N Wang

Evaluation and comparison of New 4DCT based strategies for proton treatment planning for lung tumors

PurposeTo evaluate different strategies for proton lung treatment planning based on four-dimensional CT (4DCT) scans.Methods and MaterialsTwelve cases, involving only gross tumor volumes (GTV), were evaluated. Single image sets of (1) maximum intensity projection (MIP3) of end inhale (EI), middle exhale (ME) and end exhale (EE) images; (2) average intensity projection (AVG) of all phase images; and (3) EE images from 4DCT scans were selected as primary images for proton treatment planning. Internal target volumes (ITVs) outlined by a clinician were imported into MIP3, AVG, and EE images as planning targets. Initially, treatment uncertainties were not included in planning. Each plan was imported into phase images of 4DCT scans. Relative volumes of GTVs covered by 95% of prescribed dose and mean ipsilateral lung dose of a phase image obtained by averaging the dose in inspiration and expiration phases were used to evaluate the quality of a plan for a particular case. For comparing different planning strategies, the mean of the averaged relative volumes of GTVs covered by 95% of prescribed dose and its standard deviation for each planning strategy for all cases were used. Then, treatment uncertainties were included in planning. Each plan was recalculated in phase images of 4DCT scans. Same strategies were used for plan evaluation except dose-volume histograms of the planning target volumes (PTVs) instead of GTVs were used and the mean and standard deviation of the relative volumes of PTVs covered by 95% of prescribed dose and the ipsilateral lung dose were used to compare different planning strategies.ResultsMIP3 plans without treatment uncertainties yielded 96.7% of the mean relative GTV covered by 95% of prescribed dose (standard deviations of 5.7% for all cases). With treatment uncertainties, MIP3 plans yielded 99.5% of mean relative PTV covered by 95% of prescribed dose (standard deviations of 0.7%). Inclusion of treatment uncertainties improved PTV dose coverage but also increased the ipsilateral mean lung dose in general, and reduced the variations of the PTV dose coverage among different cases. Plans based on conventional axial CT scan (CVCT) gave the poorest PTV dose coverage (about 96% of mean relative PTV covered by 95% isodose) compared to MIP3 and EE plans, which resulted in 100% of PTV covered by 95% isodose for tumors with relatively large motion. AVG plans demonstrated PTV dose coverage of 89.8% and 94.4% for cases with small tumors. MIP3 plans demonstrated superior tumor coverage and were least sensitive to tumor size and tumor location.ConclusionMIP3 plans based on 4DCT scans were the best planning strategy for proton lung treatment planning.

Individualized 4-dimensional computed tomography proton treatment for pancreatic tumors

BACKGROUND The goal of this study is to determine whether a phase or reconstruction of a 10-phase 4 dimensional computed tomography (4D CT) scan can be used as the primary planning scan for proton treatment of the pancreas, thus eliminating the need for second a slow CT or free breathing CT. METHODS Ten patients with pancreatic adenocarcinoma were simulated with 4D CT and a proton treatment plan generated based upon one of three primary planning scans, the T0 phase, T50 phase or average reconstruction. These plans were then exported to each of the remaining phases of the 4D CT and the dose to 95% of the target (D95) calculated. Plans were deemed adequate if the D95 remained at 99% of the prescribed dose or greater. RESULTS For the ten patients in this study anterior abdominal motion was found to range from 2-27 mm (mean 7.50±6.79 mm). For 9 of 10 patients the anterior abdominal motion was ≤8 mm and all three primary planning scans provided adequate target coverage, resulting in minimum D95 coverage per plan of T0_plan 99.7%, T50_plan 99.3% and AVE_plan 99%. However no plan provided adequate target coverage on the single patient with the largest anterior abdominal motion, 27 mm, and cranio-caudal motion, 20 mm, with minimum D95 values of T0_plan 96.3%, T50_plan 68%, and AVE_plan 68%. CONCLUSIONS The primary plans tested based up on the T0, T50 and average reconstructions provided adequate D95 coverage throughout the respiratory cycle as long as the anterior abdominal motion was ≤8 mm and can be considered for use as the primary proton planning scan.

Commissioning of a proton gantry equipped with dual x-ray imagers and a robotic patient positioner, and evaluation of the accuracy of single-beam image registration for this system

PURPOSE To check the accuracy of a gantry equipped with dual x-ray imagers and a robotic patient positioner for proton radiotherapy, and to evaluate the accuracy and feasibility of single-beam registration using the robotic positioner. METHODS One of the proton treatment rooms at their institution was upgraded to include a robotic patient positioner (couch) with 6 degrees of freedom and dual orthogonal kilovoltage x-ray imaging panels. The wander of the proton beam central axis, the wander of the beamline, and the orthogonal image panel crosswires from the gantry isocenter were measured for different gantry angles. The couch movement accuracy and couch wander from the gantry isocenter were measured for couch loadings of 50-300 lb with couch rotations from 0° to ± 90°. The combined accuracy of the gantry, couch, and imagers was checked using a custom-made 30 × 30 × 30 cm(3) Styrofoam phantom with beekleys embedded in it. A treatment in this room can be set up and registered at a setup field location, then moved precisely to any other treatment location without requiring additional image registration. The accuracy of the single-beam registration strategy was checked for treatments containing multiple beams with different combinations of gantry angles, couch yaws, and beam locations. RESULTS The proton beam central axis wander from the gantry isocenter was within 0.5 mm with gantry rotations in both clockwise (CW) and counterclockwise (CCW) directions. The maximum wander of the beamline and orthogonal imager crosswire centers from the gantry isocenter were within 0.5 and 0.8 mm, respectively, with the gantry rotations in CW and CCW directions. Vertical and horizontal couch wanders from the gantry isocenter were within 0.4 and 1.3 mm, respectively, for couch yaw from 0° to ± 90°. For a treatment with multiple beams with different gantry angles, couch yaws, and beam locations, the measured displacements of treatment beam locations from the one based on the initial setup beam registered at the gantry at 0°/180° and couch yaw at 0° were within 1.5 mm in three translations and 0.5° in three rotations for a 200 lb couch loading. CONCLUSIONS Results demonstrate that the gantry equipped with a robotic patient positioner and dual imaging panels satisfies treatment requirements for proton radiotherapy. The combined accuracy of the gantry, couch, and imagers allows a patient to be registered at one setup position and then moved precisely to another treatment position by commanding the robotic patient positioner and delivering treatment without requiring additional image registration.

SU‐E‐T‐238: Annual QA of Proton Gantry with Robotic Table

PURPOSE To perform the annual QA of proton gantry with a robotic table. METHODS A new proton gantry with robotic table has been commissioned and is being used in clinic for patient treatment. The gantry is equipped with a robotic table with 6-degrees of freedom and dual cardinal angle KV imagers for patient registration. The system allows direct movement from one beam location to another without additional image registration, which effectively reduces portal setup time and increases treatment efficiency. The annual QA has four main components: Beam parameter checks included proton depth dose, output, linearity, modulation factor, field size factor, effective source distance, compensator gap factor, and monitor unit comparison between model calibration and physical measurements for every energy. Mechanical checks included gantry and robotic table isocenter, gantry and robotic couch isocentricity, and mechanical movement of fully loaded couch and corresponding digital readout. Imaging system checks included proton, X-ray beam, laser and image receptor alignment, image quality of KV imagers, and image registration accuracy. The last were the system safety checks. Methods used to perform these checks, especially those pertaining to robotic positioner will be discussed. RESULTS The new proton gantry and robotic table had the isocentric accuracy of about 1 mm. The accuracy of mechanical movements of the robotic table was within 1mm/0.5 degree in the clinical motion range. The accuracy of proton outputs determined by IAEA TRS 398 protocol was within +/-2% and the consistency of beam range for all clinical energies at cardinal gantry angles was within 1mm. CONCLUSIONS The results of the gantry annual QA demonstrate that the machine satisfies the highest standards of quality assurance for proton radiation treatment. The annual QA verifies the proton output, robotic table movement accuracy, image registration and safety of the machine and thus increases our confidence level in the uncertainties of daily proton treatments.

SU‐E‐T‐499: Assessment of Patient Setup Accuracy at a Treatment Position after the Patient Has Been Registered at Another Location Using Robotic Positioner and a Dual KV Imaging System

Purpose: We use cardinal angle and beam eye view images with DRRs to register a patient at every treatment position. This process is time consuming; moreover, every treatment position is not amenable to imaging. This study is to assess the combined robotic positioner and gantry movement accuracy at a treatment position relative to zero yaw image registration so that additional imaging at the new location can be avoided. Methods: A Styrofoam phantom 30 cm × 30 cm × 30 cm with beekleys embedded in such a way that they could easily be distinguished on a DRR. The phantom was CT scanned with a 2.5 mm slice thickness and a 50 cm field of view. Treatment planningDRRs were created at numerous table and gantry angles. The phantom was subsequently set up in the treatment room with both table yaw and gantry at zero. The table was loaded with 200 lb weight in order to replicate a typical patient. Image registration was carried out using cardinal angle images and the DRRs. The robotic positioner and gantry were then moved to a new treatment position. The accuracy of the move was evaluated using the beam eye view image, the orthogonal images at the new position and the DRRs for this location. The process was repeated for numerous table and gantry positions as predetermined during treatment planning. Results: The accuracy was found better than a 1.0 mm for all moves except near treatment position with table yaw 90° and Gantry angle between 180° to 270°, where the accuracy was still better than 1.7 mm. Conclusions: As such, the system can be clinically used for patient registration, but we are in the process of further improving the setup accuracy for all clinically relevant robotic positioner and gantry movements without additional imaging.

SU-E-T-619: Dose Evaluation Tool for Proton Patch Field Irradiation

Purpose: Evaluate the tumor coverage and dose to organs at risk (OAR) from proton patch field irradiation using over‐irradiate and under‐irradiate modes. Methods: Proton patch field configuration is a powerful treatment planning tool used in proton therapy to spare OAR. In this beam arrangement a shoot through beam is used to irradiate a large portion of the tumor and one or more patching beams are used to treat rest of the tumor. The patching beam is distally stopped at the lateral edge of the shoot‐ through beam. For such complex beam arrangements a treatment plan done with nominal uncertainties may not be adequate to assess tumor coverage and risk to critical structures. The beam range and patient density uncertainty increase or decrease penetration of a proton beam and position uncertainty expands or contracts the irradiation area by shifting aperture and compensator during treatment delivery. In addition to nominal plan, we used the over‐ and under‐irradiation mode available in our treatment planning system to simulate two extreme circumstances during the treatment delivery. The nominal, over‐irradiate and under‐irradiate plan are used to evaluate tumor coverage and estimate the dose to OAR with the patch field irradiation. Results: In proton patch field irradiation the uncertainties can adversely affect tumor coverage and the OAR. Over‐irradiate and under‐ irradiate mode create dose distributions under extreme but likely coincidence of uncertainties. Using these dose distributions in conjunction with nominal dose distribution can help us evaluate a plan more realistically, and rethink the dose constrains on critical structures. Conclusions: In addition to nominal dose calculation, the over‐irradiate and under‐irradiate mode of dose calculation should be used to evaluate complex proton plans, especially those involving patch fields.

SU‐GG‐T‐478: A Comparison of Proton Beam Monitor Units Predicted by the Treatment Planning System and Obtained from Physical Calibration

Purpose: To compare the proton monitor units predicted by the treatment planning system and obtained from physical calibration.Method and Materials: Monitor units for proton beam calibration are product of the machine calibration factor (CF), mod factor (MF), Spread out Bragg Peak depth dose (SOBPPdd), field size factor (FSF), inverse square factor (ISF), off center ratio (OCR), and bolus gap factor (BGF). The CF is defined as 1cGy/MU at the center of modulation for the mod wheel 60. The SOBPPdd, MF, FSF, BGF and OCR were measured during the commissioning of proton machines for each beam energy. The proton beam monitor units in treatment planning system are calculated based on the OCR, MF, SOBPdd, ISF from virtual SAD and proton scatter parameters. The differences in the monitor unit calculation between the treatment planning system and from the physical calibration arise from the BGF, FSF and patient scatter. The BGF and FSF are handled correctly in the physical calibration. The patient scattering and heterogeneity are handled by the proton pencil beam dose kernel in the treatment planning system. The proton monitor units from the treatment planning system and physical calibration were compared for prostate, lung and brain patients. Results: The differences in monitor units between the treatment planning system and physical calibration were within 1% for breast cases, and 3%for most of the other cases in the study. The differences increased at higher energy and for thicker compensators. There were also larger differences for some of the cases where the field sizes were smaller than 3 cm. The treatment planning algorithm does not always accommodate special treatment situations adequately. Conclusion:Treatment planning algorithms cannot always accommodate special treatment situations. For such cases, in addition to routine MU calculation, it is important to carry out a physical calibration of treatment fields.

SU-FF-T-584: 4D Treatment Planning Strategy for Lung Tumor Irradiation with Protons

Purpose: To present a new strategy of 4D treatment planning for lung irradiation with protons.Method and Materials:Organ motion is a serious issue in radiation therapy planning, especially for proton irradiation, because the finite range of a proton beam makes the treatment delivery sensitive to density changes along the beam path. In our method the patient is immobilized supine in a whole body‐pod, and undergoes a 4D‐CT scan of the lung.Images are taken at 10 phases per breathing cycle. GTV is contoured on the maximum intensity projection (MIP) scan. The GTV is corrected for density averaging errors across lung‐bone or lung‐normal tissue boundaries using the EE (end exhale) and EI (end inhale) images. The GTV is then transferred to the EE scan which is used as the primary scan for treatment planning. Beam apertures were designed by incorporating beam specific lateral margins around GTV, and compensator bolus design incorporated set‐up and scatter uncertainties through a smear radius. Same method is used to design the aperture and compensator bolus for each CTV beam. Results: Four cases with different locations of the tumor in the lung were studied. Each case was planned four different ways: (i) EE scan as the primary planning scan (ii) MIP scan as the primary planning scan and (iii) and (iv) were 4D‐CT planning strategies published in literature. For small tumors all methods gave similar results irrespective of the tumor location in the lung. For large tumors (∼100 cc) EE method gave the best results both for tumor coverage and for sparing of normal lungtissue.Conclusion: Out of four methods compared, our method of using EE scan as a primary scan for planning and using MIP scan for contouring the GTV gave best results irrespective of the tumor location in the lung or the size.

SU-GG-T-555: Commissioning of a Treatment Planning System with Proton Therapy Capability

Purpose: To describe the commissioning process of a treatment planning system with proton therapy capability. Method and Materials: Simple proton beam arrangements can deliver very precise and highly conformal dose distributions to complex targets and simultaneously conformally avoid critical structures in the vicinity of the target. But protontreatment planning is quite challenging and less forgiving to errors, so commissioning a proton therapytreatment planning system presents its own challenges. Recently, we commissioned a new treatment planning system (Odysey™‐4.01). This treatment planning system has both proton and photon planning capability. We will present the process and results of our experience with commissioning the proton‐planning component of this system. Results: In the non‐dosimetric part of commissioning, various tools and features of the treatment planning system were tested for specifications and were found to be within specs. The fidelity of the data and image transfer from CT to the treatment planning system and from treatment planning system to the treatment rooms was checked and found to be accurate. The treatment devices (compensator boluses and apertures) created by the planning system were checked for accuracy. In the dosimetric commissioning we compared depth dose profiles and beam profiles generated by the treatment planning system with the measured data for regular fields. Dose distributions generated by patch and match field arrangements, and for small stereotactic radio‐surgery (SRS) fields were verified by direct measurements in a phantom. The dose monitor units from treatment planning and hand calculations agreed within 2% for fields larger than 5 cm. Smaller fields needed portal specific calibration.Conclusion: All tests performed as part of commissioning were within specifications. For patch and match fields, dosimetric verification of the plan generated dose distributions is highly recommended.