The ArcCHECK diode array for dosimetric verification of HybridArc

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

PURPOSE

It is necessary to perform gantry quality assurance (QA) in high precision radiotherapy. However, the O-ring type linear accelerator (Halcyon) does not have a light field and laser as a reference of isocenter point. The aim of this study is to investigate the usefulness of a three-dimensional diode array detector for gantry angle QA, and an O-ring type linear accelerator.

METHOD

The gantry angle and rotational center were verified using the ArcCHECK 3D diode array on the general linear accelerator (TrueBeam) as a reference and Halcyon. The gantry angles were measured at 0, 90, 180, and 270°. The accuracy of the gantry rotational center was evaluated using rotational irradiation in the clockwise and counterclockwise directions between 181° and 179°.

RESULTS

The QA system with ArcCHECK was able to apply on the TrueBeam and Halcyon. As a result of the accuracy of the gantry angle, the maximum error of value calculated from ArcCHECK was 0.1° compared with the nominal gantry angle of Halcyon. As a result of the accuracy of the gantry rotation isocenter of Halcyon, the distance between the isocenter and the gantry rotation center was 0.45 mm and 0.41 mm in the clockwise and counterclockwise directions, respectively.

CONCLUSION

The QA system with ArcCHECK was useful for the gantry angle and the rotation center accuracy on the Halcyon.

Comparison of global and local gamma evaluation results using isodose levels

The aim of this study is to evaluate the behaviour of global and local gamma analyses with isodose levels. Global and local gamma evaluation were performed on patient-specific quality assurance (PSQA) data from 100 volumetric modulated arc therapy (VMAT) arcs and 100 helical tomotherapy (HT) plans, using an in-house gamma code. Gamma pass rates versus isodose levels were plotted and evaluated. Other than a slightly increased skew towards higher pass rates for the global gamma evaluation, minimal differences were observed between the results of evaluating all VMAT arcs separately and the results of evaluating over VMAT treatment plans by combining arcs from each plan. Generally, the VMAT results showed average pass rates that increase with decreasing isodose level, for both global and local gamma evaluations. The HT results differed systematically from the VMAT results, with the results of performing global and local gamma evaluations agreeing more closely at all isodose levels and with the highest gamma pass rates being achieved at intermediate dose levels, between the 40 and 70% isodose levels. These results demonstrate the complex of relationships between global and local gamma evaluation results that can arise when clinical PSQA data are analysed and exemplify how the local gamma evaluation does not necessarily produce disproportionately reduced gamma pass rates in low dose regions. Performing gamma evaluation with different isodose levels is suggested as a useful method to improve understanding of specific PSQA data and as well as the broader features of gamma evaluation results.

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

PURPOSE

Intensity modulated radiation therapy (IMRT) has become a widely accepted and efficient treatment technique for many types of cancers. Patient's specific quality assurance (QA) should be performed with QA devices. Stability and sensitivity tests conducted on the ArcCHECK (AC) 3D diode array were performed.

METHODS

Set-up error test with AC was performed. The set-up position moved to lateral (mm), longitudinal (mm) and rotational (°) were 0.5, 1.0, 2.0 and 3.0, respectively. Sensitivity change test of diode array with AC through 230 days was also performed. Same array calibration data was applied to all measurements of volumetric-modulated arc therapy benchmark test through 230 days. Gamma method (2 mm/2% criteria) was performed to analyze the result of all measurements.

RESULTS

In the results of positional error, gamma pass rate become degenerate according to positional error became larger. With 0.5 mm or 0.5° positional error, decreasing rate of the pass rate of lateral, longitudinal and rotational were 1.0%, 2.5% and 4.2%, respectively. In the sensitivity change test, the gamma pass rate decreased 2.2%/100 days with same calibration data.

CONCLUSION

AC has highly sensitivity against positional error. Sensitivity of AC has been changed and pass rate was decreased 2.2%/100 days through 230 days. Array calibration should be performed in consideration of change of sensitivity.

Evaluation of the reconstructed dose from the three-dimensional dose module of a helical diode array: factors of influence and error detection

The 3D-dose module (3DVH) of the ArcCHECK-phantom reconstructs the dose distribution in the phantom volume and transfers it to the patient geometry. Our aim was to evaluate the 3DVH-reconstructed dose systematically building up from simple to complex cases. Therefore, the influence of different field sizes without and with blocking the isocenter was tested. The dose distributions of different radiation techniques, error-free and error-induced VMAT-plans were verified by measuring with films and other detectors in the phantom. It was checked how the inclusion of the dose measured separately in the ArcCHECK-isocenter affects the reconstruction. Thus it was also investigated which detector should be used for the dosimetry in the isocenter. Without including the isocentrically measured dose, the reconstruction for the smallest field (2  ×  2 cm²) was 5% (6 MV) and 3.7% (10 MV) higher than measured with an ionization chamber. With increasing field size, the deviation decreased. For fields with blocked isocenters, the reconstructed dose was between  -10.6% and  -24% lower than determined with a microDiamond. Measurements with the Semiflex of the spinal plan resulted in higher doses than calculated by the treatment planning system (TPS) and measured with the film and the other detectors. Through the inclusion of the isocentric dose in the reconstruction its accordance with the film increased mostly. With exception of an error-induced head and neck plan, the induced errors in the reconstructed dose volume histogram became visible, but were underestimated. With the 3DVH-algorithm not every induced-error was detected. The 3DVH underestimated the dose in blocked areas. To protect organs at risk (OAR), these are often blocked. Consequently, there is a risk that a clinical decision is based on a 3DVH that underestimated the dose for the OAR. We recommend including the isocentric dose in the reconstruction. The detector used for the isocentric measurements should be carefully chosen.

Primo software as a tool for Monte Carlo simulations of intensity modulated radiotherapy: a feasibility study

Background

IMRT provides higher dose conformation to the target and dose sparing to surrounding tissues than 3DCRT. Monte Carlo method in Medical Physics is not a novelty to approach dosimetric problems. A new PENELOPE based code named PRIMO recently was published. The most intriguing features of PRIMO are the user-friendly approach, the stand-alone property and the built-in definition of different linear accelerators models. Nevertheless, IMRT simulations are not yet implemented.

Methods

A Varian Trilogy with a Millennium120 MLC and a Varian Novalis with 120HD MLC were studied. A RW3 multi-slab phantom was irradiated with Gafchromic films inserted between slabs. An Expression 10000XL scanner (Seiko Epson Corp., Nagano, Japan) was used to digitalize the films. PTW-Verisoft software using the global Gamma Function (2%, 2 mm) was used to compare simulated and experimental results.

The primary beam parameters were adjusted to best match reference data previously obtained in a water phantom. Static MLC simulations were performed to validate the MLC models in use. Two Dynamic IMRT preliminary tests were performed with leaves moving with constant and variable speed. A further test of an in phantom delivery of a real IMRT field allowed simulating a clinical-like MLC modulation.

Results

Simulated PDD, X- and Y-profiles in reference conditions showed respectively 100.0%, 100.0% and 99.4% of Gamma points < 1 (2%, 2 mm). Static MLC simulations showed 100.0% of Gamma points < 1 with the 120HD MLC and 99.1% with the Millennium compared with the scanned images.

The fixed speed test showed 99.5 and 98.9% of Gamma points < 1 respectively with two different MLC configuration-sampling algorithms when the 120HD MLC was used. The higher modulation MLC motion simulation showed 99.1% of Gamma points < 1 with respect to the experimental. This result depends on the number of the fields to reproduce the MLC motion, as well as calculation time. The clinical-like simulation showed 96.2% of Gamma points < 1 using the same analysis conditions.

Conclusions

The numerical model of the Varian Trilogy and Novalis in the PRIMO software was validated. The algorithms to simulate MLC motion were considered reliable. A clinical-like procedure was successfully simulated.

Treatment plan quality and delivery accuracy assessments on 3 IMRT delivery methods of stereotactic body radiotherapy for spine tumors

Stereotactic body radiotherapy (SBRT) for spine tumors has demonstrated clinical effectiveness. The treatment planning and delivery techniques have evolved from dynamic conformal arc therapy, to fixed gantry angle intensity modulated radiotherapy (IMRT), and most recently to volumetric modulated arc therapy (VMAT). A hybrid-arc (HARC) planning and delivery method combining dynamic conformal arc therapy delivery with a number of equally spaced IMRT beams is proposed. In this study we investigated plan quality, delivery accuracy, and efficiency of 3 delivery techniques: IMRT, HARC, and VMAT. Patients who underwent spine SBRT treatments were randomly selected from an Institutional Review Board-approved registry. For each patient, the prescription dose was 14 to 16 Gy in a single fraction to cover >90% of the tumor (without planning margin) while constraining V_10Gy ≤ 10% of the spinal cord and the maximum point dose (MPD) of the spinal cord ≤ 14 Gy. All cases were clinically treated with fixed gantry step-shoot IMRT plans and then re-planned with VMAT using Pinnacle 9.0 and with HARC using Brainlab iPlan 4.5. Student t-test was used to compare the dosimetric end points, including V_16Gy to the planning target volume, homogeneity index, MPD_PTV, the conformity index, V_10Gy of the spinal cord, and MPD_cord. To compare the accuracy of delivery, we delivered all plans on a phantom and conducted gamma index (GI) comparisons with 3 mm/3% and 2 mm/2% criteria. All plans met our clinical requirements. Among 3 techniques, there were no differences on dose coverage to the tumor volume, maximum dose to the spinal cord, and plan homogeneity index (p > 0.05). The average V_10Gy of the spinal cord was 6.66 ± 0.03%, 5.49 ± 0.03%, and 4.76 ± 0.02% for IMRT, HARC, and VMAT plans, respectively. Accordingly, the conformity indices were 1.30 ± 0.11 and 1.29 ± 0.20, 1.53 ± 0.29, respectively. VMAT plans were significantly (p < 0.05) less conformal but significantly (p < 0.05) lower V_10Gy of the spinal cord than those from HARC and IMRT plans. With delivery accuracy measured by GIs, the average GIs of 3%/3 mm were 92.6 ± 1.1%, 96.5 ± 2.7%, 99.0 ± 1.1% for IMRT, HARC, and VMAT plans, respectively. The differences were significant (p < 0.05). Accordingly, the average monitor units were 9238 ± 2242, 9853 ± 2548 and 5091 ± 910. The plan quality created from the 3 planning techniques can meet the clinical requirement. Adding arc beams in delivery such as in HARC and VMAT plans improves the delivery accuracy. VMAT is the most efficient delivery method.

The Impact of Dose Rate on the Accuracy of Step-and-Shoot Intensity-modulated Radiation Therapy Quality Assurance Using Varian 2300CD

Introduction

Intensity-modulated radiation therapy (IMRT) delivery using "step-and-shoot" technique on Varian C-Series linear accelerator (linac) is influenced by the communication frequency between the multileaf collimator and linac controllers. Hence, the dose delivery accuracy is affected by the dose rate.

Aim

Our aim was to quantify the impact of using two dose rates on plan quality assurance (QA).

Materials and Methods

Twenty IMRT patients were selected for this study. The plan QA was measured at two different dose rates. A gamma analysis was performed, and the degree of plan modulation on the QA pass rate was also evaluated in terms of average monitor unit per segment (MU/segment) and the total number of segments.

Results

The mean percentage gamma pass rate of 94.9% and 93.5% for 300 MU/min and 600 MU/min dose rate, respectively, was observed. There was a significant (P = 0.001) decrease in percentage gamma pass rate when the dose rate was increased from 300 MU/min to 600 MU/min. There was a weak, but significant association between the percentage pass rate at both dose rate and total number of segments. The total number of MU was significantly correlated to the total number of segments (r = 0.59). We found a positive correlation between the percentage pass rate and mean MU/segment, r = 0.52 and r = 0.57 for 300 MU/min and 600 MU/min, respectively.

Conclusion

IMRT delivery using step-and-shoot technique on Varian 2300CD is impacted by the dose rate and the total amount of segments.

Validation of an improved helical diode array and dose reconstruction software using TG-244 datasets and stringent dose comparison criteria

The original helical ArcCHECK (AC) diode array and associated software for 3D measurement‐guided dose reconstruction were characterized and validated; however, recent design changes to the AC required that the subject be revisited. The most important AC change starting in 2014 was a significant reduction in the overresponse of diodes to scattered radiation outside of the direct beam, accomplished by reducing the amount of high‐Z materials adjacent to the diodes. This change improved the diode measurement accuracy, but in the process invalidated the dose reconstruction models that were assembled based on measured data acquired with the older version of the AC. A correction mechanism was introduced in the reconstruction software (3DVH) to accommodate this and potential future design changes without requiring updating model parameters. For each permutation of AC serial number and beam model, the user can define in 3DVH a single correction factor which will be used to compensate for the difference in the out‐of‐field response between the new and original AC designs. The exact value can be determined by minimizing the dose‐difference with an ionization chamber or another independent dosimeter. A single value of 1.17, corresponding to the maximum measured out‐of‐field response difference between the new and old AC, provided satisfactory results for all studied energies (6X, 15X, and flattening filter‐free 10XFFF). A library of standard cases recommended by the AAPM TG‐244 Report was used for reconstructed dose verification. The overall difference between reconstructed dose and an ion chamber in a water‐equivalent phantom in the targets was 0.0% ± 1.4% (1 SD). The reconstructed dose on a homogeneous phantom was also compared to a biplanar diode dosimeter (Delta4) using gamma analysis with 2% (local dose‐error normalization)/2 mm/10% cutoff criteria. The mean agreement rate was 96.7% ± 3.7%. For the plans common with the previous comparison, the mean agreement rate was 98.3% ± 0.8%, essentially unchanged. We conclude that the proposed software modification adequately addresses the change in the dosimeter response.

PACS number(s): 87.55Qr

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate

The main objective of this study is to demonstrate the performance characteristics of the Magic Plate (MP) system when operated upstream of the patient in transmission mode (MPTM). The MPTM is an essential component of a real‐time QA system designed for operation during radiotherapy treatment. Of particular interest is a quantitative study into the influence of the MP on the radiation beam quality at several field sizes and linear accelerator potential differences. The impact is measured through beam perturbation effects such as changes in the skin dose and/or percentage depth dose (PDD) (both in and out of field). The MP was placed in the block tray of a Varian linac head operated at 6, 10 and 18 MV beam energy. To optimize the MPTM operational setup, two conditions were investigated and each setup was compared to the case where no MP is positioned in place (i.e., open field): (i) MPTM alone and (ii) MPTM with a thin passive contamination electron filter. The in‐field and out‐of‐field surface doses of a solid water phantom were investigated for both setups using a Markus plane parallel (Model N23343) and Attix parallel‐plate, MRI model 449 ionization chambers. In addition, the effect on the 2D dose distribution measured by the Delta⁴ QA system was also investigated. The transmission factor for both of these MPTM setups in the central axis was also investigated using a Farmer ionization chamber (Model 2571A) and an Attix ionization chamber. Measurements were performed for different irradiation field sizes of 5×5 cm2 and 10×10 cm2. The change in the surface dose relative to dmax was measured to be less than 0.5% for the 6 MV, 10 MV, and 18 MV energy beams. Transmission factors measured for both set ups (i & ii above) with 6 MV, 10 MV, and 18 MV at a depth of dmax and a depth of 10 cm were all within 1.6% of open field. The impact of both the bare MPTM and the MPTM with 1 mm buildup on 3D dose distribution in comparison to the open field investigated using the Delta⁴ system and both the MPTM versions passed standard clinical gamma analysis criteria. Two MPTM operational setups were studied and presented in this article. The results indicate that both versions may be suitable for the new real‐time megavoltage photon treatment delivery QA system under development. However, the bare MPTM appears to be slightly better suited of the two MP versions, as it minimally perturbs the radiation field and does not lead to any significant increase in skin dose to the patient.

PACS number(s): 87.50.up, 87.53.Bn, 87.55.N, 87.55.Qr, 87.56.Fc.

Robotic radiosurgery system patient-specific QA for extracranial treatments using the planar ion chamber array and the cylindrical diode array

Robotic radiosurgery system has been increasingly employed for extracranial treatments. This work is aimed to study the feasibility of a cylindrical diode array and a planar ion chamber array for patient‐specific QA with this robotic radiosurgery system and compare their performance. Fiducial markers were implanted in both systems to enable image‐based setup. An in‐house program was developed to postprocess the movie file of the measurements and apply the beam‐by‐beam angular corrections for both systems. The impact of noncoplanar delivery was then assessed by evaluating the angles created by the incident beams with respect to the two detector arrangements and cross‐comparing the planned dose distribution to the measured ones with/without the angular corrections. The sensitivity of detecting the translational (1–3 mm) and the rotational (1°–3°) delivery errors were also evaluated for both systems. Six extracranial patient plans (PTV 7–137 cm³⁾ were measured with these two systems and compared with the calculated doses. The plan dose distributions were calculated with ray‐tracing and the Monte Carlo (MC) method, respectively. With 0.8 by 0.8 mm² diodes, the output factors measured with the cylindrical diode array agree better with the commissioning data. The maximum angular correction for a given beam is 8.2% for the planar ion chamber array and 2.4% for the cylindrical diode array. The two systems demonstrate a comparable sensitivity of detecting the translational targeting errors, while the cylindrical diode array is more sensitive to the rotational targeting error. The MC method is necessary for dose calculations in the cylindrical diode array phantom because the ray‐tracing algorithm fails to handle the high‐Z diodes and the acrylic phantom. For all the patient plans, the cylindrical diode array/ planar ion chamber array demonstrate 100%/>;92%(3%/3 mm) passing rates. The feasibility of using both systems for robotic radiosurgery system patient‐specific QA has been demonstrated. For gamma evaluation, 2%/2 mm criteria for cylindrical diode array and 3%/3 mm criteria for planar ion chamber array are suggested. The customized angular correction is necessary as proven by the improved passing rate, especially with the planar ion chamber array system.

PACS number: 29.40.‐n

Dosimetric verification by using the ArcCHECK system and 3DVH software for various target sizes

Objective

To investigate the usefulness of the 3DVH software with an ArcCHECK 3D diode array detector in newly designed plans with various target sizes.

Methods

The isocenter dose was measured with an ion-chamber and was compared with the planned and 3DVH predicted doses. The 2D gamma passing rates were evaluated at the diode level by using the ArcCHECK detector. The 3D gamma passing rates for specific regions of interest (ROIs) were also evaluated by using the 3DVH software. Several dose-volume histograms (DVH)-based predicted metrics for all structures were also obtained by using the 3DVH software.

Results

The isocenter dose deviation was <1% in all plans except in the case of a 1 cm target. Besides the gamma passing rate at the diode level, the 3D gamma passing rate for specific ROIs tended to decrease with increasing target size; this was more noticeable when a more stringent gamma criterion was applied. No correlation was found with the gamma passing rates and the DVH-based metrics especially in the ROI with high-dose gradients.

Conclusions

Delivery quality assurance by using 3DVH and ArcCHECK can provide substantial information through a simple and easy approach, although the accuracy of this system should be judged cautiously.

A novel method for routine quality assurance of volumetric-modulated arc therapy

PURPOSE

Volumetric-modulated arc therapy (VMAT) is delivered through synchronized variation of gantry angle, dose rate, and multileaf collimator (MLC) leaf positions. The delivery dynamic nature challenges the parameter setting accuracy of linac control system. The purpose of this study was to develop a novel method for routine quality assurance (QA) of VMAT linacs.

METHODS

ArcCheck is a detector array with diodes distributing in spiral pattern on cylindrical surface. Utilizing its features, a QA plan was designed to strictly test all varying parameters during VMAT delivery on an Elekta Synergy linac. In this plan, there are 24 control points. The gantry rotates clockwise from 181° to 179°. The dose rate, gantry speed, and MLC positions cover their ranges commonly used in clinic. The two borders of MLC-shaped field seat over two columns of diodes of ArcCheck when the gantry rotates to the angle specified by each control point. The ratio of dose rate between each of these diodes and the diode closest to the field center is a certain value and sensitive to the MLC positioning error of the leaf crossing the diode. Consequently, the positioning error can be determined by the ratio with the help of a relationship curve. The time when the gantry reaches the angle specified by each control point can be acquired from the virtual inclinometer that is a feature of ArcCheck. The gantry speed between two consecutive control points is then calculated. The aforementioned dose rate is calculated from an acm file that is generated during ArcCheck measurements. This file stores the data measured by each detector in 50 ms updates with each update in a separate row. A computer program was written in MATLAB language to process the data. The program output included MLC positioning errors and the dose rate at each control point as well as the gantry speed between control points. To evaluate this method, this plan was delivered for four consecutive weeks. The actual dose rate and gantry speed were compared with the QA plan specified. Additionally, leaf positioning errors were intentionally introduced to investigate the sensitivity of this method.

RESULTS

The relationship curves were established for detecting MLC positioning errors during VMAT delivery. For four consecutive weeks measured, 98.4%, 94.9%, 89.2%, and 91.0% of the leaf positioning errors were within ± 0.5 mm, respectively. For the intentionally introduced leaf positioning systematic errors of -0.5 and +1 mm, the detected leaf positioning errors of 20 Y1 leaf were -0.48 ± 0.14 and 1.02 ± 0.26 mm, respectively. The actual gantry speed and dose rate closely followed the values specified in the VMAT QA plan.

CONCLUSIONS

This method can assess the accuracy of MLC positions and the dose rate at each control point as well as the gantry speed between control points at the same time. It is efficient and suitable for routine quality assurance of VMAT.

3D evaluation of 3DVH program using BANG3 polymer gel dosimeter

PURPOSE

With the recent introduction of intensity modulated arc therapy techniques, there is an increasing need for validation of treatment delivery in three-dimensional (3D) space. A commercial dosimetry device ArcCHECK™ (Sun Nuclear Corporation, Melbourne, FL, USA) can be used in conjunction with 3DVH program. With this system, one can reconstruct the 3D dose distribution produced in the actual patient. In this work the authors evaluate the relative accuracy of the ArcCHECK™-3DVH system using BANG3 (MGS Research, Guilford, CT, USA) polymer gel dosimeter.

METHODS

About 15-cm diameter and 20-cm long cylindrical phantoms filled with BANG3 was used to simulate a patient, to which a volumetrically modulated arc therapy plan was created with Pinnacle3 treatment planning software (Philips Healthcare, Andover, MA, USA). The plan (76 Gy total in 38 fractions) was designed for prostate radiotherapy using a 6 MV photon beam from an Elekta Synergy linear accelerator (Elekta AB, Stockholm, Sweden). The treatment was delivered to the simulated patient. The same plan was used to irradiate an ArcCHECK™ device with an insert plug. The point dose at the isocenter was measured using a Farmer-type ionization chamber. The measured dose data were imported into the 3DVH program, which generated the 3D dose distributions projected onto the simulated patient. The dose data recorded in the polymer gel were read out using a MRI scanner and the 3D dose distribution delivered to the simulated patient was analyzed and compared with those from the 3DVH program and the Pinnacle3 software. The comparison was accomplished by using the gamma index, overlaying the isodose lines for a set of data on selected planes, and computing dose-volume histogram of structures.

RESULTS

The dose at the center of the ArcCHECK™ device measured with an ionization chamber was 1.82% lower than the dose predicted by Pinnacle3. The 3D dose distribution generated by Pinnacle3 was compared with those obtained by the ArcCHECK™-3DVH system and BANG3. The gamma passing rates for criteria of 3% dose difference, 3 mm distance-to-agreement, and 25% lower dose threshold were 99.1% for the former and 95.7% for the latter. The mean and maximum PTV doses estimated by the 3DVH were 74.0 and 79.3 Gy in comparison to 74.4 and 76.5 Gy with Pinnacle3. Those values for BANG3 measurements were 74.7 and 79.5 Gy. The mean doses to rectum were 40.2, 39.8, and 38.8 Gy for Pinnacle3, 3DVH, and BANG3, whereas the mean doses to the bladder were 26.7, 25.7, and 21.7 Gy, respectively.

CONCLUSIONS

The ArcCHECK™-3DVH system provides an accurate estimation of 3D dose distribution in an actual patient within a clinically meaningful tolerance level. However, both 3DVH and BANG3 showed two noticeable differences from Pinnacle3. First, the measured dose throughout the PTV region was less uniform than Pinnacle3. Second, the dose gradient at the interface between PTV and rectum was steeper than Pinnacle3 prediction. Further investigation may be able to identify the cause for these findings.

FusionArc optimization: a hybrid volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) planning strategy

PURPOSE

To introduce a hybrid volumetric modulated arc therapy/intensity modulated radiation therapy (VMAT/IMRT) optimization strategy called FusionArc that combines the delivery efficiency of single-arc VMAT with the potentially desirable intensity modulation possible with IMRT.

METHODS

A beamlet-based inverse planning system was enhanced to combine the advantages of VMAT and IMRT into one comprehensive technique. In the hybrid strategy, baseline single-arc VMAT plans are optimized and then the current cost function gradients with respect to the beamlets are used to define a metric for predicting which beam angles would benefit from further intensity modulation. Beams with the highest metric values (called the gradient factor) are converted from VMAT apertures to IMRT fluence, and the optimization proceeds with the mixed variable set until convergence or until additional beams are selected for conversion. One phantom and two clinical cases were used to validate the gradient factor and characterize the FusionArc strategy. Comparisons were made between standard IMRT, single-arc VMAT, and FusionArc plans with one to five IMRT∕hybrid beams.

RESULTS

The gradient factor was found to be highly predictive of the VMAT angles that would benefit plan quality the most from beam modulation. Over the three cases studied, a FusionArc plan with three converted beams achieved superior dosimetric quality with reductions in final cost ranging from 26.4% to 48.1% compared to single-arc VMAT. Additionally, the three beam FusionArc plans required 22.4%-43.7% fewer MU∕Gy than a seven beam IMRT plan. While the FusionArc plans with five converted beams offer larger reductions in final cost--32.9%-55.2% compared to single-arc VMAT--the decrease in MU∕Gy compared to IMRT was noticeably smaller at 12.2%-18.5%, when compared to IMRT.

CONCLUSIONS

A hybrid VMAT∕IMRT strategy was implemented to find a high quality compromise between gantry-angle and intensity-based degrees of freedom. This optimization method will allow patients to be simultaneously planned for dosimetric quality and delivery efficiency without switching between delivery techniques. Example phantom and clinical cases suggest that the conversion of only three VMAT segments to modulated beams may result in a good combination of quality and efficiency.

Comparison of dose calculations between pencil-beam and Monte Carlo algorithms of the iPlan RT in arc therapy using a homogenous phantom with 3DVH software

BACKGROUND

To create an arc therapy plan, certain current general calculation algorithms such as pencil-beam calculation (PBC) are based on discretizing the continuous arc into multiple fields to simulate an arc. The iPlan RT™ treatment planning system incorporates not only a PBC algorithm, but also a more recent Monte Carlo calculation (MCC) algorithm that does not need beam discretization. The objective of this study is to evaluate the dose differences in a homogenous phantom between PBC and MCC by using a three-dimensional (3D) diode array detector (ArcCHECK™) and 3DVH software.

METHODS

A cylindrically shaped 'target' region of interest (ROI) and a 'periphery ROI' surrounding the target were designed. An arc therapy plan was created to deliver 600 cGy to the target within a 350° rotation angle, calculated using the PBC and MCC algorithms. The radiation doses were measured by the ArcCHECK, and reproduced by the 3DVH software. Through this process, we could compare the accuracy of both algorithms with regard to the 3D gamma passing rate (for the entire area and for each ROI).

RESULTS

Comparing the PBC and MCC planned dose distributions directly, the 3D gamma passing rates for the entire area were 97.7% with the gamma 3%/3 mm criterion. Comparing the planned dose to the measured dose, the 3D gamma passing rates were 98.8% under the PBC algorithm and 100% under the MCC algorithm. The difference was statistically significant (p = 0.034). Furthermore the gamma passing rate decreases 7.5% in the PBC when using the 2%/2 mm criterion compared to only a 0.4% decrease under the MCC. Each ROI as well as the entire area showed statistically significant higher gamma passing rates under the MCC algorithm. The failure points that did not satisfy the gamma criteria showed a regular pattern repeated every 10°.

CONCLUSIONS

MCC showed better accuracy than the PBC of the iPlan RT in calculating the dose distribution in arc therapy, which was validated with the ArcCHECK and the 3DVH software. This may suggest that the arc step of 10° is too large in the PBC algorithm in the iPlan RT.

Quantitative analysis of geometric information from an end-to-end examination of IMRT and VMAT using the optimal selection method

PURPOSE

Gamma index, distance-to-agreement, and dose difference (DD) are commonly used to evaluate planar dose distributions. In this evaluation, the agreement between calculated and measured dose distributions can be susceptible to steep dose gradients along another axis perpendicular to the evaluation plane. Visual registration of the measured dose distribution may be performed to achieve better agreement, although doing so might lose geometric information related to beam targeting in an end-to-end test of intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). The optimal selection (OS) method was developed to take into consideration a dose distribution in three-dimensions, and also to quantitatively analyze geometric information along with better agreement.

METHODS

The OS method was composed of two steps. These steps were based on two algorithms, the gamma index and DD, to (1) find the best-matched plane, which is parallel to the planar measured dose distribution and is reconstructed by a volumetric dose distribution calculated by a treatment planning system; and (2) to get shifts and rotation along with better agreement between the calculated and measured dose distribution, compared with the planar dose distribution from the test. The OS method computes shifts and rotation against a user-defined coregistered location for the measured dose distribution. Thirteen prostate IMRT plans (two planes per plan for a total of 26 planes) were analyzed retrospectively to compare the pass ratios of DD and gamma index evaluations with and without the OS method. The computed shifts and rotations were evaluated.

RESULTS

Compared with the method without OS, the average pass ratios of DD and gamma index with the OS method increased by 8.2% and 5.7%, respectively, in the dose region from 30% to 100%. A particular result from one of the planes showed an increase of 43.5% and 32.5% in the pass ratios of DD and gamma, respectively, with the OS method in the same dose region. The shifts in the x-, y-, z-axes and rotation, which were computed using the OS method, were 0.5 ± 0.6, 0.3 ± 0.5, 1.0 ± 1.1 mm, and 0.3 ± 0.3°, respectively. In terms of the comparatively large difference between the z-shift and the x- and y-shifts, an additional geometric test was performed. A systematic error of 0.7 mm in the z-axis was found at the location of the film placed in the phantom that we used.

CONCLUSIONS

The OS method improved the quality of the end-to-end test of IMRT and VMAT plans by providing additional information regarding shifts and rotation, which were calculated and found to be in better agreement.

Development of a novel ArcCHECK(™) insert for routine quality assurance of VMAT delivery including dose calculation with inhomogeneities

PURPOSE

To design a versatile, nonhomogeneous insert for the dose verification phantom ArcCHECK(™) (Sun Nuclear Corp., FL) and to demonstrate its usefulness for the verification of dose distributions in inhomogeneous media. As an example, we demonstrate it can be used clinically for routine quality assurance of two volumetric modulated arc therapy (VMAT) systems for lung stereotactic body radiation therapy (SBRT): SmartArc(®) (Pinnacle(3), Philips Radiation Oncology Systems, Fitchburg, WI) and RapidArc(®) (Eclipse(™), Varian Medical Systems, Palo Alto, CA).

METHODS

The cylindrical detector array ArcCHECK(™) has a retractable homogeneous acrylic insert. In this work, we designed and manufactured a customized heterogeneous insert with densities that simulate soft tissue, lung, bone, and air. The insert offers several possible heterogeneity configurations and multiple locations for point dose measurements. SmartArc(®) and RapidArc(®) plans for lung SBRT were generated and copied to ArcCHECK(™) for each inhomogeneity configuration. Dose delivery was done on a Varian 2100 ix linac. The evaluation of dose distributions was based on gamma analysis of the diode measurements and point doses measurements at different positions near the inhomogeneities.

RESULTS

The insert was successfully manufactured and tested with different measurements of VMAT plans. Dose distributions measured with the homogeneous insert showed gamma passing rates similar to our clinical results (∼99%) for both treatment-planning systems. Using nonhomogeneous inserts decreased the passing rates by up to 3.6% in the examples studied. Overall, SmartArc(®) plans showed better gamma passing rates for nonhomogeneous measurements. The discrepancy between calculated and measured point doses was increased up to 6.5% for the nonhomogeneous insert depending on the inhomogeneity configuration and measurement location. SmartArc(®) and RapidArc(®) plans had similar plan quality but RapidArc(®) plans had significantly higher monitor units (up to 70%).

CONCLUSIONS

A versatile, nonhomogeneous insert was developed for ArcCHECK(™) for an easy and quick evaluation of dose calculations with nonhomogeneous media and for comparison of different treatment planning systems. The device was tested for SmartArc(®) and RapidArc(®) plans for lung SBRT, showing the uncertainties of dose calculations with inhomogeneities. The new insert combines the convenience of the ArcCHECK(™) and the possibility of assessing dose distributions in inhomogeneous media.

Implementation of HybridArc treatment technique in preoperative radiotherapy of rectal cancer: dose patterns in target lesions and organs at risk as compared to helical Tomotherapy and RapidArc

Purpose

HybridArc is a novel treatment technique blending aperture-enhanced optimized arcs with discrete IMRT-elements, allowing selection of arcs with a set of static IMRT-beams. This study compared this new technique to helical Tomotherapy, and RapidArc, in preoperative radiotherapy of rectal cancer.

Material and methods

Twelve rectal cancer patients treated consecutively with Tomotherapy Hi-Art II system were simulated with HybridArc and RapidArc. Treatment plans were designed to deliver homogeneous dose of 46.0Gy to mesorectum and draining lymph nodes, with a simultaneous-integrated-boost to the primary tumor up to a total dose of 55.2Gy. Planning objectives were 95% of prescribed dose to 95% of PTVs, while minimizing the volume of small bowel receiving more than 15Gy (V15) and the mean bladder dose. Dose gradient towards simultaneous-integrated-boost (GI), calculated by dividing the volume receiving more then 52.4Gy (95% of PTV55.2Gy)to the volume of PTV55.2Gy, was kept below 1.5. Mean beam-on time and amount of MUs were also analyzed.

Results

PTV swere adequately covered by all plans. Significant advantage was found for Tomotherapy in sparing small bowel (V15 = 112.7cm³SD73.4cm³) compared to RapidArc (133.4cm³SD75.3cm³) and HybridArc (143.7cm³SD74.4cm³) (p < 0.01). The mean bladder dose was better with RapidArc (20.6GySD2.2Gy) compared to HybridArc (24.2Gy SD4.3Gy) and Tomotherapy (23.0GySD4.7Gy) (p < 0.01). The mean beam-on time was significantly lower (p < 0.01) for HybridArc (2.7min SD0.8) and RapidArc (2.5min SD0.5) compared to Tomotherapy (11.0min SD0.7). The total amount of MUs was significantly (p < 0.01) lower for RapidArc (547SD44)compared to HybridArc (949 SD153).

Conclusions

HybridArc is a feasible solution for preoperative RT with a simultaneous-integrated-boost in rectal cancer patients. It achieved similar PTV coverage with significant lower beam-on time, but less efficient in sparing small bowel and bladder compared to Tomotherapy and RapidArc. The added value of HybridArc is that the treatment modality can be implemented on every LINAC equipped with Dynamic-Conform-Arc and IMRT treatment techniques, while maintaining the same QA-schemes.