Sensitivity of a helical diode array dosimeter to Volumetric Modulated Arc Therapy delivery errors

PurposeTo study the sensitivity of an ArcCHECK dosimeter in detecting delivery errors during the delivery of Volumetric Modulated Arc Therapy (VMAT).MethodsThree types of errors in Multi Leaf Collimator (MLC) position and dose delivery were simulated separately in the delivery of five prostate and five head and neck (H&N) VMAT plans: (i) Gantry independent: a systematic shift in MLC position and variation in output to the whole arc; (ii) Gantry dependent: sag in MLC position and output variation as a function of gantry angle; (iii) Control point specific MLC and output errors introduced to only a specific number of Control Points (CP). The difference in local and global gamma (γ) pass rate between the no-error and error-simulated measurements with 2%/2 mm and 3%/3 mm tolerances was calculated to assess the sensitivity of ArcCHECK. The clinical impact of these errors was also calculated.ResultsArcCHECK was able to detect a minimum 3 mm MLC error and 3% output error for Gantry independent errors using either local or global gamma with 2%/2 mm tolerance. For the Gantry dependent error scenario a minimum 3 mm MLC error and 3% dose error was identifiable by ArcCHECK using either global or local gamma with 2%/2 mm tolerance. In errors introduced to specific CPs a MLC error of 10 mm and dose error of 100% introduced to 4CPs were detected by ArcCHECK.ConclusionArcCHECK used with either local or global gamma analysis and 2%/2 mm criteria can be confidently used in the clinic to detect errors above the stated error values.     

Multi-institutional comparison of simulated treatment delivery errors in ssIMRT,manually planned VMAT and autoplan-VMAT plans for nasopharyngeal radiotherapy

PurposeTo quantify the impact of simulated errors for nasopharynx radiotherapy across multiple institutions and planning techniques (auto-plan generated Volumetric Modulated Arc Therapy (ap-VMAT), manually planned VMAT (mp-VMAT) and manually planned step and shoot Intensity Modulated Radiation Therapy (mp-ssIMRT)).MethodsTen patients were retrospectively planned with VMAT according to three institution’s protocols. Within one institution two further treatment plans were generated using differing treatment planning techniques. This resulted in mp-ssIMRT, mp-VMAT, and ap-VMAT plans. Introduced treatment errors included Multi Leaf Collimator (MLC) shifts, MLC field size (MLCfs), gantry and collimator errors. A change of more than 5% in most selected dose metrics was considered to have potential clinical impact. The original patient plan total Monitor Units (MUs) were correlated to the total number of dose metrics exceeded.ResultsThe impact of different errors was consistent, with ap-VMAT plans (two institutions) showing larger dose deviations than mp-VMAT created plans (one institution). Across all institutions’ VMAT plans the significant errors included; ±5° for the collimator angle, ±5 mm for the MLC shift and +1, ±2 and ±5 mm for the MLC field size. The total number of dose metrics exceeding tolerance was positively correlated to the VMAT total plan MUs (r = 0.51, p < 0.001), across all institutions and techniques.ConclusionsDifferences in VMAT robustness to simulated errors across institutions occurred due to planning method differences. Whilst ap-VMAT was most sensitive to MLC errors, it also produced the best quality treatment plans. Mp-ssIMRT was most robust to errors. Higher VMAT treatment plan complexity led to less robust plans.     

Quantification of residual dose estimation error on log file-based patient dose calculation

PurposeThe log file-based patient dose estimation includes a residual dose estimation error caused by leaf miscalibration, which cannot be reflected on the estimated dose. The purpose of this study is to determine this residual dose estimation error.Methods and materialsModified log files for seven head-and-neck and prostate volumetric modulated arc therapy (VMAT) plans simulating leaf miscalibration were generated by shifting both leaf banks (systematic leaf gap errors: ±2.0, ±1.0, and ±0.5 mm in opposite directions and systematic leaf shifts: ±1.0 mm in the same direction) using MATLAB-based (MathWorks, Natick, MA) in-house software. The generated modified and non-modified log files were imported back into the treatment planning system and recalculated. Subsequently, the generalized equivalent uniform dose (gEUD) was quantified for the definition of the planning target volume (PTV) and organs at risks.ResultsFor MLC leaves calibrated within ±0.5 mm, the quantified residual dose estimation errors that obtained from the slope of the linear regression of gEUD changes between non- and modified log file doses per leaf gap are in head-and-neck plans 1.32 ± 0.27% and 0.82 ± 0.17 Gy for PTV and spinal cord, respectively, and in prostate plans 1.22 ± 0.36%, 0.95 ± 0.14 Gy, and 0.45 ± 0.08 Gy for PTV, rectum, and bladder, respectively.ConclusionsIn this work, we determine the residual dose estimation errors for VMAT delivery using the log file-based patient dose calculation according to the MLC calibration accuracy.     

Error detection during VMAT delivery using EPID-based 3D transit dosimetry

PurposeTo investigate the effectiveness of an EPID-based 3D transit dosimetry system in detecting deliberately introduced errors during VMAT delivery.MethodsAn Alderson phantom was irradiated using four VMAT treatment plans (one prostate, two head-and-neck and one lung case) in which delivery, thickness and setup errors were introduced. EPID measurements were performed to reconstruct 3D dose distributions of “error” plans, which were compared with “no-error” plans using the mean gamma (γ_mean), near-maximum gamma (γ_1%) and the difference in isocenter dose (ΔD_isoc) as metrics.ResultsOut of a total of 42 serious errors, the number of errors detected was 33 (79%), and 27 out of 30 (90%) if setup errors are not included. The system was able to pick up errors of 5 mm movement of a leaf bank, a wrong collimator rotation angle and a wrong photon beam energy. A change in phantom thickness of 1 cm was detected for all cases, while only for the head-and-neck plans a 2 cm horizontal and vertical shift of the phantom were alerted. A single leaf error of 5 mm could be detected for the lung plan only.ConclusionAlthough performed for a limited number of cases and error types, this study shows that EPID-based 3D transit dosimetry is able to detect a number of serious errors in dose delivery, leaf bank position and patient thickness during VMAT delivery. Errors in patient setup and single leaf position can only be detected in specific cases.     

An investigation of the IQM signal variation and error detection sensitivity for patient specific pre-treatment QA

PurposeTo evaluate the Integral Quality Monitor (IQM) as a clinical dosimetry device for detecting photon beam delivery errors in clinically relevant conditions.Materials and methodsThe IQM’s ability to detect delivery errors introduced into clinical VMAT plans for two different treatment sites was assessed. This included measuring 103 nasopharynx VMAT plans and 78 lung SBRT VMAT plans with introduced errors in gantry angle (1–5°) and in MLC-defined field size and field shift (1–5 mm). The IQM sensitivity was compared to ArcCheck detector performance. Signal dependence on field position for on-axis and asymmetrically offset square field sizes from 1 × 1 cm² to 30 × 30 cm² was also investigated.ResultsThe IQM detected almost all introduced clinically-significant MLC field size errors, but not some small gantry angle errors or most MLC field shift errors. The IQM sensitivity was comparable to the ArcCheck for lung SBRT, but worse for the nasopharynx plans. Differences between IQM calculated/predicted and measured signals were within ± 2% for all on-axis square fields, but up to 60% for the smallest asymmetrically offset fields at large offsets.Conclusion The IQM performance was consistent and reproducible. It showed highest sensitivity to the field size errors for these plans, but did not detect some clinically-significant introduced gantry angle errors or most MLC field shift errors. The IQM calculation model is still being developed, which should improve small offset-field performance. Care is required in IQM use for plan verification or online monitoring, especially for small fields that are off-axis in the detector gradient direction.     

Delta4 Discover transmission detector: A comprehensive characterization for in-vivo VMAT monitoring

ObjectiveTo investigate the dosimetric behaviour, influence on photon beam fluence and error detection capability of Delta⁴ Discover transmission detector.MethodsThe transmission detector (TRD) was characterized on a TrueBeam linear accelerator with 6 MV beams. Linearity, reproducibility and dose rate dependence were investigated. The effect on photon beam fluence was evaluated in terms of beam profiles, percentage depth dose, transmission factor and surface dose for different open field sizes. The transmission factor of the 10x10 cm² field was entered in the TPS’s configuration and its correct use in the dose calculation was verified recalculating 17 clinical IMRT/VMAT plans. Surface dose was measured for 20 IMRT fields. The capability to detect different delivery errors was investigated evaluating dose gamma index, MLC gamma index and leaf position of 15 manually modified VMAT plans.ResultsTRD showed a linear dependence on MU. No dose rate dependence was observed. Short-term and long-term reproducibility were within 0.1% and 0.5%. The presence of the TRD did not significantly affect PDDs and profiles. The transmission factor of the 10x10 cm² field size was 0.985 and 0.983, for FF and FFF beams respectively. The 17 recalculated plans met our clinical gamma-index passing rate, confirming the correct use of the transmission factor by the TPS. The surface dose differences for the open fields increase for shorter SSDs and greater field size. Differences in surface dose for the IMRT beams were less than 2%. Output variation ≥2%, collimator angle variations within 0.3°, gantry angle errors of 1°, jaw tracking and leaf position errors were detected.ConclusionsDelta⁴ Discover shows good linearity and reproducibility, is not dependent on dose rate and does not affect beam quality and dose profiles. It is also capable to detect dosimetric and geometric errors and therefore it is suitable for monitoring VMAT delivery.     

Evaluation of MLC performance in VMAT and dynamic IMRT by log file analysis

PurposeThis multi-institution study assessed the positioning accuracy of multileaf collimators (MLC) by analyzing log files. It determined the main machine parameters that affect MLC positioning errors for pre-TrueBeam (Clinac) and TrueBeam linacs.MethodsAround 30,000 dIMRT and VMAT log files belonging to 6 linacs from 4 different centers were analyzed. An in-house software was used to calculate 95th percentile and RMS error values and their correlation with certain parameters such as maximum leaf speed, mean leaf speed and gantry angle. The effect of MLC communication delay on error statistics was assessed in Clinac linacs. To that end MLC positioning error statistics were calculated with and without the delay effect.ResultsFor dIMRT treatments in Clinac linacs the mean leaf RMS error was 0.306 mm with and 0.030 mm without the delay effect. Leaf RMS error was closely linked to maximum and mean leaf speeds, but without the delay effect that link was weaker. No trend was observed between bank RMS error and gantry angle. Without the delay effect larger bank RMS errors were obtained for gantry angles with leaf movements against gravity. For VMAT treatments in TrueBeam linacs the mean leaf RMS error was 0.038 mm. A link was also observed between leaf RMS error and maximum and mean leaf speeds.ConclusionTrueBeam MLC positioning errors are substantially lower than those of Clinac linacs. In Clinac machines the analysis of dynalogs without the delay effect allows us to study the influence of factors that are masked by the delay effect.     

Gamma index comparison of three VMAT QA systems and evaluation of their sensitivity to delivery errors

PurposeTo compare detectors for dosimetric verification before VMAT treatments and evaluate their sensitivity to errors.Methods and materialsMeasurements using three detectors (ArcCheck, 2d array 729 and EPID) were used to validate the dosimetric accuracy of the VMAT delivery. Firstly, performance of the three devices was studied. Secondly, to assess the reliability of the detectors, 59 VMAT treatment plans from a variety of clinical sites were considered. Thirdly, systematic variations in collimator, couch and gantry angle plus MLC positioning were applied to four clinical treatments (two prostate, two head and neck cases) in order to establish the detection sensitivity of the three devices. Measurements were compared with TPS computed doses via gamma analysis (3%/3 mm and 2%/2 mm) with an agreement of at least 95% and 90% respectively in all pixels. Effect of the errors on the dose distributions was analyzed.ResultsRepeatability and reproducibility were excellent for the three devices. The average pass rate for the 59 cases was superior to 98% for all devices with 3%/3 mm criteria. It was found that for the plans delivered with errors, the sensitivity was quite similar for all devices. Devices were able to detect a 2 mm opened or closed MLC error with 3%/3 mm tolerance level. An error of 3° in collimator, gantry or couch rotation was detected by the three devices using 2%/2 mm criteria.ConclusionsAll three devices have the potential to detect errors with more or less the same threshold. Nevertheless, there is no guarantee that pretreatment QA will catch delivery errors.     

DEMAT: A multi-institutional dosimetry audit of rotational and static intensity-modulated radiotherapy

PurposeStatic beam intensity-modulated-radiation-therapy (IMRT) and/or Volumetric-Modulated-Arc-Therapy (VMAT) are now available in many regional radiotherapy departments. The aim of this multi-institutional audit was to design a new methodology based on radiochromic films to perform an independent quality control.MethodsA set of data were sent to all participating centres for two clinical localizations: prostate and Head and Neck (H&N) cancers. The agreement between calculations and measurements was verified in the Octavius phantom (PTW) by point measurements using ionization chambers and by 2D measurements using EBT3 radiochromic films. Due to uncertainties in the whole procedure, criteria were set to 5% and 3% in local dose and 3 mm in distance excluding doses lower than 10% of the maximum doses. No normalization point or area was used for the quantitative analysis.Results13 radiotherapy centres participated in this audit involving 28 plans (12 IMRT, 16 VMAT). For point measurements, mean errors were −0.18 ± 1.54% and 0.00 ± 1.58% for prostate and H&N cases respectively. For 2D measurements with 5%/3 mm criteria, gamma map analysis showed a pixel pass rate higher than 95% for prostate and H&N. Mean gamma index was lower than 0.4 for prostate and 0.5 for H&N. Both techniques yielded similar results.ConclusionThis study showed the feasibility of an independent quality control by peers for conventional IMRT and VMAT. Results from all participating centres were found to be in good agreement. This regional study demonstrated the feasibility of our new methodology based on radiochromic films without dose normalization on a specific point.     

Comparison of multi-institutional pre-treatment verification for VMAT of nasopharynx with delivery errors

PurposeMeasurement-based pre-treatment verification with phantoms frequently uses gamma analysis to assess acceptable delivery accuracy. This study evaluates the sensitivity of a commercial system to simulated machine errors for three different institutions’ Volumetric Modulated Arc Therapy (VMAT) planning approaches.MethodsVMAT plans were generated for ten patients at three institutions using each institution’s own protocol (manually-planned at institution 1; auto-planned at institutions 2 and 3). Errors in Multi-Leaf Collimator (MLC) field size (FS), MLC shift (S), and collimator angle (C) of −5, −2, −1, 1, 2 and 5 mm or degrees were introduced.Dose metric constraints discriminated which error magnitudes were considered unacceptable. The smallest magnitude error treatment plans deemed clinically unacceptable (typically for a 5% dose change) were delivered to the ArcCHECK for all institutions, and with a high-dose point ion chamber measurement in 2 institutions. Error detection for different gamma analysis criteria was compared.ResultsNot all deliberately introduced VMAT plan errors were detected using a typical 3D 3%/3 mm global gamma pass rate of 95%. Considering all institutions, gamma analysis was least sensitive to negative FS errors. The most sensitive was a 2%/2 mm global analysis for institution 1, whilst for institution 2 it was 3%/3 mm global analysis. The majority of errors (58/59 for institution 1, 54/60 for institution 3) were detected using ArcCHECK and ion chamber measurements combined.ConclusionsNot all clinically unacceptable errors are detected. Combining ion chamber measurements with gamma analysis improved sensitivity and is recommended. Optimum gamma settings varied across institutions.     

Evaluation of the ability of three commercially available dosimeters to detect systematic delivery errors in step-and-shoot IMRT plans

BackgroundThere is limited data on error detectability for step-and-shoot intensity modulated radiotherapy (sIMRT) plans, despite significant work on dynamic methods. However, sIMRT treatments have an ongoing role in clinical practice. This study aimed to evaluate variations in the sensitivity of three patient-specific quality assurance (QA) devices to systematic delivery errors in sIMRT plans.Materials and methodsFour clinical sIMRT plans (prostate and head and neck) were edited to introduce errors in: Multi-Leaf Collimator (MLC) position (increasing field size, leaf pairs offset (1–3 mm) in opposite directions; and field shift, all leaves offset (1–3 mm) in one direction); collimator rotation (1–3 degrees) and gantry rotation (0.5–2 degrees). The total dose for each plan was measured using an ArcCHECK diode array. Each field, excluding those with gantry offsets, was also measured using an Electronic Portal Imager and a MatriXX Evolution 2D ionisation chamber array. 132 plans (858 fields) were delivered, producing 572 measured dose distributions. Measured doses were compared to calculated doses for the no-error plan using Gamma analysis with 3%/3 mm, 3%/2 mm, and 2%/2 mm criteria (1716 analyses).ResultsGenerally, pass rates decreased with increasing errors and/or stricter gamma criteria. Pass rate variations with detector and plan type were also observed. For a 3%/3 mm gamma criteria, none of the devices could reliably detect 1 mm MLC position errors or 1 degree collimator rotation errors.ConclusionsThis work has highlighted the need to adapt QA based on treatment plan type and the need for detector specific assessment criteria to detect clinically significant errors.     

An external dosimetry audit programme to credential static and rotational IMRT delivery for clinical trials quality assurance

PurposeExternal dosimetry audits give confidence in the safe and accurate delivery of radiotherapy. The RTTQA group have performed an on-site audit programme for trial recruiting centres, who have recently implemented static or rotational IMRT, and those with major changes to planning or delivery systems.MethodsMeasurements of reference beam output were performed by the host centre, and by the auditor using independent equipment. Verification of clinical plans was performed using the ArcCheck helical diode array.ResultsA total of 54 measurement sessions were performed between May 2014 and June 2016 at 28 UK institutions, reflecting the different combinations of planning and delivery systems used at each institution. Average ratio of measured output between auditor and host was 1.002 ± 0.006. Average point dose agreement for clinical plans was −0.3 ± 1.8%. Average (and 95% lower confidence intervals) of gamma pass rates at 2%/2 mm, 3%/2 mm and 3%/3 mm respectively were: 92% (80%), 96% (90%) and 98% (94%). Moderately significant differences were seen between fixed gantry angle and rotational IMRT, and between combination of planning systems and linac manufacturer, but not between anatomical treatment site or beam energy.ConclusionAn external audit programme has been implemented for universal and efficient credentialing of IMRT treatments in clinical trials. Good agreement was found between measured and expected doses, with few outliers, leading to a simple table of optimal and mandatory tolerances for approval of dosimetry audit results. Feedback was given to some centres leading to improved clinical practice.     

Exit fluence analysis using portal dosimetry in volumetric modulated arc therapy

AimIn measuring exit fluences, there are several sources of deviations which include the changes in the entrance fluence, changes in the detector response and patient orientation or geometry. The purpose of this work is to quantify these sources of errors.BackgroundThe use of the volumetric modulated arc therapy treatment with the help of image guidance in radiotherapy results in high accuracy of delivering complex dose distributions while sparing critical organs. The transit dosimetry has the potential of Verifying dose delivery by the linac, Multileaf collimator positional accuracy and the calculation of dose to a patient or phantom.Materials and methodsThe quantification of errors caused by a machine delivery is done by comparing static and arc picket fence test for 30 days. A RapidArc plan, created for the pelvis site was delivered without and with Rando phantom and exit portal images were acquired. The day to day dose variation were analysed by comparing the daily exit dose images during the course of treatment. The gamma criterion used for analysis is 3% dose difference and 3 mm distance to agreement with a threshold of 10% of maximum dose.ResultsThe maximum standard deviation for the static and arc picket fence test fields were 0.19 CU and 1.3 CU, respectively. The delivery of the RapidArc plans without a phantom shows the maximum standard deviation of 1.85 CU and the maximum gamma value of 0.59. The maximum gamma value for the RapidArc plan delivered with the phantom was found to be 1.2. The largest observed fluence deviation during the delivery to patient was 5.7% and the maximum standard deviation was 4.1 CU.ConclusionIt is found from this study that the variation due to patient anatomy and interfraction organ motion is significant.     

A new split arc VMAT technique for lymph node positive breast cancer

PurposeTo investigate different volumetric modulated arc therapy (VMAT) field designs for lymph node positive breast cancer patients when compared to conventional static fields and standard VMAT designs.MethodsNineteen breast cancer patients with lymph node involvement (eleven left and eight right sided) were retrospectively analyzed with different arc designs. Proposed split arc designs with total rotations of 2 × 190° and 2 × 240° were compared to conventional field in field (FinF) and previously published non-split arc techniques with the same amount of total rotations.ResultsAll VMAT plans were superior in dose conformity, when compared to the FinF plans. Split arc design decreased significantly ipsilateral lung dose and heart V5Gy for both left and right sided cases, when compared to non-split VMAT designs. For left sided cases no significant differences were seen in contralateral lung mean dose or V5Gy between different VMAT designs. For right sided cases the contralateral lung dose V5Gy was significantly higher in split VMAT group, when compared to non-split VMAT designs. The contralateral breast dose V5Gy increased significantly for split VMAT plans for both sides, when compared to non-split VMAT designs or FinF plans.ConclusionsThe proposed split VMAT technique was shown to be superior to previously published non-split VMAT and conventional FinF techniques significantly reducing dose to the ipsilateral lung and heart. However, this came with the expense of an increase in the dose to the contralateral breast and for right-sided cases to the contralateral lung.     

Characterization of robust optimization for VMAT plan for liver cancer

PurposeWe investigated the feasibility of robust optimization for volumetric modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT) for liver cancer in comparison with planning target volume (PTV)-based optimized plans. Treatment plan quality, robustness, complexity, and accuracy of dose delivery were assessed.MethodsTen liver cancer patients were selected for this study. PTV-based optimized plans with an 8-mm PTV margin and robust optimized plans with an 8-mm setup uncertainty were generated. Plan perturbed doses were evaluated using a setup error of 8 mm in all directions from the isocenter. The dosimetric comparison parameters were clinical target volume (CTV) doses (D_98%, D_50%, and D_2%), liver doses, and monitor unit (MU). Plan complexity was evaluated using the modulation complexity score for VMAT (MCSv).ResultsThere was no significant difference between the two optimizations with respect to CTV doses and MUs. Robust optimized plans had a higher liver dose than did PTV-based optimized plans. Plan perturbed dose evaluations showed that doses to the CTV for the robust optimized plans had small variations. Robust optimized plans were less complex than PTV-based optimized plans. Robust optimized plans had statistically significant fewer leaf position errors than did PTV-based optimized plans.ConclusionsComparison of treatment plan quality, robustness, and plan complexity of both optimizations showed that robust optimization could be feasibile for VMAT of liver cancer.     

Analysis of a volumetric-modulated arc therapy (VMAT) single phase prostate template as a class solution

AimTo assess a class solution template for volumetric-modulated arc therapy (VMAT) for prostate cancer using plan analysis software.BackgroundVMAT is a development of intensity-modulated radiotherapy (IMRT) with potential advantages for the delivery of radiotherapy (RT) in prostate cancer. Class solutions are increasingly used for facilitating RT planning. Plan analysis software provides an objective tool for evaluating class solutions.Materials and methodsThe class solution for VMAT was based on the current static field IMRT template. The plans of 77 prostate cancer patients were evaluated using a set of in-house plan quality metrics (scores) (PlanIQ™, Sun Nuclear Corporation). The metrics compared the class solution for VMAT planning with the IMRT template and the delivered clinical plan (CP). Eight metrics were associated with target coverage and ten with organs-at-risk (OAR). Individual metrics were summed and the combined scores were subjected to non-parametric analysis. The low-dose wash for both static IMRT and VMAT plans were evaluated using 40 Gy and 25 Gy isodose volumes.ResultsVMAT plans were of equal or better quality than the IMRT template and CP for target coverage (combined score) and OAR combined score. The 40 Gy isodose volume was marginally higher with VMAT than IMRT (4.9%) but lower than CP (−6.6%)(P = 0.0074). The 25 Gy volume was significantly lower with VMAT than both IMRT (−32.7%) and CP (−34.4%)(P < 0.00001).ConclusionsAutomated VMAT planning for prostate cancer is feasible and the plans are equal to or better than the current IMRT class solution and the delivered clinical plan.     

Error detection using a convolutional neural network with dose difference maps in patient-specific quality assurance for volumetric modulated arc therapy

The aim of this study was to evaluate the use of dose difference maps with a convolutional neural network (CNN) to detect multi-leaf collimator (MLC) positional errors in patient-specific quality assurance for volumetric modulated radiation therapy (VMAT). A cylindrical three-dimensional detector (Delta4, ScandiDos, Uppsala, Sweden) was used to measure 161 beams from 104 clinical prostate VMAT plans. For the simulation used error-free plans plus plans with two types of MLC error were introduced: systematic error and random error. A total of 483 dose distributions in a virtual cylindrical phantom were calculated with a treatment planning system. Dose difference maps were created from two planar dose distributions from the measured and calculated dose distributions, and these were used as the input for the CNN, with 375 datasets assigned for training and 108 datasets assigned for testing. The CNN model had three convolution layers and was trained with five-fold cross-validation. The CNN model classified the error types of the plans as “error-free,” “systematic error,” or “random error,” with an overall accuracy of 0.944. The sensitivity values for the “error-free,” “systematic error,” and “random error” classifications were 0.889, 1.000, and 0.944, respectively, and the specificity values were 0.986, 0.986, and 0.944, respectively. This approach was superior to those based on gamma analysis. Using dose difference maps with a CNN model may provide an effective solution for detecting MLC errors for patient-specific VMAT quality assurance.     

Advanced optimization methods for whole pelvic and local prostate external beam therapy

PurposeRadiation treatment planning inherently involves multiple conflicting planning goals, which makes it a suitable application for multicriteria optimization (MCO). This study investigates a MCO algorithm for VMAT planning (VMAT–MCO) for prostate cancer treatments including pelvic lymph nodes and uses standard inverse VMAT optimization (sVMAT) and Tomotherapy planning as benchmarks.MethodsFor each of ten prostate cancer patients, a two stage plan was generated, consisting of a stage 1 plan delivering 22 Gy to the prostate, and a stage 2 plan delivering 50.4 Gy to the lymph nodes and 56 Gy to the prostate with a simultaneous integrated boost. The single plans were generated by three planning techniques (VMAT–MCO, sVMAT, Tomotherapy) and subsequently compared with respect to plan quality and planning time efficiency.ResultsPlan quality was similar for all techniques, but sVMAT showed slightly better rectum (on average D_mean −7%) and bowel sparing (D_mean −17%) compared to VMAT–MCO in the whole pelvic treatments. Tomotherapy plans exhibited higher bladder dose (D_mean +42%) in stage 1 and lower rectum dose (D_mean −6%) in stage 2 than VMAT–MCO. Compared to manual planning, the planning time with MCO was reduced up to 12 and 38 min for stage 1 and 2 plans, respectively.ConclusionMCO can generate highly conformal prostate VMAT plans with minimal workload in the settings of prostate-only treatments and prostate plus lymph nodes irradiation. In the whole pelvic plan manual VMAT optimization led to slightly improved OAR sparing over VMAT–MCO, whereas for the primary prostate treatment plan quality was equal.