Irina Fotina

Rotational IMRT techniques compared to fixed gantry IMRT and tomotherapy: multi-institutional planning study for head-and-neck cases.

BackgroundRecent developments enable to deliver rotational IMRT with standard C-arm gantry based linear accelerators. This upcoming treatment technique was benchmarked in a multi-center treatment planning study against static gantry IMRT and rotational IMRT based on a ring gantry for a complex parotid gland sparing head-and-neck technique.MethodsTreatment plans were created for 10 patients with head-and-neck tumours (oropharynx, hypopharynx, larynx) using the following treatment planning systems (TPS) for rotational IMRT: Monaco (ELEKTA VMAT solution), Eclipse (Varian RapidArc solution) and HiArt for the helical tomotherapy (Tomotherapy). Planning of static gantry IMRT was performed with KonRad, Pinnacle and Panther DAO based on step&shoot IMRT delivery and Eclipse for sliding window IMRT. The prescribed doses for the high dose PTVs were 65.1Gy or 60.9Gy and for the low dose PTVs 55.8Gy or 52.5Gy dependend on resection status. Plan evaluation was based on target coverage, conformity and homogeneity, DVHs of OARs and the volume of normal tissue receiving more than 5Gy (V5Gy). Additionally, the cumulative monitor units (MUs) and treatment times of the different technologies were compared. All evaluation parameters were averaged over all 10 patients for each technique and planning modality.ResultsDepending on IMRT technique and TPS, the mean CI values of all patients ranged from 1.17 to 2.82; and mean HI values varied from 0.05 to 0.10. The mean values of the median doses of the spared parotid were 26.5Gy for RapidArc and 23Gy for VMAT, 14.1Gy for Tomo. For fixed gantry techniques 21Gy was achieved for step&shoot+KonRad, 17.0Gy for step&shoot+Panther DAO, 23.3Gy for step&shoot+Pinnacle and 18.6Gy for sliding window.V5Gy values were lowest for the sliding window IMRT technique (3499 ccm) and largest for RapidArc (5480 ccm). The lowest mean MU value of 408 was achieved by Panther DAO, compared to 1140 for sliding window IMRT.ConclusionsAll IMRT delivery technologies with their associated TPS provide plans with satisfying target coverage while at the same time respecting the defined OAR criteria. Sliding window IMRT, RapidArc and Tomo techniques resulted in better target dose homogeneity compared to VMAT and step&shoot IMRT. Rotational IMRT based on C-arm linacs and Tomotherapy seem to be advantageous with respect to OAR sparing and treatment delivery efficiency, at the cost of higher dose delivered to normal tissues. The overall treatment plan quality using Tomo seems to be better than the other TPS technology combinations.

Feasibility of CBCT-based target and normal structure delineation in prostate cancer radiotherapy: Multi-observer and image multi-modality study

BACKGROUND AND PURPOSE In-room cone-beam CT (CBCT) imaging and adaptive treatment strategies are promising methods to decrease target volumes and to spare organs at risk. The aim of this work was to analyze the inter-observer contouring uncertainties of target volumes and organs at risks (oars) in localized prostate cancer radiotherapy using CBCT images. Furthermore, CBCT contouring was benchmarked against other image modalities (CT, MR) and the influence of subjective image quality perception on inter-observer variability was assessed. METHODS AND MATERIALS Eight prostate cancer patients were selected. Seven radiation oncologists contoured target volumes and oars on CT, MRI and CBCT. Volumes, coefficient of variation (COV), conformity index (cigen), and coordinates of center-of-mass (COM) were calculated for each patient and image modality. Reliability analysis was performed for the support of the reported findings. Subjective perception of image quality was assessed via a ten-scored visual analog scale (VAS). RESULTS The median volume for prostate was larger on CT compared to MRI and CBCT images. The inter-observer variation for prostate was larger on CBCT (CIgen=0.57±0.09, 0.61 reliability) compared to CT (CIgen=0.72±0.07, 0.83 reliability) and MRI (CIgen=0.66±0.12, 0.87 reliability). On all image modalities values of the intra-observer reliability coefficient (0.97 for CT, 0.99 for MR and 0.94 for CBCT) indicated high reproducibility of results. For all patients the root mean square (RMS) of the inter-observer standard deviation (σ) of the COM was largest on CBCT with σ(x)=0.4 mm, σ(y)=1.1 mm, and σ(z)=1.7 mm. The concordance in delineating OARs was much stronger than for target volumes, with average CIgen>0.70 for rectum and CIgen>0.80 for bladder. Positive correlations between CIgen and VAS score of the image quality were observed for the prostate, seminal vesicles and rectum. CONCLUSIONS Inter-observer variability for target volume delineation in prostate cancer is larger for CBCT-based contouring compared to CT and MRI. This factor of influence needs to be considered when defining safety margins for CBCT-based Adaptive Radiotherapy (ART).

Feasibility of CBCT-based dose calculation: Comparative analysis of HU adjustment techniques

BACKGROUND AND PURPOSE The aim of this work was to compare the accuracy of different HU adjustments for CBCT-based dose calculation. METHODS AND MATERIALS Dose calculation was performed on CBCT images of 30 patients. In the first two approaches phantom-based (Pha-CC) and population-based (Pop-CC) conversion curves were used. The third method (WAB) represents override of the structures with standard densities for water, air and bone. In ROI mapping approach all structures were overridden with average HUs from planning CT. All techniques were benchmarked to the Pop-CC and CT-based plans by DVH comparison and γ-index analysis. RESULTS For prostate plans, WAB and ROI mapping compared to Pop-CC showed differences in PTV D(median) below 2%. The WAB and Pha-CC methods underestimated the bladder dose in IMRT plans. In lung cases PTV coverage was underestimated by Pha-CC method by 2.3% and slightly overestimated by the WAB and ROI techniques. The use of the Pha-CC method for head-neck IMRT plans resulted in difference in PTV coverage up to 5%. Dose calculation with WAB and ROI techniques showed better agreement with pCT than conversion curve-based approaches. CONCLUSIONS Density override techniques provide an accurate alternative to the conversion curve-based methods for dose calculation on CBCT images.

Advanced kernel methods vs. Monte Carlo-based dose calculation for high energy photon beams.

PURPOSE The aim of this study was to compare the dose calculation accuracy of advanced kernel-based methods and Monte Carlo algorithms in commercially available treatment planning systems. MATERIALS AND METHODS Following dose calculation algorithms and treatment planning (TPS) systems were compared: the collapsed cone (CC) convolution algorithm available in Oncentra Masterplan, the XVMC Monte Carlo algorithm implemented in iPlan and Monaco, and the analytical anisotropic algorithm (AAA) implemented in Eclipse. Measurements were performed with a calibrated ionization chamber and radiochromic EBT type films in a homogenous polystyrene phantom and in heterogeneous lung phantoms. Single beam tests, conformal treatment plans and IMRT plans were validated. Dosimetric evaluations included absolute dose measurements, 1D gamma-evaluation of depth-dose curves and profiles using 2mm and 2% dose difference criteria for single beam tests, and gamma-evaluation of axial planes for composite treatment plans applying 3mm and 3% dose difference criteria. RESULTS Absolute dosimetry revealed no large differences between MC and advanced kernel dose calculations. 1D gamma-evaluation showed significant discrepancies between depth-dose curves in different phantom geometries. For the CC algorithm gamma(mean) values were 0.90+/-0.74 vs. 0.43+/-0.41 in heterogeneous vs. homogeneous conditions and for the AAA gamma(mean) values were 1.13+/-0.91 vs. 0.41+/-0.28, respectively. In general, 1D gamma results obtained with both MC TPS were similar in both phantoms and on average equal to 0.5 both for profiles and depth-dose curves. The results obtained with the CC algorithm in heterogeneous phantoms were slightly better in comparison to the AAA algorithm. The 2D gamma-evaluation results of IMRT plans and four-field plans showed smaller mean gamma-values for MC dose calculations compared to the advanced kernel algorithms (gamma(mean) for four-field plan and IMRT obtained with Monaco MC were 0.28 and 0.5, respectively, vs. 0.40 and 0.54 for the AAA). CONCLUSION All TPS investigated in this study demonstrated accurate dose calculation in homogenous and heterogeneous phantoms. Commercially available TPS with Monte Carlo option performed best in heterogeneous phantoms. However, the difference between the CC and the MC algorithms was found to be small.

Experimental verification of a commercial Monte Carlo-based dose calculation module for high-energy photon beams

The dosimetric performance of a Monte Carlo algorithm as implemented in a commercial treatment planning system (iPlan, BrainLAB) was investigated. After commissioning and basic beam data tests in homogenous phantoms, a variety of single regular beams and clinical field arrangements were tested in heterogeneous conditions (conformal therapy, arc therapy and intensity-modulated radiotherapy including simultaneous integrated boosts). More specifically, a cork phantom containing a concave-shaped target was designed to challenge the Monte Carlo algorithm in more complex treatment cases. All test irradiations were performed on an Elekta linac providing 6, 10 and 18 MV photon beams. Absolute and relative dose measurements were performed with ion chambers and near tissue equivalent radiochromic films which were placed within a transverse plane of the cork phantom. For simple fields, a 1D gamma (gamma) procedure with a 2% dose difference and a 2 mm distance to agreement (DTA) was applied to depth dose curves, as well as to inplane and crossplane profiles. The average gamma value was 0.21 for all energies of simple test cases. For depth dose curves in asymmetric beams similar gamma results as for symmetric beams were obtained. Simple regular fields showed excellent absolute dosimetric agreement to measurement values with a dose difference of 0.1% +/- 0.9% (1 standard deviation) at the dose prescription point. A more detailed analysis at tissue interfaces revealed dose discrepancies of 2.9% for an 18 MV energy 10 x 10 cm(2) field at the first density interface from tissue to lung equivalent material. Small fields (2 x 2 cm(2)) have their largest discrepancy in the re-build-up at the second interface (from lung to tissue equivalent material), with a local dose difference of about 9% and a DTA of 1.1 mm for 18 MV. Conformal field arrangements, arc therapy, as well as IMRT beams and simultaneous integrated boosts were in good agreement with absolute dose measurements in the heterogeneous phantom. For the clinical test cases, the average dose discrepancy was 0.5% +/- 1.1%. Relative dose investigations of the transverse plane for clinical beam arrangements were performed with a 2D gamma-evaluation procedure. For 3% dose difference and 3 mm DTA criteria, the average value for gamma(>1) was 4.7% +/- 3.7%, the average gamma(1%) value was 1.19 +/- 0.16 and the mean 2D gamma-value was 0.44 +/- 0.07 in the heterogeneous phantom. The iPlan MC algorithm leads to accurate dosimetric results under clinical test conditions.

Clinical Comparison of Dose Calculation Using the Enhanced Collapsed Cone Algorithm vs. a New Monte Carlo Algorithm

Purpose:Comparison of the dosimetric accuracy of the enhanced collapsed cone (eCC) algorithm with the commercially available Monte Carlo (MC) dose calculation for complex treatment techniques.Material and Methods:A total of 8 intensity-modulated radiotherapy (IMRT) and 2 stereotactic body radiotherapy (SBRT) lung cases were calculated with eCC and MC algorithms with the treatment planning systems (TPS) Oncentra MasterPlan 3.2 (Nucletron) and Monaco 2.01 (Elekta/CMS). Fluence optimization as well as sequencing of IMRT plans was primarily performed using Monaco. Dose prediction errors were calculated using MC as reference. The dose–volume histrogram (DVH) analysis was complemented with 2D and 3D gamma evaluation. Both algorithms were compared to measurements using the Delta4 system (Scandidos).Results:Recalculated with eCC IMRT plans resulted in lower planned target volume (PTV) coverage, as well as in lower organs-at-risk (OAR) doses up to 8%. Small deviations between MC and eCC in PTV dose (1–2%) were detected for IMRT cases, while larger deviations were observed for SBRT (up to 5%). Conformity indices of both calculations were similar; however, the homogeneity of the eCC calculated plans was slightly better. Delta4 measurements confirmed high dosimetric accuracy of both TPS.Conclusion:Mean dose prediction errors < 3% for PTV suggest that both algorithms enable highly accurate dose calculations under clinical conditions. However, users should be aware of slightly underestimated OAR doses using the eCC algorithm.Ziel:Vergleich der Dosisberechnungsgenauigkeit eines erweiterten Collapsed-Cone-(eCC-)Algorithmus mit einem kommerziell verfügbaren Monte-Carlo-(MC-)Algorithmus anhand von komplexen Bestrahlungstechniken.Material und Methodik: 8 IMRT- und 2 SBRT-Pläne wurden jeweils mit dem eCC-Algorithmus des Bestrahlungsplanungssystems Oncentra 3.2 (Nucletron) und mit dem Monte-Carlo-Algorithmus von Monaco 2.01 (Elekta/CMS) berechnet. Die Fluenz-Optimierung und Sequenzierung der IMRT-Pläne wurden mittels Monaco durchgeführt. Zusätzlich zur DVH-Analyse wurden eine 2D und 3D Gamma-Index-Evaluierung durchgeführt. Darüber hinaus erfolgte eine dosimetrische Überprüfung der Bestrahlungspläne mit dem Delta4-System (Scandidos).Ergebnisse:Im Vergleich zum MC-Algorithmus resultierte die Nachberechnung der IMRT-Pläne mit dem eCC in einer geringeren Abdeckung der Zielvolumina sowie in einer um bis zu 8% niedrigeren Dosis für Risikoorgane (OAR). Bei den IMRT-Fällen waren die Abweichungen zwischen den PTV-Dosen der beiden Algorithmen gering (1–2%), wohingegen die größten Abweichungen (bis zu 5%) für SBRT festgestellt wurden. Die ermittelten Konformitätsindizes waren für beide Algorithmen sehr ähnlich; die Homogenität war für eCC-Pläne geringfügig besser. Die Delta4-Messungen bestätigten die hohe Dosisberechnungsgenauigkeit beider Algorithmen.Schlussfolgerung:Die mittlere Unsicherheit der berechneten PTV-Dosen betrug weniger als 3%, d.h. mit beiden Algorithmen kann eine zuverlässige Dosisvorhersage unter klinischen Bedingungen erzielt werden. Bei der Beurteilung von mit eCC berechneten Plänen sollte die geringfügige Unterschätzung der Dosis in OAR berücksichtigt werden.

Value of Magnetic Resonance Imaging Without or With Applicator in Place for Target Definition in Cervix Cancer Brachytherapy

PURPOSE To define, in the setting of cervical cancer, to what extent information from additional pretreatment magnetic resonance imaging (MRI) without the brachytherapy applicator improves conformity of CT-based high-risk clinical target volume (CTVHR) contours, compared with the MRI for various tumor stages (International Federation of Gynecology and Obstetrics [FIGO] stages I-IVA). METHODS AND MATERIALS The CTVHR was contoured in 39 patients with cervical cancer (FIGO stages I-IVA) (1) on CT images based on clinical information (CTVHR-CTClinical) alone; and (2) using an additional MRI before brachytherapy, without the applicator (CTVHR-CTpre-BT MRI). The CT contours were compared with reference contours on MRI with the applicator in place (CTVHR-MRIref). Width, height, thickness, volumes, and topography were analyzed. RESULTS The CT-MRIref differences hardly varied in stage I tumors (n=8). In limited-volume stage IIB and IIIB tumors (n=19), CTVHR-CTpre-BT MRI-MRIref volume differences (2.6 cm(3) [IIB], 7.3 cm(3) [IIIB]) were superior to CTVHR-CTClinical-MRIref (11.8 cm(3) [IIB], 22.9 cm(3) [IIIB]), owing to significant improvement of height and width (P<.05). In advanced disease (n=12), improved agreement with MR volume, width, and height was achieved for CTVHR-CTpre-BT MRI. In 5 of 12 cases, MRIref contours were partly missed on CT. CONCLUSIONS Pre-BT MRI helps to define CTVHR before BT implantation appropriately, if only CT images with the applicator in place are available for BT planning. Significant improvement is achievable in limited-volume stage IIB and IIIB tumors. In more advanced disease (extensive IIB to IVA), improvement of conformity is possible but may be associated with geographic misses. Limited impact on precision of CTVHR-CT is expected in stage IB tumors.

Comparison of Monte Carlo Dose Calculation with Advanced Kernel Methods in External Photon Beam Treatment Planning

After successful implementation in commercial treatment planning systems (TPS) for high energy electron beams, Monte Carlo dose calculation algorithms are becoming also commercially available for high energy photon beams. On the other hand, advanced kernel based methods are in clinical use for many years. The aim of this study was to compare the accuracy of both types of dose calculation in phantom studies and clinical situations. The treatment planning systems investigated were Oncentra Masterplan (Nucletron, V3.0) with a collapsed cone (CC) convolution algorithm, Eclipse (Varian, V8.1), iPlan (BrainLAB, V4.0.0) and Monaco (CMS, V1.0.2). The latter two TPS offer XVMC based Monte Carlo (MC) dose calculation. Commissioning of the first three systems was performed for an ELEKTA Synergy platform providing 6, 10 and 18MV beams. The Eclipse system was commissioned for a Varian Clinac with 6 and 15 MV. Measurements were performed with a calibrated ionization chamber and radiochromic EBT type films. Single field test cases and IMRT treatments were delivered to an inhomogeneous phantom where cork was used to simulate lung. In addition, the calculation was compared in patient geometry using 7 different IMRT cases. More specifically, MC generated plans were recalculated with the CC algorithm. Dosimetric comparisons included single points, and 1D as well as 2D gamma evaluations together with the use of independent fluence recalculation software. Absolute dosimetry revealed no large differences between MC and advance kernel dose calculation. 1D γ -evaluation showed that CC and the AAA algorithms were not able to model the penumbra broadening, build up and re-build up region behind the polystyrene plate and target as correctly as the MC algorithms. The dose calculation accuracy for the IMRT cases in patient geometry was found to be similar for CC and MC. Dose calculation speed was also assessed in the light of future demands for on-line treatment planning.