J Craig

Intensity modulated radiotherapy of non-small-cell lung cancer incorporating SPECT ventilation imaging.

PURPOSE The authors performed this retrospective study to investigate the impact of using ventilation scans obtained from single photon emission computed tomography (SPECT) in selecting beam directions in intensity modulated radiation therapy (IMRT) planning in lung cancer radiotherapy to spare dosimetrically well ventilated lung. METHODS For ten consecutive stage III non-small-cell lung cancer patients, the authors obtained both ventilation/perfusion SPECT scans and four-dimensional CT scans for treatment planning purposes. Each ventilation scan was registered with the corresponding planning CT and ventilation volumes corresponding to either > or = 50% (vv50) or > or = 70% (vv70) of the maximum SPECT count were automatically segmented. For each patient, three IMRT plans were generated: One using nine equally spaced beams optimized according to nonfunctional lung based mean lung dose and lung v20; a second using nine equally spaced beams optimized to avoid vv50 and vv70; and a third plan using only three beams with gantry angles chosen based on minimum mean ventilated lung dose calculated for each conformal beam at every 10 degrees gantry angle avoiding vv50 and vv70. Resultant dose volume histogram indices were calculated for each plan and were compared with respect to calculated SPECT-based ventilation parameters in order to quantify the potential utility of ventilation SPECT in this setting. RESULTS Two patient groups were identified based on (i) the overlap volume between PTV and vv50 and (ii) the average angular mean ventilated lung dose (AAMvLD). The first parameter quantifies the proximity of the PTV to well ventilated lung and the second parameter quantifies the degree of ventilation that surrounds the PTV. For group 1 patients, < or = 5% of the vv50 overlapped with the PTV. For group 2 patients, > 5% of the vv50 overlapped the PTV. Group 1 was further classified into subgroups 1A and 1B: For subgroup 1A, AAMvLD is >18 Gy, implying that the functional lung surrounds the PTV; for subgroup 1B, AAMvLD is <18 Gy, implying that the well ventilated lung does not completely surround PTV. For subgroup 1A, the plans generated using ventilated lung avoidance reduced dose to vv50 and vv70, with below tolerance dose to normal lung and acceptable coverage of the PTV. For subgroup 1B, the dose to the total lung and well ventilated lung are reduced with the beam direction optimization for the three-beam plan. For group 2, there was no significant dosimetric advantage of using SPECT-based ventilation information in IMRT plan optimization. CONCLUSIONS In conclusion, it is feasible to use SPECT ventilation scans to optimize IMRT beam direction and, subsequently, to reduce dose to ventilated lung when overlap of the PTV and the ventilated lung is minimal and that the PTV is not surrounded by the ventilated lung. The potential benefit of ventilation SPECT scanning can be determined by preplanning assessment of overlap volumes and the AAMvLD.

Segmentation and leaf sequencing for intensity modulated arc therapy

A common method in generating intensity modulated radiation therapy (IMRT) plans consists of a three step process: an optimized fluence intensity map (IM) for each beam is generated via inverse planning, this IM is then segmented into discrete levels, and finally, the segmented map is translated into a set of MLC apertures via a leaf sequencing algorithm. To date, limited work has been done on this approach as it pertains to intensity modulated arc therapy (IMAT), specifically in regards to the latter two steps. There are two determining factors that separate IMAT segmentation and leaf sequencing from their IMRT equivalents: (1) the intrinsic 3D nature of the intensity maps (standard 2D maps plus the angular component), and (2) that the dynamic multileaf collimator (MLC) constraints be met using a minimum number of arcs. In this work, we illustrate a technique to create an IMAT plan that replicates Tomotherapy deliveries by applying IMAT specific segmentation and leaf-sequencing algorithms to Tomotherapy output sinograms. We propose and compare two alternative segmentation techniques, a clustering method, and a bottom-up segmentation method (BUS). We also introduce a novel IMAT leaf-sequencing algorithm that explicitly takes leaf movement constraints into consideration. These algorithms were tested with 51 angular projections of the output leaf-open sinograms generated on the Hi-ART II treatment planning system (Tomotherapy Inc.). We present two geometric phantoms and 2 clinical scenarios as sample test cases. In each case 12 IMAT plans were created, ranging from 2 to 7 intensity levels. Half were generated using the BUS segmentation and half with the clustering method. We report on the number of arcs produced as well as differences between Tomotherapy output sinograms and segmented IMAT intensity maps. For each case one plan for each segmentation method is chosen for full Monte Carlo dose calculation (NumeriX LLC) and dose volume histograms (DVH) are calculated. In all cases, the BUS method outperformed the clustering, method. We recommend using the BUS algorithm and discuss potential improvements to the clustering algorithms.

Monte Carlo dose calculation of segmental IMRT delivery to a moving phantom using dynamic MLC and gating log files

Respiratory gating is emerging as a tool to limit the effect of motion for liver and lung tumors. In order to study the impact of target motion and gated intensity modulated radiation therapy (IMRT) delivery, a computer program was developed to simulate segmental IMRT delivery to a moving phantom. Two distinct plans were delivered to a rigid-motion phantom with a film insert in place under four conditions: static, sinusoidal motion, gated sinusoidal motion with a duty cycle of 25% and gated sinusoidal motion with duty cycle of 50% under motion conditions of a typical patient (A = 1 cm, T = 4 s). The MLC controller log files and gating log files were retained to perform a retrospective Monte Carlo dose calculation of the plans. Comparison of the 2D planar dose distributions between simulation and measurement demonstrated that our technique had at least 94% of the points passing gamma criteria of 3% for dose difference and 3 mm as the distance to agreement. This note demonstrates that the use of dynamic multi-leaf collimator and respiratory monitoring system log files together with a fast Monte Carlo dose calculation algorithm is an accurate and efficient way to study the dosimetric effect of motion for gated or non-gated IMRT delivery on a rigidly-moving body.

Experimental measurements and Monte Carlo simulations for dosimetric evaluations of intrafraction motion for gated and ungated intensity modulated arc therapy deliveries.

Respiratory gated radiation therapy allows for a smaller margin expansion for the planning target volume (PTV) to account for respiratory induced motion and is emerging as a common method to treat lung and liver tumors. We investigated the dosimetric effect of free motion and gated delivery for intensity modulated arc therapy (IMAT) with experimental measurements and Monte Carlo simulations. The impact of PTV margin and duty cycle for gated delivery is studied with Monte Carlo simulations. A motion phantom is used for this study. Two sets of contours were drawn on the mid-inspiration CT scan of this motion phantom. For each set of contours, an IMAT plan to be delivered with constant dose rate was created. The plans were generated on a CT scan of the phantom in the static condition with 3 mm PTV margin and applied to the motion phantom under four conditions: static, full superior-inferior (SI) motion (A = 1 cm, T = 4 s) and gating conditions (25% and 50% duty cycles) with full SI motion. A 6 by 15 cm piece of radiographic film was placed in the sagittal plane of the phantom and then irradiated under all measurement conditions. Film calibration was performed with a step-wedge method to convert optical density to dose. Gated IMAT delivery was first validated in 2D by comparing static film with that from gating and full motion. A previously verified simulation tool for IMRT that takes the log files from the multileaf collimator (MLC) controller and the gating system were adapted to simulate the delivered IMAT treatment for full 3D dosimetric analysis. The IMAT simulations were validated against the 2D film measurements. The resultant IMAT simulations were evaluated with dose criteria, dose-volume histograms and 3D gamma analysis. We validated gated IMAT deliveries when we compared the static film with the one from gating using 25% duty cycle using 2D gamma analysis. Within experimental and setup uncertainties, film measurements agreed with their corresponding simulated plans using 2D gamma analysis. Finally, when planning with margins designed for gating with 25% duty cycle and applying 50% or no gating during treatment, the dose differences in D(min,) D(99%) and D(95%) of the clinical target volume can be up to 27 cGy, 20 cGy and 18 cGy, respectively, for a plan with 200 cGy prescription dose. We have experimentally delivered gated IMAT with constant dose rate to a motion phantom and assessed their accuracies with film dosimetry and Monte Carlo simulations. Film dosimetry demonstrated that 25% gating and static plans are within 2%, 2 mm. The Monte Carlo simulation method was employed to generate dose delivered in 3D to a motion phantom, and the dosimetric results were reported. Since our film measurements agreed well with Monte Carlo simulations, we can reliably use this simulation tool to further study the dosimetric effects of target motion and effectiveness of gating for IMAT deliveries.

Experimental assessments of intrafractional prostate motion on sequential and simultaneous boost to a dominant intraprostatic lesion

PURPOSE To investigate experimentally the impact of intrafractional prostate motion on the delivered dose to a dominant intraprostatic lesion (DIL) using volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) with sequential and simultaneous boost. METHODS A series of six IMRT and VMAT treatment plans were generated, evaluated, and compared for two patient CT scans with dissimilar anatomies. Plans were generated for the prostate with and without the DIL. Plans were delivered using a Varian CLINAC and 2D dose distributions were measured using mapcheck(TM)-mapphan(TM) system. The effect of the prostate intrafractional motion on the delivery of the plans was studied by delivering the plans to the mapcheck(TM)-mapphan(TM) system on a programmable motion platform. Prostate intrafractional motion was simulated based on six different motion patterns from the literature obtained on Calypso system (Calypso System, Calypso Medical, Seattle, WA, USA) in a clinical study that provided continuous, real-time localization, and monitoring of the prostate. Absolute dose differences and Gamma analysis were used to assess the quality of a total of 42 plans with motion and without motion. RESULTS Dose escalation to the whole prostate from 76 to 86 Gy caused the rectum and bladder to exceed normal tissue tolerances in both patients. All the DIL boost plans satisfied the planning criteria and delivery quality assurance when motion was not present. For a single fraction, the motion pattern with large constant shift caused the largest dose delivery discrepancy with mean Gamma value (1.14-1.44) and the lowest plan passing percentage (18.9%-35.7%), while the motion pattern with continuous random changes during treatment had the least impact on dose delivery with mean Gamma value (0.33-0.55) and the highest passing percentage (81.9%-100%) for all the investigated plans. For dose escalation to DIL in the presence of intrafractional prostate motion, a significant difference was observed between the different motion patterns (p < 0.05), but no significant difference in the sensitivity to motion between the various plans was observed (p = 0.30). Based on Gamma analysis, treatment courses in which 15% of the fractions are dominated by severe motion proved to be significantly different from those dominated by random motion (p < 0.05). CONCLUSIONS The impact of intrafractional prostate motion on dose delivery is sensitive to different motion patterns but not to different delivery techniques. Dose escalation to DIL using either sequential or simultaneous boost plans with 7 mm PTV margin is achievable in the presence of intrafractional prostate motion, even if the severe motion comprised 8.6% (3 out of the 35) treatment fractions.

Incorporating geometric ray tracing to generate initial conditions for intensity modulated arc therapy optimization.

PURPOSE AND BACKGROUND Intensity modulated arc therapy (IMAT) is a rotational variant of Intensity modulated radiation therapy (IMRT) that is achieved by allowing the multileaf collimator (MLC) positions to vary as the gantry rotates around the patient. This work describes a method to generate an IMAT plan through the use of a fast ray tracing technique based on dosimetric and geometric information for setting initial MLC leaf positions prior to final IMAT optimization. METHODS AND MATERIALS Three steps were used to generate an IMAT plan. The first step was to generate arcs based on anatomical contours. The second step was to generate ray importance factor (RIF) maps by ray tracing the dose distribution inside the planning target volume (PTV) to modify the MLC leaf positions of the anatomical arcs to reduce the maximum dose inside the PTV. The RIF maps were also segmented to create a new set of arcs to improve the dose to low dose voxels within the PTV. In the third step, the MLC leaf positions from all arcs were put through a leaf position optimization (LPO) algorithm and brought into a fast Monte Carlo dose calculation engine for a final dose calculation. The method was applied to two phantom cases, a clinical prostate case and the Radiological Physics Center (RPC)s head and neck phantom. The authors assessed the plan improvements achieved by each step and compared plans with and without using RIF. They also compared the IMAT plan with an IMRT plan for the RPC phantom. RESULTS All plans that incorporated RIF and LPO had lower objective function values than those that incorporated LPO only. The objective function value was reduced by about 15% after the generation of RIF arcs and 52% after generation of RIF arcs and leaf position optimization. The IMAT plan for the RPC phantom had similar dose coverage for PTV1 and PTV2 (the same dose volume histogram curves), however, slightly lower dose to the normal tissues compared to a six-field IMRT plan. CONCLUSION The use of a ray importance factor can generate initial IMAT arcs efficiently for further MLC leaf position optimization to obtain more favorable IMAT plan.

SU‐GG‐T‐425: Modeling a Multi Leaf Collimator for IMRT Monte Carlo Dose Calculations

Purpose: To investigate the effects of modeling a multi leaf collimator(MLC) with a simplified discrete block geometry for Monte Carlo(MC) simulations. Method and Materials: In our MC simulations (NxEGS NumeriX, LLC) we modeled each leaf in the MLC using 3 layers of staggered blocks to simulate the rounded tip, tongue and groove, and leaf divergence. Dose calculations were compared against measurements and the collapsed cone convolution algorithm in Pinnacle. Profiles from a water phantom were measured with ion chamber or diode both orthogonally and parallel to leaf travel directions for field sizes of 15×15cm, 10×10cm, 5×5cm, 3×3cm, and 2×2cm (with jaws always set to 20×20cm). Two abutting 10×5cm fields were measured with film on a solid water phantom, to determine how well the simulations handle the tongue and groove effect. A 3‐field IMRTlung patient was also planned and simulated on both the patient CT and a solid water phantom. Results: Gamma analysis (3mm/3%) comparing MC dose calculations and measurements for the range of the 2×2cm to 15×15cm fields collimated by the MLC showed good agreement. Results for the two abutting 10×5cm fields indicates NxEGS is modeling the tongue and groove effect, where as Pinnacle has under estimated it. When the IMRTlung patient plan was evaluated by MC calculations and Pinnacle on the solid water phantom, < 2% of points failed the gamma analysis (3%/3mm) whereas ∼5% increase in dose in the lung was observed in the MC dose calculation when compared to Pinnacle. Conclusion: A 3‐layer block geometry for modeling the MLC rounded leaf, divergence and tongue and groove is easily implemented, forgoing the minute details of a MLC, while still giving an accurate distribution as compared to both measurements. This can be used to verify dose distributions in 3D for IMRT treatment plans.

TU‐E‐ValB‐07: A Segmentation and Leaf Sequencing Algorithm for IMAT

Purpose: To develop an intensity segmentation and leaf sequencing algorithm specifically for intensity‐modulated arc therapy (IMAT), which can be applied to optimized intensity patterns derived from existing commercial IMRT inverse planning software. Methods and Materials: Three phantom cases, as well as a clinical case were planned using a Hi‐Art II (Tomotherapy Inc, WI.) planning station. The end of planning sinograms were then extracted and inputted into our IMAT conversion algorithm. The number of required arcs, deliverable MLC segments for each arc and the relative intensity weighting of each arc were outputted. The number of arcs (modulation) could be controlled by a user parameter, α. The resulting MLC segments were then fed into a fast monte‐carlo dose calculation algorithm, NXEGS (NumeriX, LLC) to obtain 3D dose distributions. Dose statistics (max, min, mean) and dose volume histograms of relevant structures were calculated and compared against the results generated by the Hi‐Art II system. Results: Each plan was converted in under three minutes on a typical desktop PC, with the arc numbers varying between 4 and 15 360° arcs. Qualitatively, the dose distributions obtained from the IMAT plans were similar to the tomotherapy results, as well as planned doses. Quantitatively, the IMAT plans were slightly degraded, with the average dose to normal structures being 7.5% higher for IMAT vs. tomotherapy. However, the IMAT plans generally met planned values, being 9.1% below for maximum doses to normal structures. The number of arcs and therefore the resulting dose distribution could be varied according to α. Conclusions: IMAT segmentation and leaf sequencing produced deliverable IMAT MLC segments and relative arc weights directly from Hi‐Art II optimized plans. The algorithm was computationally efficient, and produced similar dose distributions. Additional optimization could improve resulting dose distributions further. IMAT back‐up for tomotherapy is another potential application.

TU‐FF‐A1‐02: Commissioning Fast Monte Carlo Dose Calculation for Lung Treatment Planning

Purpose: To commission a commercial Monte Carlo (MC) simulation package, NXEGS (Numerix LLC), for photon beam dose calculations. We investigated within NXEGS the EGS4 compatibility mode, fast MC, and post processing (PostP). Method and Materials: We commissioned NXEGS and Pinnacle 6.2b with the same set of measured data. We compared its dose calculation accuracy and efficiency with the collapsed cone convolution algorithm in Pinnacle and the National Research Council EGS4. Dose distributions were compared in three phantoms: a water phantom to check the output and beam profiles; a water phantom with a lung slab to test the inhomogeneity correction; and a water phantom with 1- 3 cm diameter cylindrical air pockets to test the PostP algorithm. We also compared fast MC using PostP with Pinnacle for a three-field lung treatment plan. Number of histories is chosen to give +/− 2% dose accuracy at the isocenter. All doses were converted to cGy per MU. Results: Fast MC improves computational speed by a factor of ∼10 from the EGS4 compatibility mode. PostP decreases number of histories required and hence the computation time by another factor of ∼10. PostP adds ∼1 minute per 106 dose voxels. Inside the lung slab, fast MC with PostP differed from Pinnacle by ∼0.03cGy/MU with a misalignment of ∼2mm whereas fast MC with PostP agreed within 0.03cGy/MU of EGS4. PostP did not preserve the dose perturbation from ⩾1cm air inhomogeneities. Conclusion: Without PostP, the accuracy and computational time scaled with number of histories. When we specify ±2% accuracy in the target volume, the dose calculation time using fast MC with PostP is comparable to Pinnacle for a three-field lung plan. NXEGS fast MC with PostP predicts the dose spread due to electron transport in lung with good accuracy-to-speed ratio and is suitable for routine treatment planning.

SU-GG-T-248: Dose-Escalation for a Dominant Intraprostatic Lesion Using a Combination of IMRT and VMAT

Purpose: To study the feasibility of boosting the dosedelivered to a dominant intraprostatic lesion (DIL), identified by functional imaging, requiring higher dose for better tumorcontrol using Intensity‐Modulated Radiation Therapy(IMRT) and Volumetric Modulated Arc Therapy (VMAT). Materials and Methods:Treatment planning was performed on 2 patient CT‐scans with hypothetical DIL (1/8 the size of the prostate) in the inferior‐posterior‐left side of the prostate using Pinnacle treatment planning system (Philips Medical Systems). Three types of plan were generated, (A1) 10 Gy 5‐field IMRT boost to the DIL‐PTV followed by 76 Gy 5‐field IMRT to the Prostate‐PTV, (A2) 10 Gy VMAT boost to the DIL‐PTV followed by 76 Gy 5‐field IMRT to the Prostate‐PTV and (A3) 10 Gy VMAT boost to the DIL‐PTV followed by 76 Gy VMAT to the Prostate‐PTV. Plans were delivered using Varian clinac and verified using MapCheck and MapPhan (Sun Nuclear). Results: For plans A1, A2 and A3, the DIL‐PTV received a mean dose of 87.0±0.7 Gy, 86.4±0.4 Gy and 87.5±0.8 Gy respectively, and the Prostate‐PTV — DIL‐PTV received a mean dose of 77.9±1.1 Gy, 77.7±0.7 Gy and 78.1±1.3 Gy respectively. All three plans did not exceed the normal tissue tolerances from the guidelines of the RTOG 0126. Plan A2 had a slightly lower dose to the rectum and bladder. All the plans were delivered and verified successfully. The plan A3 had better dose conformity to DIL‐PTV. Conclusions: This study demonstrates the feasibility of using any of the three treatment methods in treating a DIL without exceeding normal tissue tolerances. However, considering the speed of VMAT delivery over the 5‐field IMRT and better conformality of the dose distribution, a VMAT boost to a DIL followed by a VMAT to the Prostate‐PTV is a preferable treatment option for DIL dose‐escalation. Research sponsored by CIHR.