Individual Intensity Modulated Radiation Therapy Research Articles

Novel, full 3D scintillation dosimetry using a static plenoptic camera.

Patient-specific quality assurance (QA) of dynamic radiotherapy delivery would gain from being performed using a 3D dosimeter. However, 3D dosimeters, such as gels, have many disadvantages limiting to quality assurance, such as tedious read-out procedures and poor reproducibility. The purpose of this work is to develop and validate a novel type of high resolution 3D dosimeter based on the real-time light acquisition of a plastic scintillator volume using a plenoptic camera. This dosimeter would allow for the QA of dynamic radiation therapy techniques such as intensity-modulated radiation therapy (IMRT) or volumetric-modulated arc therapy (VMAT). A Raytrix R5 plenoptic camera was used to image a 10 × 10 × 10 cm(3) EJ-260 plastic scintillator embedded inside an acrylic phantom at a rate of one acquisition per second. The scintillator volume was irradiated with both an IMRT and VMAT treatment plan on a Clinac iX linear accelerator. The 3D light distribution emitted by the scintillator volume was reconstructed at a 2 mm resolution in all dimensions by back-projecting the light collected by each pixel of the light-field camera using an iterative reconstruction algorithm. The latter was constrained by a beam's eye view projection of the incident dose acquired using the portal imager integrated with the linac and by physical consideration of the dose behavior as a function of depth in the phantom. The absolute dose difference between the reconstructed 3D dose and the expected dose calculated using the treatment planning software Pinnacle(3) was on average below 1.5% of the maximum dose for both integrated IMRT and VMAT deliveries, and below 3% for each individual IMRT incidences. Dose agreement between the reconstructed 3D dose and a radiochromic film acquisition in the same experimental phantom was on average within 2.1% and 1.2% of the maximum recorded dose for the IMRT and VMAT delivery, respectively. Using plenoptic camera technology, the authors were able to perform millimeter resolution, water-equivalent dosimetry of an IMRT and VMAT plan over a whole 3D volume. Since no moving parts are required in the dosimeter, the incident dose distribution can be acquired as a function of time, thus enabling the validation of static and dynamic radiation delivery with photons, electrons, and heavier ions.

Developing a novel method to analyse Gafchromic EBT2 films in intensity modulated radiation therapy quality assurance

Recently individual intensity modulated radiation therapy quality assurances (IMRT QA) have been more and more performed with Gafchromic™ EBT series films processed in red-green-blue (R-G-B) channel due to their extremely high spatial resolution. However, the efficiency of this method is relatively low, as for each box of film, a calibration curve must be established prior to the film being used for measurement. In this study, the authors find a novel method to process the Gafchromic™ EBT series, that is, to use the 16-bit greyscale channel to process the exposed film rather than the conventional 48-bit R-G-B channel, which greatly increases the efficiency and even accuracy of the whole IMRT procedure. The main advantage is that when processed in greyscale channel, the Gafchromic™ EBT2 films exhibits a linear relationship between the net pixel value and the dose delivered. This linear relationship firstly reduces the error in calibration-curve fitting, and secondly saves the need of establishing a calibration curve for each box of films if it is only to be used for relative measurements. Clinical testing for this novel method was carried out in two radiation therapy centres that involved a total of 743 IMRT cases, and 740 cases passed the 3 mm 3 % gamma analysis criteria. The cases were also tested with small ionization chambers (cc-13) and the results were convincing. Consequently the authors recommend the use of this novel method to improve the accuracy and efficiency of individual IMRT QA procedure using Gafchromic EBT2 films.

Dosimetric impact of Acuros XB deterministic radiation transport algorithm for heterogeneous dose calculation in lung cancer.

The novel deterministic radiation transport algorithm, Acuros XB (AXB), has shown great potential for accurate heterogeneous dose calculation. However, the clinical impact between AXB and other currently used algorithms still needs to be elucidated for translation between these algorithms. The purpose of this study was to investigate the impact of AXB for heterogeneous dose calculation in lung cancer for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). The thorax phantom from the Radiological Physics Center (RPC) was used for this study. IMRT and VMAT plans were created for the phantom in the Eclipse 11.0 treatment planning system. Each plan was delivered to the phantom three times using a Varian Clinac iX linear accelerator to ensure reproducibility. Thermoluminescent dosimeters (TLDs) and Gafchromic EBT2 film were placed inside the phantom to measure delivered doses. The measurements were compared with dose calculations from AXB 11.0.21 and the anisotropic analytical algorithm (AAA) 11.0.21. Two dose reporting modes of AXB, dose-to-medium in medium (Dm,m) and dose-to-water in medium (Dw,m), were studied. Point doses, dose profiles, and gamma analysis were used to quantify the agreement between measurements and calculations from both AXB and AAA. The computation times for AAA and AXB were also evaluated. For the RPC lung phantom, AAA and AXB dose predictions were found in good agreement to TLD and film measurements for both IMRT and VMAT plans. TLD dose predictions were within 0.4%-4.4% to AXB doses (both Dm,m and Dw,m); and within 2.5%-6.4% to AAA doses, respectively. For the film comparisons, the gamma indexes (± 3%∕3 mm criteria) were 94%, 97%, and 98% for AAA, AXB_Dm,m, and AXB_Dw,m, respectively. The differences between AXB and AAA in dose-volume histogram mean doses were within 2% in the planning target volume, lung, heart, and within 5% in the spinal cord. However, differences up to 8% between AXB and AAA were found at lung∕soft tissue interface regions for individual IMRT fields. AAA was found to be 5-6 times faster than AXB for IMRT, while AXB was 4-5 times faster than AAA for VMAT plan. AXB is satisfactorily accurate for the dose calculation in lung cancer for both IMRT and VMAT plans. The differences between AXB and AAA are generally small except in heterogeneous interface regions. AXB Dw,m and Dm,m calculations are similar inside the soft tissue and lung regions. AXB can benefit lung VMAT plans by both improving accuracy and reducing computation time.

Intensity modulated dose calculation with an improved experimental pencil‐beam kernel

This article presents an improved pencil-beam dose calculation formalism based on an experimental kernel obtained by deconvolution. The new algorithm makes it possible to calculate the absorbed dose for all field sizes. The authors have enhanced their previous work [J. D. Azcona and J. Burguete, Med. Phys. 35, 248-259 (2008)] by correcting the kernel tail representing the contribution to the absorbed dose far from the photon interaction point. The correction was performed by comparing the calculated and measured output factors. Dose distributions and absolute dose values calculated using the new formalism have been compared to measurements. The agreement between calculated and measured dose distributions was evaluated according to the gamma-index criteria. In addition, 35 individual intensity-modulated radiation therapy (IMRT) fields were calculated and measured in polystyrene using an ionization chamber. Furthermore, a series of 541 IMRT fields was calculated using the algorithm proposed here and using a commercial IMRT optimization and calculation software package. Comparisons were made between the calculations at single points located at the isocenter for all the beams, as well as between beams grouped by anatomic location. The percentage of points passing the gamma-index criteria (3%, 3 mm) when comparing calculated and measured dose distributions is generally greater than 99% for the cases studied. The agreement between the calculations and the experimental measurements generally lies in the +/- 2% interval for single points, with a mean value of 0.2%. The agreement between calculations using the proposed algorithm and using a commercial treatment planning system is also between +/- 5%. An improved algorithm based on an experimental pencil-beam kernel obtained by deconvolution has been developed. It has been validated clinically and promises to be a valuable tool for IMRT quality assurance as an independent calculation system for monitor units and dose distributions. An important point is that the algorithm presented here uses an experimental kernel, which is therefore independent of Monte-Carlo-calculated kernels.

SU‐GG‐T‐181: Effects of the MLC Characteristics on IMRT Dose Distribution

Purpose: The purpose of this study is to investigate the characteristics of the Multi‐leaf collimator (MLC) including the leaf thickness, leakage and the tongue‐and‐groove structure and their effects on the dose distribution of intensity‐modulated radiation therapy (IMRT) and patient‐specific IMRT quality assurance (QA). Methods: Monte Carlo dose calculations were performed based on intensity maps that were built from the actual deliverable IMRT leaf sequences with and without considering the characteristics of MLC using our in‐house Monte Carlo software. Intensity maps with a fine resolution (0.1×0.1mm2 pixel size) were used for IMRT dose calculation. The mean dose of the treatment target and the voxel doses in high dose regions of 7 IMRT plans generated by different treatment planning systems for Varian 21EX and Siemens Primus linear accelerators were used for comparison and evaluation. Results: The MLC leaf thickness, leakage and the tongue‐and‐groove structure can change the mean dose by up to 4.5%, 5.6% and 5.3%, respectively, when these factors are considered separately. They also cause significant uncertainties in the voxel doses with standard deviations of up to 0.7%, 2.1% and 1.3%, respectively. The fudge factors applied in treatment planning systems (TPS) can only compensate the mean dose change partially because the effects are different for individual IMRT plans. The uncorrected mean dose change and the voxel dose uncertainties are shown as the difference between the TPS calculation and measurements in measurement‐based IMRT QA. Conclusions: It is concluded that the MLC characteristics can affect the mean target dose and cause significant uncertainties in individual voxel doses. The dose uncertainties caused by MLC characteristics are a major source of dose discrepancies for measurement‐based IMRT QA because they are not fully considered in the TPS.

Pre-treatment verification of intensity modulated radiation therapy plans using a commercial electronic portal dosimetry system

We commissioned a commercially available portal dosimetry system for quality assurance of intensity modulated radiation therapy (IMRT) treatment plans. The system included gamma analysis software to compare the measured and predicted fluence maps from individual IMRT fields. The portal dosimetry system was tested using six head and neck IMRT patient plans, and we demonstrated that the accuracy of the alignment of measured and predicted images improved by retracting and repositioning the electronic portal imaging device (EPID) at each new gantry angle. The mean gamma score (fraction of pixels passing the gamma criteria) for the six test plans (after initial testing and using the EPID retracting and repositioning method) was 0.987 (2SD = 0.018), using gamma criteria of a dose difference of 2% of the maximum field dose and 2 mm distance to agreement. The mean gamma score was 0.989 (2SD = 0.017) for 24 head and neck IMRT patient plans carried out with portal dosimetry. Using gamma criteria of 2% maximum field dose and 2 mm distance to agreement, a gamma score tolerance of 0.980 is a useful way of highlighting only those fields requiring further analysis. Portal dosimetry is a quick way of assessing individual field fluence distributions and can be integrated into an IMRT quality assurance programme.

Discrepancy of IMRT Dose Delivery due to the Dose-Dynamic MLC Gravity Effect

The aim of the present study was to investigate the multi-leaf collimator (MLC) gravity e ect on intensity modulated radiation therapy (IMRT) dose-dynamic delivery at di erent gantry angles. The non-uniform uence for IMRT was generated by means of dose-dynamic MLC (DMLC) performances, which were multiple static segments (MSS) or fully dynamic sliding window (SW) delivery methods at di erent gantry angles. SW delivery is quite similar to MSS with regard to MLC sliding pattern through the X1 to X2 jaw direction, except that it is irradiated during DMLC movement. To determine how the DMLC gravity a ects the IMRT dose uence, we mounted a two-dimensional ion chamber matrix (MatriXX, Scanditronix-Wellhofer, Schwarzenbruck, Germany) to a linear accelerator (CL 21EX, Varian, Palo Alto, CA, USA). Individual IMRT dose-dynamic segments were applied by using MSS and SW for 6-MV photon beams at ve di erent gantry angles: 0 (neutral gravity), 45 (semi-along gravity), 90 (vertical-along gravity), 315 (semi-against gravity) and 270 (against gravity). To test the correlation of leaf speed and gravity, we delivered half monitor units (MUs) for planned doses for absolute dose comparison. Strict -index (dose di erence: 2 %; distance to agreement: 2 mm) histograms were used for quantitative analyses of the discrepancy. The dose distribution by MSS under neutral gravity (gantry 0 ) with 137 MUs at a prescribed dose (45 cGy) point, which was in the low gradient area, was used as a reference for the other results. The -index histograms showed an increased tendency of the dose discrepancy toward the gravity-along direction rather than toward the gravity-against direction. The acceptable proportional ranges below 1 of the -index were 96.2 { 99.6 % (mean: 97.6 %), 92.6 { 93.5 % (mean: 93.0 %), 90.4 { 92.6 % (mean: 91.3 %), 94.6 { 98.0 % (mean: 96.4 %) and 92.9 { 96.8 % (mean: 95.0 %) for neutral, semi-along, vertical-along, semi-against and vertical-against gravity of the positions of MLC segments, respectively. we noted that MSS deliveries were more stable than were the SW deliveries (unacceptable -index range 1.0 { 2.0: mean 3.6 % for MSS and 5.4 % for SW). For the dose measurement on the prescribed dose point, all measurements showed a good agreement within an average of 2 %. Our experiment conclusively demonstrated that the DMLC gravity a ects the

Feasibility study for linac‐based intensity modulated total marrow irradiation

Total body irradiation (TBI) is used as a preconditioning regimen prior to bone marrow transplant for treatment of hematologic malignancies. During TBI, large volumes of normal tissue are irradiated, and this can lead to toxicities, most significantly in the lungs. Intensity modulated total marrow irradiation (IMTMI) may be able to reduce these toxicities by directly targeting the bone marrow while minimizing the dose to critical structures. The goal of this study was to assess the feasibility of IMTMI by following the planning and delivery process for a Rando phantom. A three isocenter technique was used to provide a full body plan for treatment on a linear accelerator. Thermoluminescent detectors (TLDs) were placed at 22 positions throughout the phantom to compare the delivered doses to the planned doses. Individual intensity modulated radiation therapy verification plans were delivered to a solid water phantom for the three isocenters, and doses measured from an ion chamber and film were compared to the planned doses. The treatment plan indicated that target coverage was achieved with this IMTMI technique, and that the doses to critical structures were reduced by 29%-65% compared to conventional TBI. TLD readings demonstrated accurate dose delivery, with an average difference of 3.5% from the calculated dose. Ion chamber readings for the verification plans were all within 3% of the expected dose, and film measurements showed accurate dose distributions. Results from this study suggest that IMTMI using the three isocenter technique can be accurately delivered and may result in substantial dose reductions to critical structures.

A Bridge Between Academic and Community Radiation Oncology Treatment Planning

To evaluate Telesynergy (TS) as a method of interactive treatment planning between academic and community radiation oncology departments. Through a grant from the National Cancer Institute to improve cancer outcomes for underserved populations, community radiation oncologists at New Hanover Regional Medical Center (NHRMC) in Wilmington, North Carolina, partnered with those at the University of North Carolina (UNC) in Chapel Hill, North Carolina. TS suites were installed at both sites to facilitate teleconferencing and review of treatment planning for intensity-modulated radiation therapy (IMRT). Patients with locally advanced head and neck cancer at NHRMC who were enrolled on a clinical trial of chemoirradiation underwent IMRT planning utilizing commercial software. NHRMC physicians contoured tumor targets and adjacent healthy organs. Physics staff at NHRMC generated an initial IMRT plan for each patient. Radiation oncologists at UNC then reviewed individual IMRT plans via TS conferences. Between August 2004 and August 2005, seven IMRT plans were reviewed in eight TS conferences. Physician contours of tumor targets and healthy organs, dose volume histograms, IMRT beams, and isodose distributions were shared during each TS conference successfully. Median time for each session was 35 minutes (range, 30 to 75). Physician satisfaction with the interactive planning process was high at both NHRMC and UNC. A cycle would likely evolve of initial intensive use of TS conferences, to gradual use for ongoing quality control, then greater use as the treatment planning technology undergoes its next change. Complex IMRT treatment planning review was feasible between an academic and community hospital via TS with a high level of physician participant satisfaction.

Effectiveness of noncoplanar IMRT planning using a parallelized multiresolution beam angle optimization method for paranasal sinus carcinoma

To determine the effectiveness of noncoplanar beam configurations and the benefit of plans using fewer but optimally placed beams designed by a parallelized multiple-resolution beam angle optimization (PMBAO) approach. The PMBAO approach uses a combination of coplanar and noncoplanar beam configurations for intensity-modulated radiation therapy (IMRT) treatment planning of paranasal sinus cancers. A smaller number of beams (e.g. 3) are first used to explore the solution space to determine the best and worst beam directions. The results of this exploration are then used as a starting point for determining an optimum beam orientation configuration with more beams (e.g. 5). This process is parallelized using a message passing interface, which greatly reduces the overall computation time for routine clinical practice. To test this approach, treatment for 10 patients with paranasal sinus cancer was planned using a total of 5 beams from a pool of 46 possible beam angles. The PMBAO treatment plans were also compared with IMRT plans designed using 9 equally spaced coplanar beams, which is the standard approach in our clinic. Plans with these two different beam configurations were compared with respect to dose conformity, dose heterogeneity, dose-volume histograms, and doses to organs at risk (i.e., eyes, optic nerve, optic chiasm, and brain). The noncoplanar beam configuration was superior in most paranasal sinus carcinoma cases. The target dose homogeneity was better using a PMBAO 5-beam configuration. However, the dose conformity using PMBAO was not improved and was case dependent. Compared with the 9-beam configuration, the PMBAO configuration significantly reduced the mean dose to the eyes and optic nerves and the maximum dose to the contralateral optical path (e.g. the contralateral eye and optic nerve). The maximum dose to the ipsilateral eye and optic nerve was also lower using the PMBAO configuration than using the 9-beam configuration, although this difference was not significant. The mean doses to the optic chiasm and brain are marginally lower using the PMBAO configuration than using 9-beam configuration. The maximum doses to the optic chiasm and brain are the same with the PMBAO configuration and the 9-beam configuration. Parallelized multiple-resolution beam angle optimization with an optimized noncoplanar beam configuration is an effective and practical approach for IMRT treatment planning. Five-beam treatment plans optimized using the PMBAO are at least equivalent to, and overall better than, the plans using 9 equally spaced coplanar beams.

Dose verification in clinical imrt prostate incidents

Purpose In view of the need for dose-validation procedures on each individual intensity-modulated radiation therapy (IMRT) plan, dose-verification measurements by film, by ionization chamber, and by polymer gel–MRI dosimetry were performed for a prostate-treatment plan configuration. Treatment planning system (TPS) calculations were evaluated against dose measurements. Methods and materials Intensity-modulated radiation therapy (IMRT) treatments were planned on a commercial TPS. Kodak EDR-2 films were used for the verification of two-dimensional (2D) dose distributions at 1 coronal and 5 axial planes in a water-equivalent phantom. Full three-dimensional (3D) dose distributions were measured by use of a novel polymer gel formulation and a 3D magnetic resonance imaging (MRI) readout technique. Calculations were compared against measurements by means of isocontour maps, γ-index maps (3% dose difference, 3-mm distance to agreement) and dose–volume histograms. Results A good agreement was found between film measurements and TPS predictions for points within the 60% isocontour, for all the examined plans (γ-index <1 for 96% of pixels). Three-dimensional dose distributions obtained with the polymer gel–MRI method were adequately matched with corresponding TPS calculations, for measurements in a gel phantom covering the planning-target volume (PTV). Conclusions Measured 2D and 3D dose distributions suggest that, for the investigated prostate IMRT plan configuration, TPS calculations provide clinically acceptable accuracy.