Monitor Units Research Articles

Dosimetric Comparison Between Two and Six Beams Conformal Radiotherapy Plans for Paediatric Wilm's Tumour Patients at Ocean Road Cancer Institute, Tanzania

This study aimed to compare the dose distributions of three-dimensional conformal treatment planning using two and six radiation beams among paediatric patients receiving radiotherapy for Wilms tumour at Ocean Road Cancer Institute. CT scans of 53 patients were used to generate 106 treatment plans, 53 plans for two beams, and 53 plans for six beams. Planning target volumes and organs at risk parameters were compared between both plans using paired samples t-test. A p-value < 0.05 was considered statistically significant. The findings showed that the D95% and Dmean of the target are better with six beams than with the two-beam plans (p ˂ 0.001). The conformity index and monitor unit are significantly better with two-beam plans than with six-beam plans (p ˂ 0.001). No volume of the contralateral kidney received more than 12 Gy for both two and six plans. There were no differences in Dmax for the spinal cord (p = 0.208), while Dmean was observed to be lower in two compared to six beams (p ˂ 0.001) for the liver. Six beam plans have good target coverage, while two beam plans have good conformity index and monitor unit. The observed doses to the organs at risk in both plans were lower than their normal tolerance.
 Keywords: Wilm's tumour, Three-dimensional conformal radiotherapy, whole abdomen radiotherapy, paediatric radiotherapy, Dosimetry

Technical note: Consistency of IAEA's TRS‐483 and AAPM's extended TG‐51 protocols for clinical reference dosimetry of the CyberKnife M6 machine

While IAEA's TRS-483 code of practice is adapted for the calibration of CyberKnife machines, AAPM's TG-51 is still the protocol recommended by the manufacturer for their calibration. The differences between both protocols could lead to differences in absorbed dose to water during the calibrationprocess. The aims of this work are to evaluate the difference resulting from the application of TG-51 (including the manufacturer's adaptations) and TRS-483 in terms of absorbed dose to water for a CyberKnife M6, and to evaluate the consistency ofTRS-483. Measurements are performed on a CyberKnife M6 unit under machine-specific reference conditions using a calibrated Exradin A12 ionization chamber. Monte Carlo (MC) simulations are performed to estimate and using a fully modeled detector and an optimized CyberKnife M6 beam model. The latter is also estimated experimentally. Differences between the adapted TG-51 and TRS-483 protocols are identified and their impact isquantified. When using an in-house experimentally-evaluated volume averaging correction factor, a difference of 0.11% in terms of absorbed dose to water per monitor unit is observed when applying both protocols. This disparity is solely associated to the difference in beam quality correction factor. If a generic volume averaging correction factor is used during the application of TRS-483, the difference in calibration increases to 0.14%. In both cases, the disparity is not statistically significant according to TRS-483's reported uncertainties on their beam quality correction factor (i.e., 1%). MC results lead to and . Results illustrate that the generic beam quality correction factor provided in the TRS-483 might be overestimated by 0.36% compared to our specific model and that this overestimation could be due to the volume averagingcomponent. For clinical reference dosimetry of the CyberKnife M6, the application of TRS-483 is found to be consistent withTG-51.

Image-based features in machine learning to identify delivery errors and predict error magnitude for patient-specific IMRT quality assurance

ObjectiveTo identify delivery error type and predict associated error magnitude by image-based features using machine learning (ML).MethodsIn this study, a total of 40 thoracic plans (including 208 beams) were selected, and four error types with different magnitudes were introduced into the original plans, including 1) collimator misalignment (COLL), 2) monitor unit (MU) variation, 3) systematic multileaf collimator misalignment (MLCS), and 4) random MLC misalignment (MLCR). These dose distributions of portal dose predictions for the original plans were defined as the reference dose distributions (RDD), while those for the error-introduced plans were defined as the error-introduced dose distributions (EDD). Both distributions were calculated for all beams with portal dose image prediction (PDIP). Besides, 14 image-based features were extracted from RDD and EDD of portal dose predictions to obtain the feature vectors. In addition, a random forest was adopted for the multiclass classification task, and regression prediction for error magnitude.ResultsThe top five features extracted with the highest weight included 1) the relative displacement in the x direction, 2) the ratio of the absolute minimum residual error to the maximal RDD value, 3) the product of the maximum and minimum residuals, 4) the ratio of the absolute maximum residual error to the maximal RDD value, and 5) the ratio of the absolute mean residual value to the maximal RDD value. The relative displacement in the x direction had the highest weight. The overall accuracy of the five-class classification model was 99.85% for the validation set and 99.30% for the testing set. This model could be applied to the classification of the error-free plan, COLL, MU, MLCS, and MLCR with an accuracy of 100%, 98.4%, 99.9%, 98.0%, and 98.3%, respectively. MLCR had the worst performance in error magnitude prediction (70.1–96.6%), while others had better performance in error magnitude prediction (higher than 93%). In the error magnitude prediction, the mean absolute error (MAE) between predicted error magnitude and actual error ranged from 0.03 to 0.33, with the root mean squared error (RMSE) varying from 0.17 to 0.56 for the validation set. The MAE and RMSE ranged from 0.03 to 0.50 and 0.44 to 0.59 for the test set, respectively.ConclusionIt could be demonstrated in this study that the image-based features extracted from RDD and EDD can be employed to identify different types of delivery errors and accurately predict error magnitude with the assistance of ML techniques. They can be used to associate traditional gamma analysis with clinically based analysis for error classification and magnitude prediction in patient-specific IMRT quality assurance.

Monte Carlo based continuous aperture optimization for VMAT on conventional and MR-linacs: A proof of concept.

Currently, the commercial treatment planning systems for magnetic-resonance guided linear accelerators (MR-linacs) only support step-and-shoot intensity-modulated radiation therapy (IMRT). However, recent studies have shown the feasibility of delivering arc therapy on MR-linacs, which is expected to improve dose distributions and delivery speed. By accurately accounting for the electron return effect in the presence of a magnetic field, a Monte Carlo (MC) algorithm is ideally suited for the inverse treatment planning of thistechnique. We propose a novel MC-based continuous aperture optimization (MCCAO) algorithm for volumetric modulated arc therapy (VMAT), including applications to VMAT on MR-linacs and trajectory-based VMAT. A unique feature of MCCAO is that the continuous character of gantry rotation and multileaf collimator (MLC) motion is accounted for at every stage of theoptimization. The optimization process uses a multistage simulation of 4D dose distribution. A phase space is scored at the top surface of the MLC and the energy deposition of each particle history is mapped to its position in this phase space. A progressive sampling method is used, where both MLC leaf positions and monitor unit (MU) weights are randomly changed, while respecting the linac mechanical limits. Due to the continuous nature of the leaf motion, such changes affect not only a single control point, but propagate to the adjacent ones as well, and the corresponding dose distribution changes are accounted for. A dose-volume cost function is used, which includes the MC statisticaluncertainty. We applied our optimization technique to various treatment sites, using standard and flattening-filter-free (FFF) 6 MV beam models, with and without a 1.5 T magnetic field. MCCAO generates deliverable plans, whose dose distributions are in good agreement with measurements on ArcCHECK and stereotactic radiosurgery End-To-EndPhantom. We show that the novel MCCAO method generates VMAT plans that meet clinical objectives for both conventional andMR-linacs.

Development of a log file analysis tool for proton patient QA, system performance tracking, and delivered dose reconstruction.

To describe a log file-based patient-specific quality assurance (QA) method and develop an in-house tool for system performance tracking and dose reconstruction in pencil-beam scanning proton therapy that can be used for pre-treatment plan review. The software extracts beam-specific information from the treatment delivery log file and automatically compares the monitor units (MU), lateral position, and size of each spot against the intended values in the treatment plan to identify any discrepancies in the beam delivery. The software has been used to analyze 992 patients, 2004 plans, 4865 fields, and more than 32 million proton spots from 2016 to 2021. The composite doses of 10 craniospinal irradiation (CSI) plans were reconstructed based on the delivered spots and compared with the original plans as an offline plan review method. Over the course of 6 years, the proton delivery system has proved stable in delivering patient QA fields with proton energies of 69.4-221.3 MeV and an MU range of 0.003-1.473 MU per spot. The planned mean and standard deviation (SD) of the energy and spot MU were 114.4 ± 26.4 MeV and 0.010 ± 0.009 MU, respectively. The mean and SD of the differences in MU and position between the delivered and planned spots were 9.56 × 10-8 ± 2.0 × 10-4 MU and 0.029/-0.007 ± 0.049/0.044 mm on the X/Y-axis for random differences and 0.005/0.125 ± 0.189/0.175 mm on the X/Y-axis for systematic differences. The mean and SD of the difference between the commissioning and delivered spot sizes were 0.086/0.089 ± 0.131/0.166 mm on the X/Y-axis. A tool has been developed to extract crucial information about the performance of the proton delivery and monitor system and provide a dose reconstruction based on delivered spots for quality improvement. Each patient's plan was verified before treatment to ensure accurate and safe delivery within the delivery tolerance of the machine.

A Dosimetric Study on the Impact of Collimator Angles in Nasopharynx Carcinoma using VMAT Planning

Objective: This study aims to rule out the best collimator angle using Monte- Carlo algorithm in Nasopharynx Carcinoma (NPC) with adequate tumour coverage and less OAR. Methods: Twenty-nine NPC patients were selected retrospectively. VMAT with dual arc were made with same constraints but with different collimator angles for each patient. Total often plans were created for each patient with a collimator angles of 0;5;10;15;20;25;30;35;40;45 in Monaco Treatment Planning System(TPS) using Monte-Carlo algorithm. Plans were evaluated based on Dosimetric parameters such as target coverage, organ at risk (OAR), conformity index(CI) and homogeneity index(HI). Mean dose (Dmean), and Maximum dose (Dmax) of OARs were also analysed. Data were further evaluated by using SPSS software. Findings: The VMAT plans with collimator angles 15◦-30◦ demonstrated better target coverage with a range of (92-94) %0.7195 for the high-dose PTV and significant reduction in monitor units (MU) by an average of 75 with a CI (0.95-0.96)0.0068. Moreover, significant reduction in Dmax of Brainstem and Right optic nerve were obtained with increasing angles. Also,5◦-10◦ showed least PTV coverage and 45◦ got highest MUs compared to 0◦. Novelty : This study shows that collimator angles 15◦-30◦ is superior compared with other angles in Ca Nasopharynx VMAT planning by Elekta-HD Versa Linac using Monte-Carlo algorithm. Keywords: Ca Nasopharynx; Collimator angle; Montecarlo algorithm; Elekta HD Versa Linac; VMAT

Built-in wavelet-induced smoothness to reduce plan complexity in intensity modulated radiation therapy (IMRT)

Objective. Reducing plan complexity in intensity modulated radiation therapy (IMRT) to ensure dosimetric accuracy and delivery efficiency of the radiation treatment plans. We propose a novel approach by representing the beamlet intensities using an incomplete wavelet basis that explicitly excludes fluctuating intensity maps from the decision space (explicit hard constraint). This technique provides a built-in wavelet-induced smoothness that improves both dosimetric plan quality and delivery efficiency. Approach. The beamlet intensity maps need to be especially smooth in the leaf travel direction (referred to as the X-direction). We treat the intensity map of each beam as a 2D image and represent it using the wavelets corresponding to low-frequency changes in the X-direction (i.e. approximation and horizontal). The absence of wavelets corresponding to high-frequency changes (i.e. vertical and diagonal) induces built-in smoothness. We still utilize a regularization term in the objective function to promote smoothness in the Y-direction (perpendicular to the X-direction) and further possible smoothness in the X-direction. This technique has been tested on three patient cases of different disease sites (paraspinal, lung, prostate) and all final evaluations and comparisons have been performed on an FDA-approved commercial treatment planning system (Varian EclipseTM). Main results. Wavelet-induced smoothness reduced monitor units by about 10%, 45%, and 14% for paraspinal, lung, and prostate cases, respectively. It also improved organ at risk sparing, especially on the complex paraspinal case where it resulted in about 7%, 13%, and 14% less mean dose to esophagus, lung, and cord, respectively. Moreover, built-in wavelet-induced smoothness desensitizes the results to changing the weight associated to the regularization term, and thereby mitigates the weight fine-tuning difficulty. Significance. Fluence smoothness is often achieved by smoothing the beamlet intensity maps using a proper regularization term in the objective function aiming at disincentivizing fluctuation in the beamlet intensities (implicit soft constraint). This work reports a novel application of wavelets in imposing an explicit smoothness hard constraint in the search space using an incomplete wavelet basis. This idea has been successfully applied to exclude complex and clinically irrelevant radiation plans from the search space. The code and pertained models along with a sample dataset are released on our LowDimRT GitHub (https://github.com/PortPy-Project/LowDimRT).

Neutrons produced in a 12 MV LINAC working in electron mode

The electroneutron spectrum was estimated with Monte Carlo methods using two source terms. In addition, the electroneutron spectrum was measured using a passive Bonner sphere spectrometer. With estimated spectra were calculated, per history, the total neutron fluence, the ambient dose equivalent, the ambient dose, the mean energy, and the spectrum-averaged neutron fluence-to-dose conversion coefficients. With the measured spectrum were calculated, per monitor unit, the total neutron flux, the ambient dose equivalent; additionally, were calculated the mean energy and the spectrum-averaged neutron fluence-to-dose conversion coefficients. Doses were calculated using the fluence-to-dose conversion coefficients from the ICRP 74, LB 6411 neutron area monitor and those of the ambient dose, recently defined by the ICRP/ICRU joint publication. To 1 m from the isocenter calculations and measurements were carried out around a 12 MV linear accelerator for medical using with a solid water phantom.

Improving normal tissue sparing using scripting in endometrial cancer radiation therapy planning.

The aim of this study was to improve the protection of organs at risk (OARs), decrease the total planning time and maintain sufficient target doses using scripting endometrial cancer external beam radiation therapy (EBRT) planning. Computed tomography (CT) data of 14 endometrial cancer patients were included in this study. Manual and automatic planning with scripting were performed for each CT. Scripts were created in the RayStation™ (RaySearch Laboratories AB, Stockholm, Sweden) planning system using a Python code. In scripting, seven additional contours were automatically created to reduce the OAR doses. The scripted and manual plans were compared to each other in terms of planning time, dose-volume histogram (DVH) parameters, and total monitor unit (MU) values. While the mean total planning time for manual planning was 368 ± 8s, it was only 55 ± 2s for the automatic planning with scripting (p < 0.001). The mean doses of OARs decreased with automatic planning (p < 0.001). In addition, the maximum doses (D2% and D1%) for bilateral femoral heads and the rectum were significantly reduced. It was observed that the total MU value increased from 1146 ± 126 (manual planning) to 1369 ± 95 (scripted planning). It is concluded that scripted planning has significant time and dosimetric advantages over manual planning for endometrial cancer EBRT planning.

Dosimetric effects of the custom dose iteration times on stereotactic radiotherapy for lung cancer

PurposeTo evaluate the dosimetric effects of the custom dose iteration function in the Monaco 6.0 planning system on stereotactic radiotherapy for lung cancer. MethodsUsing the custom dose iterative function segment shape optimization (SSO) times in the Monaco 6.0 treatment planning system, the iterative times were set as 3, 5, 8, 10, and 12, respectively. Five volumetric modulated arc stereotactic body radiation therapy (SBRT) plans were made for each patient, and other optimization parameters and optimization functions were kept consistent. The dose volume histogram (DVH) statistical table was used to evaluate the dose of the target area and organs at risk (OAR). Statistical analysis was performed using SPSS 25 software, and the results were statistically different when p < 0.05. ResultsFor the target area dose analysis, SSO8, SSO10, and SSO12 groups showed similar results, significantly better than both SSO3 and SSO5 groups (p < 0.05). With the increase in the times of SSO, the protection of the heart was better, except for SSO3, the maximum dose of the esophagus and V47Gy of great vessels were significantly lower than SSO5 in all groups. The mean dose of heart was significantly lower in SSO10 and SSO12 than SSO5 (p < 0.05). There was no significant difference in other OAR among the groups. The optimization time (OT) of each group was statistically different, the maximum OT exceeded 1000s, the maximum deviation between groups exceeded 600s, and the OT increased significantly with the times of SSO (p < 0.05). There was no statistically significant difference between the 5 groups of monitor unit (MU), and segment was not significantly different between the groups except SSO3 which was significantly higher than SSO5 (p < 0.05). The delivery time (DT) was statistically different between the groups except for SSO3. ConclusionsIn the treatment of left-sided lung cancer patients with volumetric modulated arc SBRT, considering both calculation accuracy and clinical treatment efficiency, SSO times of 8 is recommended for designing SBRT for lung cancer with the Monaco treatment planning system.

Implementation, Dosimetric Assessment, and Treatment Validation of Knowledge-Based Planning (KBP) Models in VMAT Head and Neck Radiation Oncology

The aim of this study was to evaluate knowledge-based treatment planning (KBP) models in terms of their dosimetry and deliverability and to investigate their clinical benefits. Three H&N KBP models were built utilizing RapidPlan™, based on the dose prescription, which is given according to the planning target volume (PTV). The training set for each model consisted of 43 clinically acceptable volumetric modulated arc therapy (VMAT) plans. Model quality was assessed and compared to the delivered treatment plans using the homogeneity index (HI), conformity index (CI), structure dose difference (PTV, organ at risk-OAR), monitor units, MU factor, and complexity index. Model deliverability was assessed through a patient-specific quality assurance (PSQA) gamma index-based analysis. The dosimetric assessment showed better OAR sparing for the RapidPlan™ plans and for the low- and high-risk PTV, and the HI, and CI were comparable between the clinical and RapidPlan™ plans, while for the intermediate-risk PTV, CI was better for clinical plans. The 2D gamma passing rates for RapidPlan™ plans were similar or better than the clinical ones using the 3%/3 mm gamma-index criterion. Monitor units, the MU factors, and complexity indices were found to be comparable between RapidPlan™ and the clinical plans. Knowledge-based treatment plans can be safely adapted into clinical routines, providing improved plan quality in a time efficient way while minimizing user variability.

Data Communication Stability Test a Data Acquisition System (DAQ) for Inpatient Rooms

In inpatient services, patient beds are usually limited by curtains or partitions to maintain the patient's safety. At the time of data collection, the barrier or barrier is considered to the effectiveness of the results and validation of medical device sensor data. This study aims to explain the results of the reliability testing of Bluetooth and the internet on hardware and CovWatch applications. This study is action research; comparison is used using devices (tools) with different specifications. Testing internet and Bluetooth connectivity from the CovWatch unit and applications installed on the Samsung A01 devices, Redmi Note 10 and Oppo A57, have mixed results. Hardware CovWatch and Samsung A01 are considered the best in acquiring vital sign data, while Redmi Note 10 and OPPO A57 are not good because some data cannot be obtained so that it does not appear on the monitor unit.

Evaluation of plan quality and treatment efficiency in cranial stereotactic radiosurgery treatment plans with a variable source-to-axis distance.

Radiotherapy deliveries with dynamic couch motions that shorten the source-to-axis distance (SAD) on a C-arm linac have the potential to increase treatment efficiency through the increase of the effective dose rate. In this investigation, we convert clinically deliverable volumetric modulated arc therapy (VMAT) and dynamic conformal arc (DCA) plans for cranial radiosurgery into virtual isocenter plans through implementation of couch trajectories that maintain the target at a shortened but variable SAD throughout treatment. A randomly sampled population of patients treated with cranial radiosurgery from within the last three years were separated into groups with one, two, and three lesions. All plans had a single isocenter (regardless of number of targets), and a single prescription dose. Patient treatment plans were converted from their original delivery at a standard isocenter to a dynamic virtual isocenter in MATLAB. The virtual isocenter plan featured a variable isocenter position based upon the closest achievable source-to-target distance (referred to herein as a virtual source-to-axis distance [vSAD]) which avoided collision zones on a TrueBeam STx platform. Apertures were magnified according to the vSAD and monitor units at a given control point were scaled based on the inverse square law. Doses were calculated for the plans with a virtual isocenter in the Eclipse (v13.6.23) treatment planning system (TPS) and were compared with the clinical plans. Plan metrics (MU, Paddick conformity index, gradient index, and the volume receiving 12 Gy or more), normal brain dose-volume differences, as well as maximum doses received by organs at risk (OARs) were assessed. The values were compared between standard and virtual isocenter plans with Wilcoxon Sign Ranked tests to determine significance. A subset of the plans were mapped to the MAX-HD anthropomorphic phantom which contained an insert housing EBT3 GafChromic film and a PTW 31010 microion chamber for dose verification on a linac. Delivering plans at a virtual isocenter resulted in an average reduction of 20.9% (p = 3×10-6 ) and 20.6% (p = 3.0×10-6 ) of MUs across all VMAT and all DCA plans, respectively. There was no significant change in OAR max doses received by plans delivered at a virtual isocenter. The low dose wash (1.0-2.0 Gy or 5-11% of the prescription dose) was increased (by approximately 20 cc) for plans with three lesions. This was equivalent to a 2.7%-3.8% volumetric increase in normal tissue receiving the respective dose level when comparing the plan with a virtual isocenter to a plan with a standard isocenter. Gamma pass rates with a 5%/1mm analysis criteria were 96.40% ± 2.90% and 95.07% ± 3.10% for deliveries at standard and virtual isocenter, respectively. Absolute point dose agreements were within -0.36% ± 3.45% and -0.55% ± 3.39% for deliveries at a standard and virtual isocenter, respectively. Potential time savings per arc were found to have linear relationship with the monitor units delivered per arc (savings of 0.009 s/MU with an r2 = 0.866 when fit to plans with a single lesion). Converting clinical plans at standard isocenter to a virtual isocenter design did not show any losses to plan quality while simultaneously improving treatment efficiency through MU reductions.

Dosimetry comparison with helical tomotherapy, volumetric modulated arc therapy, and intensity-modulated radiotherapy for grade II gliomas: A single‑institution case series.

Radiotherapy is an essential postoperative treatment for grade II gliomas. However, comparative dosimetric studies of different radiotherapy plans for grade II gliomas are still lacking. Therefore, we conducted this case series analysis to compare the dosimetric differences among helical tomotherapy (TOMO), volumetric modulated arc therapy (VMAT), and intensity-modulated radiotherapy (IMRT) for grade II gliomas. To achieve that, seven diagnosed postoperative patients with grade II gliomas were analyzed by computed tomography and then planned with TOMO, VMAT, and IMRT. The plan target volume (PTV) prescribed dose was 50 Gy (daily fraction of 2.0 Gy, 5 days/week). The expected treatment efficiency was measured by monitor units (MUs) scoring. Treatment plans of the patients were compared in the quality of target volumes dosage coverage, the efficiency of dosage delivery, and the dosage exposure of normal adjacent organs at risk (OAR). Differences in each method were measured by utilizing the Nonparametric ANOVA. The study shows that TOMO achieved a significantly higher PTV-D98% (doses received by 98% of the PTV volume) than VMAT and IMRT (50.30 ± 0.13 vs 49.21 ± 0.19, p = 0.006; 50.30 ± 0.13 vs 49.78 ± 0.18, p = 0.014), while there was no difference in PTV-D2% (doses received by 2% of the PTV volume). IMRT achieved a conformity index (CI) preferably, and TOMO generated a favorable homogeneity index (HI) (p < 0.05 for both). The MUs were fewer for VMAT than IMRT and TOMO (294 ± 19, 572 ± 24, 317 ± 97, respectively). IMRT achieved better protection for the lens and brain stems. Our case series study indicated that TOMO, VMAT, and IMRT achieved a comparatively good target dosimetric coverage, and most OARs were protected well. IMRT is not inferior to TOMO and VMAT and is still very suitable for treating most grade II glioma patients.

Spot delivery error predictions for intensity modulated proton therapy using robustness analysis with machine learning.

The purpose of this work is to assess the robustness of treatment plans when spot delivery errors were predicted with a machine learning (ML) model for intensity modulated proton therapy (IMPT). Over 6000 machine log files from delivered IMPT treatment plans were included in this study. From these log files, over 4.1 106 delivered proton spots were used to train the ML model. The presented model was tested and used to predict the spot position as well as the monitor units (MU) per spot, based on the original planning parameters. Two patient plans (one accelerated partial breast irradiation [APBI] and one ependymoma) were recalculated with the predicted spot position/MUs by the ML model and then were re-analyzed for robustness. Plans with ML predicted spots were less robust than the original clinical plans. In the APBI plan, dosimetric changes to the left lung and heart were not clinically relevant. In the ependymoma plan, the hot spot in the brainstem decreased and the hot spot in the cervical cord increased. Despite these differences, after robustness analysis, both ML spot delivery error plans resulted in >95% of the CTV receiving >95% of the prescription dose. The presented workflow has the potential benefit of including realistic spots information for plan quality checks in IMPT. This work demonstrates that in the two example plans, the plans were still robust when accounting for spot delivery errors as predicted by the ML model.

Dosimetric comparison of two dose expansion methods in intensity modulated radiotherapy for breast cancer

BackgroundTo explore the dosimetric difference between IMRT-VB plan based on the establishment of external expansion structure and virtual bolus (VB) and IMRT-SF based on the skin flash (SF) tool of the Eclipse treatment planning system in postoperative chest wall target intensity modulation radiotherapy plan of breast cancer.MethodsTwenty patients with breast cancer were randomly selected as subjects to develop IMRT-VB plan based on virtual bolus and IMRT-SF plan based on skin flash tool of Eclipse treatment planning system. The planning target volume, monitor unit (MU) of every single treatment and the dosimetric parameters of organ at risk (OARs) were recorded. Paired t-test was used for normal distribution data while nonparametric paired Wilcoxon rank sum test was used for non-normal distribution data.ResultsBoth IMRT-VB and IMRT-SF plan can expand outward to the chest wall skin and meet the dose requirements of clinical prescription. The conformal index, the homogeneity index, D2%, D98% and D50% were significantly better in IMRT-SF plan than those in IMRT-VB plan (P < 0.05). The average MU of the IMRT-SF plan was much higher than that of the IMRT-VB plan (866.0 ± 68.1 MU vs. 760.9 ± 50.4 MU, P < 0.05). In terms of organ at risk protection, IMRT-SF plan had more advantages in the protection of ipsilateral lung and spinal cord than IMRT-VB plan (P < 0.05).ConclusionOur study indicated that IMRT-SF plan displayed clinical application superiority compared to IMRT-VB plan, and the operation steps of which are simpler and faster. Besides, IMRT-SF plan took advantages in achieve effective external expansion of skin dose intensity and OARs protection.

Clinical experience on patient-specific quality assurance for CBCT-based online adaptive treatment plan.

Ethos CBCT-based adaptive radiotherapy (ART) system can generate an online adaptive plan by re-optimizing the initial reference plan based on the patient anatomy at the treatment. The optimization process is fully automated without any room for human intervention. Due to the change in anatomy, the ART plan can be significantly different from the initial plan in terms of plan parameters such as the aperture shapes and number of monitor units (MUs). In this study, we investigated the feasibility of using calculation-based patient specific QA for ART plans in conjunction with measurement-based and calculation-based QA for initial plans to establish an action level for the online ART patient-specific QA. A cohort of 98 cases treated on CBCT-based ART system were collected for this study. We performed measurement-based QA using ArcCheck and calculation-based QA using Mobius for both the initial plan and the ART plan for analysis. For online the ART plan, Mobius calculation was conducted prior to the delivery, while ArcCheck measurement was delivered on the same day after the treatment. We first investigated the modulation factors (MFs) and MU numbers of the initial plans and ART plans, respectively. The γ passing rates of initial and ART plan QA were analyzed. Then action limits were derived for QA calculation and measurement for both initial and online ART plans, respectively, from 30 randomly selected patient cases, and were evaluated using the other 68 patient cases. The difference in MF between initial plan and ART-plan was 12.9% ± 12.7% which demonstrates their significant difference in plan parameters. Based on the patient QA results, pre-treatment calculation and measurement results are generally well aligned with ArcCheck measurement results for online ART plans, illustrating their feasibility as an indicator of failure in online ART QA measurements. Furthermore, using 30 randomly selected patient cases, the γ analysis action limit derived for initial plans and ART plans are 89.6% and 90.4% in ArcCheck QA (2%/2mm)andare 92.4% and 93.6% in Mobius QA(3%/2mm),respectively.According to the calculated action limits, the ArcCheck measurements for all the initial and ART plans passed QA successfully while the Mobius calculation action limits flagged seven and four failure cases respectively for initial plans and ART plans, respectively. An ART plan can be substantially different from the initial plan, and therefore a separate session of ART plan QA is needed to ensure treatment safety and quality. The pre-treatment QA calculation via Mobius can serve as a reliable indicator of failure in online ART plan QA. However, given that Ethos ART system is still relatively new, ArcCheck measurement of initial plan is still in practice. It may be skipped as we gain more experience and have better understanding of the system.

Monitor unit restrained hybrid approach using conformal arcs for radiosurgery of solitary brain metastasis

PurposeThis study appraises the efficacy of monitor unit (MU) restrained hybrid planning approach for solitary brain metastasis combining dynamic conformal and volumetric modulated arcs. The optimal level of monitor unit restrainment is ascertained by plan metric analysis. Besides, dependence of efficiency index (η50%) on beam modulation (BM) and association of normal brain receiving 12Gy (V12) with gradient index (GI) under the applied MU constraints is assessed. MethodsSixteen cases were planned to a dose of 20 Gy using 2–3 non-coplanar DCAT arcs and a coplanar VMAT arc, combined in a 7:3 dose ratio. All VMAT optimizations were based on DCA dose. A baseline VMAT plan was generated without using the MU-tool. Then MUs of the baseline plan were reduced using the MU-tool by 10%, 20%, 30%, and 40% to develop alternate plan types. Alternate plans were compared to baseline plan using the mean difference of plan metrics. The relationship of η50% with BM and that of V12 with GI was assessed using Pearson's r. ResultsEfficiency index, η50% reduced significantly across all plan types except MU-10. CI increased from 90.8% to 91.4% on reducing MU restraint. GI did not deteriorate for MU-10 and MU-20 but increased for MU-30 and MU-40, P < 0.006. Brain receiving 12Gy, V12 increased about 0.1 cm3 for MU-40, P = 0.001. Also, the probability of symptomatic necrosis, S-NEC(V12), increased by 0.1% for MU-40 and MU-30, P < 0.012. BM reduced from MU-20 through MU-40 by at least 5.7% (P < 0.001). Beam-on-time and MU factor reduced maximally by 0.6 min and 15.7%, for MU-40. For all plan types, η50% correlated directly with BM, whereas V12 showed an inverse relationship with GI. ConclusionsModerately constrained, MU-20 type hybrid plans demonstrated optimal overall efficiency, achieving plan objectives with reasonable delivery simplicity. Efficiency index reduced gradually with decreasing beam modulation.

Dosimetric Comparison of Different Dose Calculation Algorithms in Postmastectomy Breast Cancer Patients Using Conformal Planning Techniques.

The aim of the current study was to compare three different dose-calculating algorithms, i.e., superposition (SP), fast SP (FSP), and convolution (CV), for breast cancer patients treated with intensity-modulated radiotherapy (IMRT) and field-in-Field forward plan IMRT (FiF-FP-IMRT). The current retrospective study involved 100 postmastectomy breast cancer patients who were given radiotherapy using IMRT and FiF-FP-IMRT planning techniques. All the initially SP-calculated plans were recalculated with the same monitor units for FSP and CV algorithm without change in any of the other planning parameters. The isodose distribution and various plan evaluating parameters, for example, conformity index (CI), homogeneity index, and uniformity index target volume and normal structure doses were compared and analyzed for all the different algorithm calculated plans. In the IMRT plans, all the target and normal structure dose-volume parameters showed a significant difference between all the three different algorithms with P < 0.05. In the FiF-FP-IMRT plans, CV algorithm showed a significant difference in most of the target and normal structure dose-volume parameters. Among quality indexes, only CI showed a significant difference between all the algorithms in both the planning techniques. R50 showed a significant difference with the CV algorithm in both the planning techniques. The change in the dose calculation algorithm resulted in dosimetric changes which must be evaluated by the medical physicists and oncologists while evaluating treatment plans. In the current study with breast patients, the results obtained for target and normal structure doses using the CV algorithm are overestimated as compared to SP and FSP algorithms, producing variable results in air and bony normal structures. However, the ipsilateral lung V5 parameter and the ipsilateral humeral head mean dose were found to be underestimated by the CV algorithm as compared to the SP and FSP algorithm in both the planning techniques.

Multiple Brain Metastases Radiosurgery with CyberKnife Device: Dosimetric Comparison between Fixed/Iris and Multileaf Collimator Plans.

In our institution, stereotactic radiosurgery of multiple brain metastases is performed with the CyberKnife® (CK) device, using fixed/Iris collimators. In this study, nineteen fixed/Iris plans were recalculated with the multileaf collimator (MLC), to assess if it is possible to produce plans with comparable dosimetric overall quality. For consistent comparisons, MLC plans were re-optimized and re-normalized in order to achieve the same minimum dose for the total planning target volume (PTVtot). Conformation number (CN), homogeneity index (HI) and dose gradient index (DGI) metrics were evaluated. The dose to the brain was evaluated as the volume receiving 12 Gy (V12) and as the integral dose (ID). The normal tissue complication probability (NTCP) for brain radionecrosis was calculated as a function of V12. The reoptimized plans were reviewed by the radiation oncologist and were found clinically acceptable according to the The American Association of Physicists in Medicine (AAPM) Task Group-101 protocol. However, fixed/Iris plans provided significantly higher CN (+8.6%), HI (+2.2%), and DGI (+44.0%) values, and significantly lower ID values (-35.9%). For PTVtot less than the median value of 2.58cc, fixed/Iris plans provided significantly lower NTCP values. On the other side, MLC plans provided significantly lower treatment times (-18.4%), number of monitor units (-33.3%), beams (-46.0%) and nodes (-21.3%). CK-MLC plans for the stereotactic treatment of brain multi metastases could provide an important advantage in terms of treatment duration. However, to contain the increased risk for brain radionecrosis, it could be useful to calculate MLC plans only for patients with large PTVtot.