation algorithms have matured to a level that their differences are below common experimental detection thresholds for clinical treatment plans. Aside from residual differences which could be traced back to implementation details and fundamental cross-section data, both algorithms arrive at identical dose distributions.

Feasibility of magnetic resonance imaging-only rectum radiotherapy with a commercial synthetic computed tomography generation solution.

Background and purposeSynthetic computed tomography (sCT) images enable magnetic resonance (MR)-based dose calculations. This work investigated whether a commercially available sCT generation solution was suitable for accurate dose calculations and position verification on patients with rectal cancer.Material and methodsFor twenty rectal cancer patients computed tomography (CT) images were rigidly registered to sCT images. Clinical volumetric modulated arc therapy plans were recalculated on registered CT and sCT images. Dose deviations were determined through gamma and voxelwise analysis. The impact on position verification was investigated by identifying differences in translations and rotation between cone-beam CT (CBCT) to CT and CBCT to sCT registrations.ResultsAcross twenty patients, within a threshold of 90% of the prescription dose, a gamma analysis (2%, 2 mm) mean pass rate of 95.2 ± 4.0% (±1) and mean dose deviation of −0.3 ± 0.2% of prescription dose were obtained. The mean difference of translations and rotations over ten patients (76 CBCTs) was <1 mm and <0.5° in all directions. In the sole posterior-anterior direction a mean systematic shift of 0.7 ± 0.6 mm was found.ConclusionsAccurate MR-based dose calculations using a commercial sCT generation method were clinically feasible for treatment of rectal cancer patients. The accuracy of position verification was clinically acceptable. However, before clinical implementation future investigations will be performed to determine the origin of the systematic shift.

Automated data mining of a plan-check database and example application.

PurposeThe aim of this work was to present the development and example application of an automated data mining software platform that preforms bulk analysis of results and patient data passing through the 3D plan and delivery QA system, Mobius3D.MethodsPython, matlab, and Java were used to create an interface that reads JavaScript Object Notation (JSON) created for every approved Mobius3D pre‐treatment plan‐check. The aforementioned JSON files contain all the information for every pre‐treatment QA check performed by Mobius3D, including all 3D dose, CT, structure set information, as well as all plan information and patient demographics. Two Graphical User Interfaces (GUIs) were created, the first is called Mobius3D‐Database (M3D‐DB) and presents the check results in both filterable tabular and graphical form. These data are presented for all patients and includes mean dose differences, 90% coverage, 3D gamma pass rate percentages, treatment sites, machine, beam energy, Multi‐Leaf Collimator (MLC) mode, treatment planning system (TPS), plan names, approvers, dates and times. Group statistics and statistical process control levels are then calculated based on filter settings. The second GUI, called Mobius3D organ at risk (M3DOAR), analyzes dose‐volume histogram data for all patients and all Organs‐at‐Risk (OAR). The design of the software is such that all treatment parameters and treatment site information are able to be filtered and sorted with the results, plots, and statistics updated.ResultsThe M3D‐DB software can summarize and filter large numbers of plan‐checks from Mobius3D. The M3DOAR software is also able to analyze large amounts of dose‐volume data for patient groups which may prove useful in clinical trials, where OAR doses for large numbers of patients can be compared and correlated. Target DVHs can also be analyzed en mass.ConclusionsThis work demonstrates a method to extract the large amount of treatment data for every patient that is stored by Mobius3D but not easily accessible. With scripting, it is possible to mine this data for research and clinical trials as well as patient and TPS QA.

Estimation of extremely small field radiation dose for brain stereotactic radiotherapy using the Vero4DRT system

The purpose of this study was a dosimetric validation of the Vero4DRT for brain stereotactic radiotherapy (SRT) with extremely small fields calculated by the treatment planning system (TPS) iPlan (Ver.4.5.1; algorithm XVMC). Measured and calculated data (e.g. percentage depth dose [PDD], dose profile, and point dose) were compared for small square fields of 30 × 30, 20 × 20, 10 × 10 and 5 × 5 mm2 using ionization chambers of 0.01 or 0.04 cm3 and a diamond detector. Dose verifications were performed using an ionization chamber and radiochromic film (EBT3; the equivalent field sizes used were 8.2, 8.7, 8.9, 9.5, and 12.9 mm2) for five brain SRT cases irradiated with dynamic conformal arcs.The PDDs and dose profiles for the measured and calculated data were in good agreement for fields larger than or equal to 10 × 10 mm2 when an appropriate detector was chosen. The dose differences for point doses in fields of 30 × 30, 20 × 20, 10 × 10 and 5 × 5 mm2 were +0.48%, +0.56%, −0.52%, and +11.2% respectively. In the dose verifications for the brain SRT plans, the mean dose difference between the calculated and measured doses were −0.35% (range, −0.94% to +0.47%), with the average pass rates for the gamma index under the 3%/2 mm criterion being 96.71%, 93.37%, and 97.58% for coronal, sagittal, and axial planes respectively.The Vero4DRT system provides accurate delivery of radiation dose for small fields larger than or equal to 10 × 10 mm2.

Evaluation of target dose inhomogeneity in breast cancer treatment due to tissue elemental differences

BackgroundMonte Carlo simulations were run to estimate the dose variations generated by thedifference arising from the chemical composition of the tissues.MethodsCT datasets of five breast cancer patients were selected. Mammary gland was delineated as clinical target volume CTV, as well as CTV_lob and CTV_fat, being the lobular and fat fractions of the entire mammary gland. Patients were planned for volumetric modulated arc therapy technique, optimized in the Varian Eclipse treatment planning system. CT, structures and plans were imported in PRIMO, based on Monte Carlo code Penelope, to run three simulations: AdiMus, where the adipose and muscle tissues were automatically assigned to fat and lobular fractions of the breast; Adi and Mus, where adipose and muscle, respectively were assigned to the whole mammary gland. The specific tissue density was kept identical from the CT dataset. Differences in mean doses in the CTV_lob and CTV_fat structures were evaluated for the different tissue assignments. Differences generated by the tissue composition and estimated by Acuros dose calculations in Eclipse were also analysed.ResultsFrom Monte Carlo simulations, the dose in the lobular fraction of the breast, when adipose tissue is assigned in place of muscle, is overestimated by 1.25 ± 0.45%; the dose in the fat fraction of the breast with muscle tissue assignment is underestimated by 1.14 ± 0.51%. Acuros showed an overestimation of 0.98 ± 0.06% and an underestimation of 0.21 ± 0.14% in the lobular and fat portions, respectively. Reason of this dissimilarity resides in the fact that the two calculations, Monte Carlo and Acuros, differently manage the range of CT numbers and the material assignments, having Acuros an overlapping range, where two tissues are both present in defined proportions.ConclusionAlthough not clinically significant, the dose deposition difference in the lobular and connective fat fraction of the breast tissue lead to an improved knowledge of the possible dose distribution and homogeneity in the breast radiation treatment.

Application and set-up error of deep inspiration breath-hold (DIBH) technique for whole breast irradiation in left breast cancer

Objective To analyze the correlation between treatment time and radiotherapy plan of deep inspiration breath-hold (DIBH) technique for the whole breast irradiation (WBI) in the left breast cancer after breast-conserving surgery, verify the inter-fractional reproducibility of radiotherapy, observe the heart location and dosimetric changes and calculate the effect of DIBH upon the WBI setup error after the surgery. Methods We prospectively enrolled 15 patients with left breast cancer undergoing WBI after breast-conserving surgery, who met the requirement of DIBH.Treatment time was recorded, its correlation with the number of field and monitor unit was analyzed. Inter-fractional setup errors and PTV delineation were calculated using cone beam CT (CBCT). The accuracy of the position and dose of the heart during radiotherapy was verified by the imaging fusion of CBCT and CT images. The variables among groups were analyzed by non-parametric Firedman test. Results The average treatment time of DIBH radiotherapy was 4.6 minutes. The treatment time was correlated with the maximal and total number of sub-fields and total monitor units. During DIBH treatment, the mean cardiac displacement volume was 19.1 cm3(3.8%). The mean cardiac dose difference between CBCT and planning CT was 5.1 cGy, and there was no significant difference in the heart V5-V30.The mean inter-fractional system setup error (Σ) and random setup error (σ) in the left-right (x), superior-inferior (y) and anterior-posterior (z) direction were Σx 1.9 mm, Σy 2.1 mm, Σz 2.0 mm, σx1.3 mm, σy 1.3 mm, σz 1.4 mm, respectively. The corresponding minimal margins for setup error were 5.7 mm, 6.2 mm and 6.0 mm, respectively. Conclusion DIBH for WBI after breast-conserving surgery does not significantly prolong the treatment time. Treatment time is related to treatment plan. DIBH yields high inter-fractional reproducibility and protects the heart. Key words: Breast neoplasm; Radiotherapy dosage; Deep inspiration breath-hold technique; Setup error

(OA40) Deep Learning-Based Autosegmentation for Head and Neck Radiotherapy

To investigate the feasibility of implementing a deep learning-based autosegmentation system for head and neck cancer patients undergoing radiotherapy. The accuracy of four different contouring approaches was investigated. Two atlas-based autosegmentation (ABAS) systems, MIM (MIM Software, Inc., Cleveland, OH) (ABAS1) and RayStation (RaySearch Laboratories AB, Stockholm, Sweden) (ABAS2), one deep learning-based autosegmentation (DLBAS) system Nimble ContourTM (Nimble Therapy, LLC, San Francisco, CA), and intra-physician contouring variability (IPCV), were evaluated. 200 patients were considered in this study. 160 patients were used to train the ABAS1 and DLBAS algorithms; due to limitations of the system only 10 patients were used to train ABAS2. The IPCV was generated by expert manual contouring of the spinal cord, mandible, right parotid gland, left parotid gland, oral cavity, brainstem, larynx, esophagus, right submandibular gland, left submandibular gland, right temporomandibular joint (TMJ), and left TMJ for five head and neck cancer patients ten times each. Contouring approaches were analyzed by calculating the average Dice coefficients, root mean squared error (RMSE), maximum dose difference, and mean dose difference. The IPCV scored the best for most evaluation metrics. DLBAS performed only slightly worse than IPCV for most evaluation metrics and was able to match or beat the IPCV for some OARs. DLBAS was significantly better than ABAS1 and ABAS2 for most evaluation metrics. The ABAS systems required approximately 1.5 minutes per training patient; the DLBAS method required approximately 72 hours to train all 12 OARs. By moving towards deep learning-based algorithms, autosegmentation accuracy can approach ICPV accuracy. Deep learning-based autosegmentation is a superior alternative to atlas-based autosegmentation, given sufficient training data and training time. This technique can help reduce physician workload and standardize practice between institutions.

Impact of the Lok-bar for High-precision Radiotherapy with Tomotherapy.

Patient immobilization systems are used to establish a reproducible patient position relative to the couch. In this study, the impact of conventional lok-bars for CT-simulation (CIVCO-bar) and treatment (iBEAM-bar) were compared with a novel lok-bar (mHM-bar) in tomotherapy. Verification was obtained as follows: i. artifacts in CT images; ii. dose attenuation rate of lok-bar, compared to without lok-bar; and iii. dose differences between the calculated and measured absorbed doses. With the CIVCO-bar, there were obvious metal artifacts, while there were nearly no artifacts with the mHM-bar. The mean dose attenuation rates with the mHM-bar and iBEAM-bar were 1.31% and 2.28%, and the mean dose difference was 1.55% and 1.66% for mHM-bar and iBEAM-bar. Using the mHM-bar reduced artifacts on the CT image and improved dose attenuation are obtained. The lok-bar needs to be inserted as a structure set in treatment planning with tomotherapy.

Interplay effect on a 6-MV flattening-filter-free linear accelerator with high dose rate and fast multi-leaf collimator motion treating breast and lung phantoms.

Using a new linear accelerator with high dose rate (800MU/min), fast MLC motions (5.0cm/s), fast gantry rotation (15s/rotation), and 1cm wide MLCs, we aimed to quantify the effects of complexity, arc number, and fractionation on interplay for breast and lung treatments under target motion. To study lung interplay, eight VMAT plans (1-6 arcs) and four-nine-field sliding-window IMRT plans varying in complexity were created. For the breast plans, four-four-field sliding-window IMRT plans were created. Using the Halcyon 1.0 linear accelerator, each plan was delivered five times each under sinusoidal breathing motion to a phantom with 20 implanted MOSFET detectors; MOSFET dose (cGy), delivery time, and MU/cGy values were recorded. Maximum and mean dose deviations were calculated from MOSFET data. The number of MOSFETs with at least 19 of 20 detectors agreeing with their expected dose within 5% per fraction was calculated across 106 iterations to model dose deviation as function of fraction number for all plan variants. To put interplay plans into clinical context, additional IMRT and VMAT plans were created and delivered for the sites of head and neck, prostate, whole brain, breast, pelvis, and lung. Average modulation and interplay effect were compared to those from conventional linear accelerators, as reported from previous studies. The mean beam modulation for plans created for the Halcyon 1.0 linear accelerator was 2.9MU/cGy (two- to four-field IMRT breast plans), 6.2MU/cGy (at least five-field IMRT), and 3.6MU/cGy (four-arc VMAT). To achieve treatment plan objectives, Halcyon 1.0 VMAT plans require more arcs and modulation than VMAT on conventional linear accelerators. Maximum and mean dose deviations increased with increasing plan complexity under tumor motion for breast and lung treatments. Concerning VMAT plans under motion, maximum, and mean dose deviations were higher for one arc than for two arcs regardless of plan complexity. For plan variants with maximum dose deviations greater than 3.7%, dose deviation as a function of fraction number was protracted. For treatments on the Halcyon 1.0 linear accelerator, the convergence of dose deviation with fraction number happened more slowly than reported for conventional linear accelerators. However, if plan complexity is reduced for IMRT and if tumor motion is less than ~10-mm, interplay is greatly reduced. To minimize dose deviations across multiple fractions for dynamic targets, we recommend limiting treatment plan complexity and avoiding one-arc VMAT on the Halcyon 1.0 linear accelerator when interplay is a concern.

Personalized automated treatment planning for breast plus locoregional lymph nodes using Hybrid RapidArc

PurposeBreast cancer patients who require locoregional lymph node (LLN) irradiation can be treated using a hybrid RapidArc technique combining 2 tangential and 3 RapidArc fields. Because the creation of hybrid RapidArc plans is complex and labor-intensive, we developed an automated treatment planning workflow using the scripting application programming interface of the Eclipse treatment planning system. Methods and materialsFifteen patients (5 right- and 10 left-sided) previously treated with breast + LLN radiation therapy were replanned using the script. The automated workflow included1 optimal placement of the tangential fields based on the planning target volume and organ-at-risk contours, followed by optimization of field weights and beam energy2; positioning of the RapidArc fields; and3 subsequent RapidArc optimization using the RapidPlan knowledge-based planning solution. ResultsAverage total planning times were 163 ± 97 and 33 ± 5 minutes for the manual and automated plans, respectively, with approximately 130 and 5 minutes of user interaction. Dosimetrically, both sets of plans were very similar, with comparable planning target volume dose homogeneity values and organ-at-risk mean dose differences of ≤1.9 Gy. In 14/15 patients, the physician judged that the automated plan was either preferred (n = 4) or equal (n = 10) to the manual plan. ConclusionsThe complex hybrid RapidArc planning process for patients requiring breast + LLN irradiation was automated by optimizing the tangential field setup and integrating RapidPlan. The quality of the automated and manual plans was comparable, whereas automated planning times were substantially shorter. The principles described here could be used to automate other planning workflows.

Relationship between radiation-induced xerostomia and the volume of parotid glands exposed in radiotherapy for nasopharyngeal carcinoma

Objective To evaluate the relationship between the volume of parotid glands and radiation-induced xerostomia in patients with nasopharyngeal carcinoma (NPC). Methods A total of 199 patients with NPC who were admitted to our hospital from 2015 to 2016 were enrolled as subjects. According to the initial volume of parotid glands, all patients were divided into large parotid group and small parotid group. The incidence of xerostomia after radiotherapy and dosimetric parameters were compared between the two groups. Comparison of categorical data was made by χ2 test. Comparison of continuous data was made by t-test or nonparametric test. Comparison of dose-volume histogram index was made by nonparametric test. Results There was no difference in the severity of xerostomia between the two groups at 3 and 6 months after radiotherapy. At one year after radiotherapy, the large parotid group had significantly milder xerostomia than the small parotid group (P=0.035). The small parotid group had a higher dose delivered to both parotid glands than the large parotid group. There was no difference in the mean dose to the submandibular gland between the two groups. Conclusions The initial volume of parotid glands is one of the influencing factors for the grade of xerostomia after radiotherapy. Patients with large parotid glands have better recovery from xerostomia after radiotherapy than those with small parotid glands. For patients with small parotid glands, more attention should be paid to reducing the dose to protect parotid glands. Key words: Nasopharyngeal neoplasms/intensity-modulated radiotherapy; Radiation-induce dxerostomia; Dosimetry

Analysis of ion beam teletherapy patient-specific quality assurance

The objective of this study was to evaluate the procedures for patient-specific quality assurance measurements using modulated scanned and energy stacked beams for proton and carbon ion teletherapy. Delivery records from 1734 portal measurements were analyzed using a 3-point pass criteria: more than 22 of 24 chambers in a water phantom (WP) had to have a measured dose difference from the planned portal doses less than or equal to 3%, or the distance from the measurement point location to a point location in the plan having the same dose had to be less than or equal to 3 mm (distance to agreement [DTA]), and the mean dose deviation of all chambers had to be less than 3%. Stratification of results showed some associations between measurement parameters and pass rates. For proton portals, pass rates were high at all measurement depths, but for carbon ion portals, pass rates decreased as a function of increasing measurement depth. Pass rates of both proton and carbon ion portals with 1 WP were slightly lower than those with a second WP. The total pass rates were 97.7% and 91.9% for proton and carbon ion patient portals, respectively. In general, the measured doses exhibited good agreement with the treatment planning system (TPS) calculated doses. When the chamber position was deeper than 150 mm in carbon ion beams, a lower pass rate was observed, which may have been caused by ion chamber array setup uncertainty (lateral and depth) in highly modulated portals or incorrect modeling of scatter by the TPS. These deviations need further investigation.

Normal Tissue Complication Probability (NTCP) model for erectile dysfunction (ED) following external beam radiotherapy (RT) for prostate cancer.

135 Background: ED remains a common toxicity of prostate RT despite technological advances. Penile bulb (PB) dose has been proposed as a predictor of ED post RT. The main objective of this study was to develop NTCP models for ED. Methods: 162 men treated within the CHHiP IGRT substudy (CRUK/06/16) had baseline clinical data, PB dosimetric data &amp; evaluation of ED using EPIC-26 at least 3 years post RT. Planning CT and reference dose distributions were imported into analysis software (VODCA, MSS GmbH) and PB retrospectively contoured by one clinician. The defined endpoint (severe ED) was a standardised average value of 0-33 for EPIC-26 sexual domain. Predictive models of ED were generated using PB dose in EQD2 (α/β ratio = 3Gy) &amp; clinical data (age, diabetes, hypertension, NCCN risk group, baseline PSA, hormone therapy, IGRT, margin size, PB volume). Multivariate logistic regression method using resampling methods was applied to select model order and parameters. Models were fitted using logistic regression of the form Probability = eA(x)/1+eA(x), where A(x) = constant + sum of (variables * associated regression coefficients). Model performance was evaluated through area under the receiver operating characteristic curve (AUC) and Hosmer-Lemeshow (HL) goodness-of-fit test. Results: 101/162 (62%) men had severe ED with statistically significant difference in PB max and mean dose between those patients with or without severe ED (max: 61.8Gy vs 43Gy &amp; mean: 27.4Gy vs 14Gy respectively; p = 0.001). In the univariate analyses, age, diabetes, risk group, PB mean and max doses were significantly associated with EPIC calculated severe ED. The optimal NTCP model (AUC 0.78; CI 0.71-0.86: p for HL = 0.75) for EPIC calculated severe ED included age, PB mean dose and diabetes where A(x) = -10.13+(0.14*age)+(0.03*PB mean dose)+(2.88 if diabetic). A comparable model using clinician completed outcomes will be reported. Conclusions: This study provides the first known clinical prediction model for ED including PB dose, with good model performance. The determined predictors for the NTCP model of severe ED in this cohort were PB mean dose, age &amp; diabetes. External validation of this model is desirable. Clinical trial information: 97182923.

Dose Verification for Tumor Motion with Different Treatment Planning Systems: A Dynamic Thorax Phantom Study

During radiation therapy for lung cancer, the respiratory motion of the target increases error and affects the treatment outcome. A four-dimensional computed tomography (4D-CT) technology developed recently can be used to obtain snapshot thoracic CT images during the entire respiratory cycle, which can be helpful in visualization of the tumor displacement in respiratory motion. This study employed a dynamic phantom to simulate tumor respiratory motion, with motion amplitudes of 2, 5 and 10 mm in the superior-inferior direction and motion periods of 4 and 6 s. 4D-CT imaging was applied to record the movement of the target, and maximum-intensity projection (MIP) imaging was used to define the internal target volume (ITV). Treatment plans were generated using three different treatment techniques—tomotherapy, intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). In addition, using interchangeable insert modules (an ionization chamber for point dose measurement and Gafchromic EBT3 film for planar dose distribution) in the dynamic phantom, the dose received by the target was measured. The dose received with motion was compared with that received under stationary conditions for dose errors due to respiratory motion. The point dose differences between moving and stationary conditions for tomotherapy, IMRT and VMAT were 0.48 ± 0.51%, 0.17 ± 0.45%, and 0.68 ± 0.70%, respectively; and at motion amplitudes of 2, 5, and 10 mm, the mean dose differences were 0.27 ± 0.38%, 0.37 ± 0.55% and 0.68 ± 0.74%, respectively. Based on planar dose analysis, at motion amplitudes of 2 and 5 mm, the dose distribution differences between moving and stationary conditions using the three treatment methods were all very small, and all passed the gamma index test with a criterion of 3%/3 mm, while at an amplitude of 10 mm, the edge of the ITV was associated with significantly greater dose errors. As studies have shown that in only approximately 10% of pulmonary tumors move with an amplitude >10 mm, 4D-CT MIP imaging in combination with delineation of the ITV may resolve the issue of dose error in most cases. For tumors with large motion amplitudes due to respiration, respiratory gating method may be used to reduce healthy lung tissues inside treatment field.

Comparison of calculated dose distributions reported as dose-to-water and dose-to-medium for intensity-modulated radiotherapy of nasopharyngeal cancer patients

Advanced dose calculation algorithms for radiation therapy treatment planning can report external beam photon dose 2-sided, in terms of dose-to-medium (Dm) and dose-to-water (Dw). The purpose of our study was to determinate the effect of Dw and Dm reporting modes built in Elekta Monaco treatment planning system on intensity-modulated radiotherapy dose distributions for patients with nasopharyngeal cancer. For 13 patients involved in this retrospective study, 2 plans were created: 1 using Dw and another according to Dm reporting mode. Treatment plans were normalized such that 100% planning target volume should be covered by 95% of prescribed dose. Dose-volume constraints were assigned according to international standards. The comparison between dose distributions was performed evaluating quantities important for respective volumes of interest. For target volumes, heterogeneity index and conformity index methodology were used along with the maximum dose concept. Also, for the comparisons over particular organ at risk, maximum dose or mean dose as well as dose-volume concepts were used. For all target volumes and majority of organs at risk, the differences between 2 reporting modes are statistically insignificant, but this is not the case for bony structured organs at risks: mandible and cochlea. It was observed that Dw is higher than Dm with mean difference of 9.91% (p = 0.000009) of the mandible volume covered with 70 Gy. The same trend was observed for left and right cochlea with difference in mean dose of 8.74% (p = 0.037) and 6.87% (p = 0.029), respectively. The comparative analysis of dosimetric parameters in this study shows that the selection of reporting modes in Monaco treatment planning system can produce dose differences up to 15% in high-density volumes such as mandible and cochlea, which might have clinical consequences.

The Dosimetric Effects of Different Multileaf Collimator Widths on Physical Dose Distributions

Introduction: Geometric changes in the multileaf collimator (MLC) led to dosimetric considerations in intensity-modulated radiation therapy (IMRT) due to the number and size of the pixels in the intensity map, which are determined by the MLC leaf width. In this study, we evaluated the dosimetric effects of different MLC widths on physical dose distributions for IMRT plans. Materials and Methods: Forty-two IMRT plans based on different MLC devices were generated and analyzed to study the effect of MLC width on plan quality. Results: Improvements in IMRT plan quality using 0.4 cm leaf width in comparison with 1 cm leaf width were evaluated. The 0.4 cm leaf-based plans resulted in significantly higher Dmean, D98%, D95% , D5%, and V95 (58.86 Gy, 95.11%, 96.57%, 104%, and 97.92%, respectively) compared to the 1 cm leaf plans (58.66 Gy, 92.56%, 94.56%, 104.14%, and 95.72%, respectively). Conformation number (CN) for planning target volume in 0.4 cm leaf plans was significantly higher than the 1 cm leaf plans (0.74 vs. 0.67; P<0.05). In addition, the 0.4 cm leaf plans significantly improved dose homogeneity compared to the 1 cm leaf plans (1.08 vs. 1.10; P<0.05). We found that 0.4 cm leaf width significantly decreased the integral dose to normal tissue compared to the 1 cm leaf width (from 56.09 to 49.46 Gy.Kg P<0.05). Conclusion: No significant clinical differences were observed between the two plans for a serially functioning tissue, while the differences in mean doses were statistically significant for parallel functioning normal tissues.