Pinnacle Planning System Research Articles

SU‐E‐T‐582: Modelling of 7MV Unflat Beam of ONCOR in Pinnacle3 Planning System

Purpose: Beam modelling of 7MV unflat beam of Siemens ONCOR linear accelerator using Pinnacle3 planning system. Methods: Siemens ONCOR Impression plus linear accelerator equipped with 6MV (300MU/min, with FF) has recently approved beam of 7MV UF (2000MU/min, without FF). Beam quality index has been measured over last two years and has been consistent, varying within 1.0%. The output calibration was done with 0.65cc chamber following TRS398 protocol. KQ value was interpolated for the new beam. Output factors were measured at 10cm depth with 0.65cc chamber for field sizes above 5×5cm; and with 0.13cc chamber for smaller fields. The depth dose curves and beam profiles at different depths for various field sizes were collected using 0.13cc chamber in Blue phantom RFA (IBA dosimetry). The collected beam data (ASCII format) was imported into the Pinnacle3 V9.2 (Philips) treatment planning system. Results: Beam modelling was done in Pinnacle3 system. Measured beam data was compared against the available beam spectra of comparative beam energy. E_Tune All InSections sequence from the auto modelling option gave us a best fit for the measured data. Beam profiles were modelled as arbitrary beam profile. Few parameters obtained after completion of 7MV UF beam modelling were compared against 6MV flat beam data that was already modelled before. Beam spectrum for 7UF beam differs from 6MV spectrum. Maximum number of relative photons per energy is reached at 2MeV and 1.5MeV for 7UF and 6MV respectively. This slight increase in energy is also shown in beam quality index which is 0.681 for 7UF and 0.672 for 6MV. There is reduction in spectral off axis softening factor to 1.6878 (7UF) from 13.9628 (6MV) Conclusion: The modelled beam profiles at dmax were compared with the measured ones for three different field sizes and found be within 1% variation. This confirms the correctness of the modelling.

SU‐E‐T‐580: Effect of Lung and Target Density On Small Field Dose Coverage

Purpose: Dose coverage of small targets in SBRT treatments depends on the density of the target and the surrounding lung. We studied the relationship between field size and dose heterogeneity for different size targets as a function of target and lung density. Methods: A 50 cm3 phantom was created in the Pinnacle (V9.2) planning system. A 23 cm diameter lung ROI was inserted in the phantom. 2 or 3 cm diameter targets were placed in the lung ROI and all centered on a 15 cm deep isocenter. Dose points were set at the lateral edges of each target, normal to the beam direction (i.e., at +/− cm for the 2 cm target and +/−1.5 cm for the 3 cm target). Field size was varied from the diameter of each target up to about 5×5 cm2. 100 cGy was delivered to the isocenter via a single 6 MV field and the ratio of the average dose to the lateral dose points to the isocenter dose was recorded for each combination of lung and target density. Lung densities, relative to water, were 0.25, 0.15, 0.1, 0.05 and 0.0. Target densities were 1.0, 0.9, 0.6, 0.4, 0.3, 0.2 and 0.1. Results: We found that uniformity of dose coverage is mainly influenced by lung density. For a typical average target density of 0.4–0.6 and lung density of 0.25 a 1 cm margin is needed to achieve 90% dose uniformity. If the lung density decreases to 0.1 the margin would have to be increased to 2 cm for the same uniformity. Conclusion: It is possible to create a nomographic‐type relationship between target size and field size as a function of lung and target density which can be used as a planning aid in the selection of adequate margins for SBRT cases, particularly for patients with COPD.

SU‐E‐T‐642: A Patient Population Based Approach for Disease Site Specific Intensity‐Modulated Radiation Therapy (IMRT) Treatment Planning

Purpose: To develop and evaluate a population based planning scheme for disease site specific IMRT treatment planning. Methods: A database of previously treated locally advanced head and neck patients from our institution was retrospectively used for this study. The treatment targets involved primary tumor, high risk and low risk nodal volumes. Organs‐at‐risk (OARs) included both parotids, oral cavity, brain stem and the spinal cord. Nine‐beam IMRT plans were generated using the Pinnacle3 planning system for 80% of patients. These plans served as the population basis. Thousands of apertures from these cases were extracted. From these apertures, prospective case‐specific apertures were either selected from the population basis or derived using machine learning for each of the remaining 20% of the patients. Aperture‐based 3D dose distributions were calculated in parallel using Monte Carlo‐based Kernel Superposition (MCKS) via a high throughput computing environment. Aperture weights of each case were optimized via an in‐house developed progressive dose optimization (PDO) approach. The PDO approach involved solving the DO problem by dividing the problem into sub‐problems, each of which was solved while maintaining a global view of the full problem. Our approach was compared with conventional IMRT and two‐arc volumetric modulated arc therapy (VMAT). Results: On average, the improvements achieved using population approach in parotid gland mean dose were 16% and 15%, in the maximum dose to the spinal cord were 21% and 28%, in mean oral cavity dose were 10% and 24% and in maximum brain stem dose were 6% and 15% compared with conventional IMRT and two‐arc VMAT. t‐test showed that these improvements were significant (p < 0.001). Dose homogeneity for all PTVs (tumor and nodes) was higher for population based approach as well. Conclusion: Population based planning approach resulted in superior plan quality compared with conventional IMRT and VMAT.

SU-E-T-439: First Experience of Three Dimensional Conformai Radiotherapy (3DCRT) Planning with Helical Tomotherapy.

A three-dimensional conformal radiotherapy (3DCRT) has been recently introduced to helical tomotherapy, allowing the user to plan and treat patients that do not require sophisticated IMRT planning and delivery. This study aims to test treatment planning on this modality and evaluate its performance by comparing to conventional LINAC-based 3DCRT planning. Four clinical cases (whole brain, extremity, lung, and partial breast irradiation) were retrospectively selected from a Pinnacle planning system (Philips Medical System, Fitchburg, WI) and planned on Tomotherapy (Accuray Inc., Sunnyvale, CA). Computed tomography (CT) images together with contours of target and critical structures were exported from Pinnacle to the Tomotherapy planning station. The same prescription and fractionation scheme was adopted. The pitch factor for all clinical cases was set to 0.287. A 2.5 cm jaw was employed except in the lung case the field size was set to 1.0 cm for better dose conformity. The dose grid size was chosen to be half of that of the planning CT images. On Pinnacle 100% prescription dose was delivered to the treatment isocenter while onTomotherapy it was stipulated that at least 95% of the target volume received the prescribed dose. Comparison between two planning strategies was performed, in terms of dose volume histograms (DVH), dosimetric and radiobiological parameters, for plan quality assessment. Comparison of DVHs reveals that up to 25% healthy tissue sparing in volume can be accomplished with Tomotherapy 3DCRT while the same target coverage is ensured. Dosimetric and radiobiological indices between Tomotherapy and Pinnacle planning agree to within 3.0%. Additional beam modifiers and non-coplanar beams associated with LINAC-based 3DCRT are not needed on Tomotherapy, making it more favorable. Tomotherapy 3DCRT has similar dosimetric performance when compared to conventional LINAC-based 3DCRT while it is substantially easier to use.

SU-E-J-47: Evaluation of the Mismatches Between DRR's and Port Films for IMRT Fields.

To quantify the frequency and magnitude of mismatches between the MLC shape on the DRR created by the Pinnacle planning system and the corresponding port film MLC shape generated by the Mosaiq record-and-verify system for IMRT fields. A retrospective review was made of the most recent 60 patients to receive IMRT at our clinic. The MLC shape on the DRR created by the Philips Pinnacle planning system for each treatment field was reviewed (573 fields total) and compared with the MLC shape of the port film image, which was generated by the Mosaiq treatment planning system. Of the 60 patients studied, 20 had at least one leaf mismatched between the MLC shape on the DRR and port film (142 of the 573 fields). Of the affected cases, on average 59% of the fields had a mismatch. The affected fields had an average of 6.7 leaves mismatched with a mean discrepancy of 27mm. The average maximum discrepancy for each affected patient was 69mm. Discrepancies were most common for head and neck cancers. The MLC shape Mosaiq generates for the port film is the CIAO, or Complete Irradiated Area Outline, which is the area that is actually treated. The Pinnacle DRR displays the maximum leafmotion, which can be different for larger fields in which the MLC leaves abut within the collimator jaw opening. The discrepancy can create substantially different MLC shapes. The problem can be solved by not filming the MLC shape and only using the films for isocenter placement; however, displaying the area receiving treatment can be a useful safety check, possibly preventing a treatment error. The persons assessing the films must be aware of this issue and evaluate the films carefully.

OC-0046 SHORT TERM HORMONE THERAPY AND DOSE ESCALATED RADIATION FOR LOCALIZED PROSTATE CANCER: A RANDOMIZED PHASE III STUDY

consequences of rotational errors, which remain uncorrected if six degree of freedom couches are missing, and intra-fractional patient motion, which appears larger compared to the invasive frame-based approach, are unknown. It was the purpose of this study to investigate geometric and dosimetric accuracy of frame-less IG-RS for brain metastases. Materials and Methods: Single fraction IG-RS was practiced in 72 patients with 98 brain metastases. Patient positioning and immobilization used either double(n=72) or single-layer (n=27) thermoplastic masks. Pre-treatment set-up errors (n=98) were evaluated with cone-beam CT (CBCT) based IG and were corrected in six degrees of freedom without an action level. CBCT imaging after treatment measured intra-fractional errors (n=64). Preand posttreatment errors were simulated in the Pinnacle planning system based on the patient-specific RS treatment plans and target coverage and Paddick conformity index were evaluated. Three scenarios of 0mm, 1mm and 2mm GTV-to-PTV (gross / planning tumor volume) safety margins (SM) were simulated. Results: Errors prior to IG were 3.9mm±1.7mm (3D vector) and the maximum rotational error was 1.7°±0.8° on average. The posttreatment 3D error was 0.9mm±0.6mm. No significant differences between the more rigid double-layer (0.8mm±0.6mm) and single-layer (1.0mm±0.6mm) masks were observed. Simulation of RS without IG reduced target coverage and conformity by 25% and 40% for 0mm SM, respectively. Each 3D set-up error of 1mm decreased target coverage and dose conformity by 6% and 10% on average, respectively. After simulation of 1mm and 2mm GTV-to-PTV SM, target coverage was still reduced by 18% and 10% on average, respectively, if no IG was practiced. Pre-treatment correction of translations only but not rotations did not affect target coverage and conformity, irrespective of the SM. Post-treatment errors reduced target coverage by >5% in 14% and 0% of the patients for a 0mm and 1mm GTV-to-PTV SM, respectively. Conclusions: IG-RS with online correction of translational errors achieved high accuracy with residual errors of about 1mm on average after treatment. Application of 1mm GTV-to-PTV safety margins ensured target coverage within a 5% range compared to the planned dose distributions in all patients.

Impact of leaf motion constraints on IMAT plan quality, deliver accuracy, and efficiency

Intensity modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines the efficiency of arc based delivery with the dose painting capabilities of intensity modulated radiation therapy (IMRT). A key challenge in developing robust inverse planning solutions for IMAT is the need to account for the connectivity of the beam shapes as the gantry rotates from one beam angle to the next. To overcome this challenge, inverse planning solutions typically impose a leaf motion constraint that defines the maximum distance a multileaf collimator (MLC) leaf can travel between adjacent control points. The leaf motion constraint ensures the deliverability of the optimized plan, but it also impacts the plan quality, the delivery accuracy, and the delivery efficiency. In this work, the authors have studied leaf motion constraints in detail and have developed recommendations for optimizing the balance between plan quality and delivery efficiency. Two steps were used to generate optimized IMAT treatment plans. The first was the direct machine parameter optimization (DMPO) inverse planning module in the Pinnacle(3) planning system. Then, a home-grown arc sequencer was applied to convert the optimized intensity maps into deliverable IMAT arcs. IMAT leaf motion constraints were imposed using limits of between 1 and 30 mm∕deg. Dose distributions were calculated using the convolution∕superposition algorithm in the Pinnacle(3) planning system. The IMAT plan dose calculation accuracy was examined using a finer sampling calculation and the quality assurance verification. All plans were delivered on an Elekta Synergy with an 80-leaf MLC and were verified using an IBA MatriXX 2D ion chamber array inserted in a MultiCube solid water phantom. The use of a more restrictive leaf motion constraint (less than 1-2 mm∕deg) results in inferior plan quality. A less restrictive leaf motion constraint (greater than 5 mm∕deg) results in improved plan quality but can lead to less accurate dose distribution as evidenced by increasing discrepancies between the planned and the delivered doses. For example, the results from our patient-specific quality assurance measurements demonstrated that the average gamma analysis passing rate decreased from 98% to 80% when the allowable leaf motion increased from 3 to 20 mm∕deg. Larger leaf motion constraints also led to longer treatment delivery times (2 to 4 folds) due to the additional MLC leaf motion. Leaf motion constraints significantly impact IMAT plans in terms of plan quality, delivery accuracy, and delivery efficiency with the impact magnified for more complex cases. Our studies indicate that a leaf motion constraint of 2 to 3 mm∕deg of gantry rotation can provide an optimal balance between plan quality, delivery accuracy, and efficiency.

SU-E-T-804: Total Scalp Irradiation Using Volumetric Modulated Arc Therapy and Comparison with IMRT and Conventional Photon/Electron Field Combination

Purpose: Irradiation of total scalp with conventional photon/electron fields has been challenging. The unique target shape adds complexity to the beam setup and poses difficulties in achieving a homogeneous plan. Patient setup can be cumbersome and time consuming. IMRT provides excellent dose distributions and simplifies the process of field boundary matching; however beam delivery time is still long due to large number of fields. VMAT has the capability to deliver a homogeneous dose with reduced set‐up and treatment time. VMAT will be compared to other techniques in this work.Methods: Seven patients have been planned for total scalp irradiation using conventional photon/electron planning, IMRT, and SmartArc VMAT. After optimized plans have been achieved using Pinnacle3 planning system, dosimetric comparisons were made between different methods on each patient. Results: Compared with conventional therapy, both IMRT and VMAT yield more homogeneous plan on all the patients. V100% target coverage can easily increase to 98% or higher from approximately 75%. Maximum dose reduces to less than 108% of prescribed dose from 150%. The conventional method performs better in protecting critical organs. However, brain volume at D50%, maximum and mean doses to the cord and brainstem can all be reduced by more than 50% compared to IMRT and VMAT. As a comparison between VMAT and IMRT, in all the cases, IMRT plans are slightly more uniform, while VMAT generally demonstrates better critical organ protection. Different dose levels can be easily achieved without boost using IMRT or VMAT. An added bonus of VMAT is shortened beam delivery time, which is 2.5 minutes at maximum with double arc. Conclusions: VMAT offers a viable alternative to conventional techniques in treatment of the total scalp. Advantages are improved dose distributions to the target volumes compared to conventional, and shorter treatment times compared to IMRT.

SU-E-T-201: Issues Encountered in Film-Based IMRT QA

Purpose: To investigate characteristics of EDR2 film-based IMRT QA with an eye towards moving from isodose line comparisons to more objective gamma statistical analysis. Methods: IMRT QA was performed on Siemens Linacs using a traditional ion-chamber-and-film phantom setup for step-and-shoot IMRT plans. All beams were delivered at their actual gantry angles and to the same film. Fiducial beams were used to fix film location for comparison with calculated dose. Dose was calculated on the Pinnacle planning system (8.0m). The EDR2 films were developed using a Kodak PP-XOMAT film processor that has been remanufactured by Picker. Films were scanned with a VIDAR VXR-16 Dosimetry Pro film digitizer using RIT 113 v5.2 software. Calibration films were created using crossed step wedges with doses calculated by Pinnacle. Gamma was calculated using the standard software settings of 3% / 4 mm. Results: For each QA film, the normalization doses (target to measured), mean gamma, and percentage of points failing the gamma limit (gamma < 1) were recorded. These percentages were found to be considerably higher in many cases than the often-used limit of 5%. This problem was reduced when a new calibration film was used every week and developed together with the QA films; however, the mean fraction of points failing the gamma criterion was still 7%, with 45% of films failing. The mean gamma was 0.4. The product of (target dose) * (mean gamma) * (3%/100%) was relatively constant and averaged 2.3 cGy. Conclusions: Various factors were found to be relevant to the deviations observed, especially film calibration and the target dose (which sets the dose scale for gamma). In many cases, the problem was due to a poorly functioning film processor, resulting in films with streaks and artifacts; this problem was solved by service but recurs on occasion.

SU-E-T-751: Clinical Comparison of Point Source and Line Source Approximations for I-125 Brachytherapy Seeds

Purpose: Currently, the point source approximation is used in our institution to calculate the dose distribution of I‐125 seeds. The purpose of this study was to compare the isodose distributions of both point source and line source models in prostate implant and eye plaque procedures. Methods: Using Pinnacle Planning System (V8.0m, dose grid size 2×2×2 mm3), the point source and line source models for Iodine‐125 were commissioned according to the AAPM TG43 report. Values of function g(r) and F(r, &#952;) were obtained from reference tables inTG‐43. The CTimages for 3 post‐implant prostate cases were obtained in 3mm slices. In addition, the plans for 5 different sized eye plaques (diameters 12–20 mm) were used. The point source was positioned at the center of each seed, and the orientation of the line source was defined by both ends of the seed. Dose volume histograms for the prostate, bladder, and rectum were calculated for the prostate implants, while the dose at points 5mm and 10mm from the sclera were calculated for the eye plaque cases. Results: Target prescription dose coverage for the prostate post‐implants increased by ∼4% using the line source over the point source, while mean target dose decreased by ∼9.1%. D90 increased by ∼7.5%. Mean dose to rectum increased by as much as 6.5% and mean dose to the bladder decreased as much as 4Gy in one patient. For the eye plaque cases, a significant difference in dose at 5mm and 10mm existed between the point and line source models. This discrepancy can be explained by large differences in g(r), between the two models, at small distances. Conclusions: Further investigation is required to determine the extent to which the point source approximation can be implemented in clinical settings while ignoring the discrepancy between the two models.

SU-E-T-744: Dose Distribution of a Hip Prosthesis Phantom in Photon Beams

Purpose: A hip prosthesis phantom was used to investigate the two‐dimensional dose distribution below the hip prosthesis irradiated by photon beams and to compare measured dose distribution with calculated one. Methods: A hip prosthesis phantom included in paraffin was home‐made, which was embedded in an acrylic material. The hip prosthesis consisted of Co‐Cr‐Mo alloys. With 4cm‐thickness build‐up solid water phantom both 6MV and 10MV photon beam of field size 20×20cm were irradiated on the hip prosthesis phantom. We measured two‐dimensional dose distributions with a two‐dimensional ion chamber array I'mRT MatriXX (IBA dosimetry, Schwarzenbruck, Germany), which were compared with commercial treatment planning system Pinnacle3 (Phillips, Madison, WI). In our study, CT numbers from metal artifacts were corrected using the density override option in the Pinnacle3. Results: The dose distribution just behind the prosthesis was severely underestimated in the Pinnacle3. The dose discrepancy between measured and calculated dose was more significant for higher energy. For 1 0MV, there were dose discrepancies over 20% around the prosthesis, while for 6MV below 20%. Conclusions: In the clinical aspect there must be more careful considerations on the calculated dose for patients with hip prosthesis irradiated by photon beam.

Craniospinal Irradiation Using Helical Tomotherapy: Evaluation of Acute Toxicity and Dose Distribution

The purpose of this study was to evaluate acute toxicity of craniospinal irradiation (CSI) using helical tomotherapy (HT) and compare its dose distribution with that of conventional linac-based plans. Twelve patients with various brain tumors were treated with HT-CSI. Median patient age was 14 years (range: 4-37 years). Median CSI dose was 30.6 Gy in 18 fractions (range: 23.4-40 Gy in 13-25 fractions). Toxicities were assessed according to the Common Terminology Criteria for Adverse Events version 4.0. Before CSI, 11 patients (92%) received neoadjuvant chemotherapy, so acute toxicity was evaluated by comparing patient status before and after CSI. HT-CSI plans were compared with linac-based CSI plans made using Pinnacle(3) planning system in 9 patients. All patients completed planned CSI without interruption. Grade 3 or higher toxicities were leukopenia seen in 11 patients (92%), anorexia in 6 (50%), anemia in 5 (42%), and thrombopenia in 5 (42%). Administration of granulocyte colony-stimulating factor, platelet transfusion and total parenteral nutrition were required in 8 (67%), 5 (42%) and 5 (42%) patients, respectively. HT plans were superior to linac-based plans in terms of homogeneity and conformality in planning target volume (PTV). For most organs at risk (OARs), volumes receiving more than 10 Gy (V10 Gy) or 20 Gy (V20 Gy) were lower in HT plans. However, HT plans significantly increased mean doses to the lung, kidneys and liver, and V5 Gy of 6 OARs including the lung. Despite intensive neoadjuvant chemotherapy, acute toxicity of HT-CSI was acceptable. HT provided better dose distribution in PTV than conventional linac. In most OARs, smaller volumes received >10-20 Gy in HT plans, although larger volumes received 5-10 Gy.

SU-GG-T-120: The Role of IMRT and IMAT in Targets of Varying Complexity

Purpose: To evaluate the treatment plan quality and delivery efficiency of fixed‐field IMRT and IMAT with different target complexities Method and Materials: Four target shapes with different complexities (circular, “L”, inverse “U” and “V” shapes) for a head and neck phantom and a pelvic‐prostate phantom were used for treatment planning of IMRT and IMAT. Dose prescriptions of 70 Gy and 75.6Gy were given to the head and neck and prostate, respectively. Seven/nine‐beam IMRT plans were generated using Pinnacle3 planning system and in‐house developed sequential linear program (SLP). Single‐arc and double‐arc IMAT plans were generated using Pinnacle SmartArc 9.0. Plan quality was compared with DVHs and isodose curves and delivery efficiency was compared with MUs and aperture number. Results: Two‐arc plans improve the plan quality compared to single‐arc plans. For simple targets and large volume cases such as prostate, IMAT provides better plans than IMRT. As the target complexity increases, conventional fixed‐field IMRT can provide better plan quality than IMAT plans. SLP provides even better quality plans than conventional IMRT and IMAT, especially for complex cases. 180 and 360 apertures were used in single‐arc and double‐arc plans respectively. Conventional IMRT plans reduce the number of aperture by 25–80% comparing to single‐arc plans. SLP can further reduce 5–20% of the apertures, but used 1–7% more MUs. Two‐arc plans used 10–50% more MUs than single‐arc plans. IMAT plans used fewer MUs compared with IMRT.Conclusion: Our results indicate that (1) as the complexity of the target increases, fixed‐field IMRT plans (particularly the ones optimized using the SLP approach) become more competitive in dosimetric quality compared with IMAT and (2) IMAT plans result in greater delivery efficiency compared with IMRT. Thus, for complex cases involving concave targets, dosimetric plan quality is inversely proportional to plan delivery efficiency. Conflict of Interest: NIH grant CA130814

WE-E-BRA-02: Clinical Evaluation of Fixed-Field IMRT Plans and IMAT Plans in Locally Advanced Head and Neck Cancer

Purpose: To evaluate the treatment plan quality and delivery efficiency of fixed‐field IMRT and IMAT in locally advanced head and neck cancer cases. Method and Materials: Ten locally advanced head and neck cases were included. For each case, the treatment targets involve primary tumor, positive lymph node(s), high risk and low risk nodal volumes. Organs‐at‐risk included both parotids and spinal cord. Seven‐beam IMRT plans were generated using the Pinnacle3 planning system and an in‐house developed sequential linear program (SLP) optimization approach. The SLP approach solved the corresponding dose optimization problem by dividing the problem into sub‐problems, each of which was solved using LP while maintaining a global view of the full problem. Single‐arc and double‐arc IMAT plans with 180 and 360 total apertures, respectively were generated using Pinnacle SmartArc 9.0. Plan quality was compared with DVHs and quantitative dose‐volume measures and delivery efficiency was compared with MUs and aperture number. Results: Single‐arc plans did not provide enough coverage for all three targets. Two‐arc plans improved the plan quality compared to single‐arc plans. Conventional fixed‐field IMRT and SLP plans resulted in better plan quality than IMAT plans with SLP plans outperforming the other approaches. Improvements in parotid dose‐volume levels and maximum cord dose were 12% and 90% when SLP was compared to two‐arc plans. Approximately 160 to 230 apertures were used with conventional IMRT plans. SLP used fewer apertures than conventional IMRT but used more monitor units. Double‐arc plan used more monitor units than the single‐arc plan. Overall, IMAT plans used fewer MUs than IMRT plans. Conclusion: Our results indicate that (1) fixed‐field IMRT plans (particularly the ones optimized using the SLP approach) result in superior dosimetric quality compared with IMAT and (2) IMAT plans result in greater delivery efficiency compared with IMRT. Conflict of Interest: NIH grant CA130814

SU‐GG‐T‐375: A Custom‐Developed Method for Accurate Dose Recalculation of Patient Plans Entered into Clinical Trials

Purpose: To compare a custom‐developed method for accurate dose recalculation of patient plans entered into clinical trials with results from a common treatment planning system. Method and Materials: A measurement‐driven multiple‐source model with the Dose Planning Method (DPM) Monte Carlo (MC) dose calculation algorithm was previously developed, validated, and benchmarked for the Varian 6 MV and 10 MV photon beams. Several patient cases have been recalculated and compared to the calculations from a Pinnacle planning system. Intensity modulated radiation therapy (IMRT) prostate, IMRT abdomen, stereotactic body radiotherapy lung, and IMRT lung patient cases were selected. Results: Field sizes from 4 cm × 4 cm to 40 cm × 40 cm were validated to within 2% of the maximum dose and 2 mm distance to agreement. At least 95% of the data tested met the validation criteria. Benchmark treatments planned using anthropomorphic phantoms were tested to within 3% of the target dose and 2 mm distance to agreement. At least 85% of the data tested met the benchmark criteria. Disagreement in the patient plan evaluation tended to occur at heterogeneity interfaces where electronic disequilibrium occurred, and in the beam penumbra, where scattered radiation was more prominent. The ratios of planning system calculation to MC calculation for the mean dose of the gross and planning target volumes for the patient plans ranged from 0.984 to 1.016. Conclusion: These results show that this MC software code generates answers similar to the Pinnacle system for IMRT and SBRT treatment plans. Differences are consistent with the superior physics modeling inherent in the Monte Carlo code. We believe the method will be useful for recalculating dose distributions for patients entered into clinical trials. Work supported by PHS CA010953, CA081647, and R01 CA85181 awarded by NCI, DHHS

SU-DD-A4-01: Phantom Study of Cone Beam Computed Tomography Setup Accuracy in Stereotactic Body Radiation Therapy for Lung Cancer Treatment

Purpose: To evaluate the accuracy of cone beam computed tomography(CBCT) guided positioning in the case of motion patterns associated with regular and irregular breathing for lungcancer stereotactic body radiation therapy(SBRT).Methods and Materials: Five irregular motion patterns have been used in phantom‐simulated cases using a solid ball to create four‐dimensional (4D) CT datasets. Images are transferred to the Pinnacle Planning System 8.0m (Phillips) where a planning treatment volume (PTV) is constructed for planning. Subsequently the same phantom simulated cases are imaged with a CBCT (Elekta XVI) in volumetric reconstruction mode. Images defining the PTV using CBCT are compared with PTV determined from 4DCT images. Breathing motion patterns used in these phantom studies were from actual patients' data taken at random. For comparisons regular breathing motion data have been also used for reference in PTV matching derived from 4DCT and CBCT. The actual time dependant target position was known in each phantom‐simulated case from the driver files associated with the motion generator. Results For regular motion, CBCT exhibited up to 2mm shift between 4DCT derived PTV and CBCT defined PTV. For irregular motion, the CBCT derived PTV exhibited up to3.7mm shift relative to 4DCT derived PTV. However, we found that neither the 4DCT nor the CBCT accurately reflect of the actual PTV motion and geometry (up to 4.9mm) derived from known target dimensions and knowledge of motion data. Dosimetric impact of this error will be reported. Conclusions The setup of target for SBRTtreatment of lungtumor based on CBCT derived PTV is consistent with treatment plan based on 4DCT. This clinically positive result is not based, however, on accurate representation of clinically used PTV with true geometrical representation of PTV target derived from known target dimensions and known pattern of target motion in the phantom.

Is a single arc sufficient in volumetric-modulated arc therapy (VMAT) for complex-shaped target volumes?

Purpose To compare step-and-shoot intensity-modulated radiotherapy (ss-IMRT) with volumetric-modulated arc therapy (VMAT) for complex-shaped target volumes with a simultaneous integrated boost (SIB). Materials and methods This retrospective planning study was based on 20 patients composed of prostate cancer ( n = 5), postoperative ( n = 5) or primary ( n = 5) radiotherapy for pharyngeal cancer and for cancer of the paranasal sinuses ( n = 5); a SIB with two or three dose levels was planned in all patients. For each patient, one ss-IMRT plan with direct-machine-parameter optimization (DMPO) and VMAT plans with one to three arcs (SmartArc technique) were generated in the Pinnacle planning system. Results Single arc VMAT improved target coverage and dose homogeneity in radiotherapy for prostate cancer. Two and three VMAT arcs were required to achieve equivalent results compared to ss-IMRT in postoperative and primary radiotherapy for pharyngeal cancer, respectively. In radiotherapy for cancer of the paranasal sinuses, multiarc VMAT resulted in increased spread of low doses to the lenses and decreased target coverage in the region between the orbits. Conclusions The complexity of the target volume determined whether single arc VMAT was equivalent to ss-IMRT. Multiple arc VMAT improved results compared to single arc VMAT at cost of increased delivery times, increased monitor unites and increased spread of low doses.

Poster — Wed Eve—26: Dose‐Volume Histogram Analyses on the Prostate IMRT Plan for Interfraction Organ Motion Using the Gaussian Error Function

The Gaussian error function (GEF) model was used to carry out cumulative dose‐volume histogram (cDVH) analysis on prostate IMRT plans with interfraction organ motion. cDVHs for CTVs, shifted in the anterior‐posterior directions based on 7‐beam IMRT plans for three patients (small, medium and large prostate), were calculated and modeled using the Pinnacle3 planning system and GEF. To simulate the interfraction prostate motion, the CTV was shifted 1 cm in the anterior‐posterior directions in 2 mm steps, using the dose distribution for the plan without prostate motion. As parameters in the GEF model, namely, a, b and c, were related to the shape of the cDVH curve, evaluation of cDVHs corresponding to the prostate motion becomes possible. cDVH analysis for the CTV shifting in the anterior‐posterior directions using the GEF model showed that parameters , which were related to the maximum relative volume of the cDVH, changed symmetrically when the prostate was shifted in the anterior‐posterior directions. This change was more significant for larger prostate. For parameters b related to the slope of the cDVH, changed symmetrically from the isocenter, when the CTV was within the PTV. This was different from parameters c ( related to the maximum dose of the cDVH), which did not vary significantly with the prostate motion in the anterior‐posterior directions and prostate volume. Using the patient data, this analysis validates the GEF model, and further verified the clinical application of this mathematical model on treatment plan evaluation.

MO‐D‐BRB‐01: Study of Systemic and Random Errors On VMAT and IMRT Plan Quality and Deliver Accuracy

Purpose: During the delivery of volumetric modulated arc therapy (VMAT), random errors exist in both the MLC leaf positions and the gantry angle. In this work, we investigated the impact of such random errors on VMAT plan quality and delivery accuracy. The impact of system calibration errors was also examined. For comparison purposes, we performed a similar study on step‐and‐shoot IMRT plans. Material and Method: VMAT plans for three treatment sites (prostate, pancreas and head‐&amp;‐neck) were created using a home‐grown arc sequencer. Next, random and systematic leaf position errors were introduced into these plans with the random errors sampled from Gaussian distributions of varying widths ranging from 1 to 3mm. Two types of systematic errors, including MLC bank shifts in the same direction (Type I: leaf gap unchanged) and MLC bank shifts in opposite directions (Type II: leaf gap increase/decrease). The plan quality variations were compared in the Pinnacle3 planning system. Plans with systematic errors were verified using the MatriXX ion chamber array with gamma evaluation criteria of 3%/3mm. Results: The plan degradation observed for VMAT plans was slightly less as compared to that for fixed‐field IMRT plans when random errors up to 3mm to the leaf positions were introduced. With type I systematic errors of 3mm on leaf positions, the average standard deviation of PTV dose increased by 10.2%. This value increased to 18.4% for the corresponding fixed‐field IMRT plans. A larger impact on the IMRT plans was also observed when type II systematic errors were introduced. The above results were confirmed by plan verification measurements with higher gamma passing rates for VMAT plans when systematic errors were applied. Conclusion: The VMAT delivery technique has better tolerance to random and systematic errors in gantry angle and MLC leaf position errors as compared with step‐and‐shoot IMRT.Research supported by Elekta.

SU‐FF‐T‐113: Effect of the Same Plan On Doses to Patients During Radiotherapy with Nasopharyngeal Carinoma

Purpose To evaluate the feasibility of primitive IMRT plan was used throughout the treatment course in the nasopharyngeal carcinoma. Methods 10 patients with nasopharyngeal carcinoma who had undergone radiotherapy by IMRT was selected. A IMRT plan was designed by Pinnacle3 planning system. During the middle period of the irradiation, the secondary scans were made with the same CT parameters of the first, and then to copy the primitive plan to secondary CT image, The same energy, beam angles, control points, and optimization parameters were used fordose calculations for each paired plans using a treatment optimization system. The two plans were compared using a set of parameters derived from DVHs and dose‐volume statistics. Detailed analysis of the dose distribution of the treatment volume. Result the secondary plan was compared with the first plan. At the Isocenter layer, the mean length was diminute to 8% in the anterior‐posterior direction and 3% in the left‐right direction. PTV1(D95) was decreased to 0.6%–5.3%; the extent of the averaged volume to the right/left parotids was decreased to 13.1%–41.4% and 12.0%–49.0% respectively; The averaged dose to the right/left parotids was increase to 5.6%–45.1% and 3.3%–32.2% respectively; the waving extent of Dmax To the spine cord and brain stem are −4.1%–13.9%, −3.9%–9.3% respectively. Conclusion Our planning study showed that there were the essentiality to modify the plan to correcting the target and normal tissue variation in treatment courses in the nasopharyngeal carcinoma.