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Relation of external surface to internal tumor motion studied with cine CT

The accuracy of delivering gated-radiation therapy to lung tumors using an external respiratory surrogate relies on not only interfractional and intrafractional reproducibility, but also a strong correlation between external motion and internal tumor motion. The purpose of this work was to use the cine images acquired by four-dimensional computed tomography acquisition protocol to study the relation between external surface motion and internal tumor motion. The respiratory phase information of tumor motion and chest wall motion was measured on the cine images using a proposed region-of-interest (ROI) method and compared to measurement of an external respiratory monitoring device. On eight lung patient data sets, the phase shifts were measured between (1) the signal of a real-time positioning-management (RPM) respiratory monitoring device placed in the abdominal region and four surface locations on the chest wall, (2) the RPM signal in the abdominal region and tumor motions, and (3) chest wall surface motions and tumor motions. Respiratory waveforms measured at different surface locations during the same respiratory cycle often varied and had significant phase shifts. Seven of the 8 patients showed the abdominal motion leading chest wall motion. The best correlation (smallest phase shift) was found between the abdominal motion and the superior-inferior (S-I) tumor motion. A wide range of phase shifts was observed between external surface motion and tumor anterior-posterior (A-P)/lateral motion. The result supported the placement of the RPM block in the abdominal region and suggested that during a gated therapy utilizing the RPM system, it is necessary to place the RPM block at the same location as it is during treatment simulation in order to reduce potential errors introduced by the position of the RPM block. Correlations between external motions and lateral/A-P tumor motions were inconclusive due to a combination of patient selection and the limitation of the ROI method.

Design of respiration averaged CT for attenuation correction of the PET data from PET/CT

Our previous patient studies have shown that the use of respiration averaged computed tomography (ACT) for attenuation correction of the positron emission tomography (PET) data from PET/CT reduces the potential misalignment in the thorax region by matching the temporal resolution of the CT to that of the PET. In the present work, we investigated other approaches of acquiring ACT in order to reduce the CT dose and to improve the ease of clinical implementation. Four-dimensional CT (4DCT) data sets for ten patients (17 lung/esophageal tumors) were acquired in the thoracic region immediately after the routine PET/CT scan. For each patient, multiple sets of ACTs were generated based on both phase image averaging (phase approach) and fixed cine duration image averaging (cine approach). In the phase approach, the ACTs were calculated from CT images corresponding to the significant phases of the respiratory cycle: ACT(050phs) from end-inspiration (0%) and end-expiration (50%), ACT(2070phs) from mid-inspiration (20%) and mid-expiration (70%), ACT(4phs) from 0%, 20%, 50% and 70%, and ACT(10phs) from all ten phases, which was the original approach. In the cine approach, which does not require 4DCT, the ACTs were calculated based on the cine images from cine durations of 1 to 6 s at 1 s increments. PET emission data for each patient were attenuation corrected with each of the above mentioned ACTs and the tumor maximum standard uptake value (SUVmax), average SUV (SUVavg), and tumor volume measurements were compared. Percent differences were calculated between PET data corrected with various ACTs and that corrected with ACT(10phs). In the phase approach, the ACT(10phs) can be approximated by the ACT(4phs) to within a mean percent difference of 2% in SUV and tumor volume measurements. In cine approach, ACT(10phs) can be approximated to within a mean percent difference of 3% by ACTs computed from cine durations > or =3 s. Acquiring CT images only at the four significant phases for the ACT can reduce radiation dose to 1/3 of the current 4DCT dose; however, the implementation of this approach requires additional hardware that is not standard equipment on PET/CT scanners. In the cine approach, we recommend a duration of 6 +/- 1 s in order to include variations of respiratory patterns in a larger population. This approach can be easily implemented because cine acquisition mode is available on all GE PET/CT scanners. The CT dose in the cine approach can be reduced to approximately 5 mGy by using the lowest mA setting (10 mA), while still maintaining good quality CT data for PET attenuation correction. In our scanning protocol, the ACT is only acquired if respiration-induced misregistration is observed (determined before the PET scan is completed), and therefore patients do not receive unnecessary CT radiation dose.

Cine Computed Tomography Without Respiratory Surrogate in Planning Stereotactic Radiotherapy for Non–Small-Cell Lung Cancer

PURPOSE To determine whether cine computed tomography (CT) can serve as an alternative to four-dimensional (4D)-CT by providing tumor motion information and producing equivalent target volumes when used to contour in radiotherapy planning without a respiratory surrogate. METHODS AND MATERIALS Cine CT images from a commercial CT scanner were used to form maximum intensity projection and respiratory-averaged CT image sets. These image sets then were used together to define the targets for radiotherapy. Phantoms oscillating under irregular motion were used to assess the differences between contouring using cine CT and 4D-CT. We also retrospectively reviewed the image sets for 26 patients (27 lesions) at our institution who had undergone stereotactic radiotherapy for Stage I non-small-cell lung cancer. The patients were included if the tumor motion was >1 cm. The lesions were first contoured using maximum intensity projection and respiratory-averaged CT image sets processed from cine CT and then with 4D-CT maximum intensity projection and 10-phase image sets. The mean ratios of the volume magnitude were compared with intraobserver variation, the mean centroid shifts were calculated, and the volume overlap was assessed with the normalized Dice similarity coefficient index. RESULTS The phantom studies demonstrated that cine CT captured a greater extent of irregular tumor motion than did 4D-CT, producing a larger tumor volume. The patient studies demonstrated that the gross tumor defined using cine CT imaging was similar to, or slightly larger than, that defined using 4D-CT. CONCLUSION The results of our study have shown that cine CT is a promising alternative to 4D-CT for stereotactic radiotherapy planning.

Effects of Respiration-Averaged Computed Tomography on Positron Emission Tomography/Computed Tomography Quantification and its Potential Impact on Gross Tumor Volume Delineation

PURPOSE Patient respiratory motion can cause image artifacts in positron emission tomography (PET) from PET/computed tomography (CT) and change the quantification of PET for thoracic patients. In this study, respiration-averaged CT (ACT) was used to remove the artifacts, and the changes in standardized uptake value (SUV) and gross tumor volume (GTV) were quantified. METHODS AND MATERIALS We incorporated the ACT acquisition in a PET/CT session for 216 lung patients, generating two PET/CT data sets for each patient. The first data set (PET(HCT)/HCT) contained the clinical PET/CT in which PET was attenuation corrected with a helical CT (HCT). The second data set (PET(ACT)/ACT) contained the PET/CT in which PET was corrected with ACT. We quantified the differences between the two datasets in image alignment, maximum SUV (SUV(max)), and GTV contours. RESULTS Of the patients, 68% demonstrated respiratory artifacts in the PET(HCT), and for all patients the artifact was removed or reduced in the corresponding PET(ACT). The impact of respiration artifact was the worst for lesions less than 50 cm(3) and located below the dome of the diaphragm. For lesions in this group, the mean SUV(max) difference, GTV volume change, shift in GTV centroid location, and concordance index were 21%, 154%, 2.4 mm, and 0.61, respectively. CONCLUSION This study benchmarked the differences between the PET data with and without artifacts. It is important to pay attention to the potential existence of these artifacts during GTV contouring, as such artifacts may increase the uncertainties in the lesion volume and the centroid location.

Average CT in PET studies of colorectal cancer patients with metastasis in the liver and esophageal cancer patients

Average CT (ACT) and PET have a similar temporal resolution and it has been shown to improve registration of the CT and PET data for PET/CT imaging of the thorax. The purpose of this study was to quantify the effect of ACT attenuation correction on PET for gross tumor volume (GTV) delineation with standardized uptake value (SUV) for liver and esophageal lesions. Our study included 48 colorectal cancer patients with metastasis in the liver and 52 esophageal cancer patients. These patients underwent a routine PET/CT scan followed by a cine CT scan of the thoracic region for ACT. Differences between the two PET data sets (PETHCT and PETACT) corrected with the helical CT (HCT) and ACT were quantified by analyzing image alignment, maximum SUV (SUVmax), and GTV. The 67% of the colorectal and 73% of the esophageal studies demonstrated misregistration between the PETHCT and HCT data. ACT was effective in removing misregistration artifacts in 65% of the misregisted colorectal and in 76% of the misregisted esophageal cancer patients. Misregistration between the CT and PET data affected GTVs due to the change in SUVmax with ACT. A change of SUVmax greater than 20% between PETHCT and PETACT was found in 15% of the colorectal and 17% of the esophageal cases. Our results demonstrated a more pronounced effect of misregistration for the smaller lesions (<5cm3) near the diaphragm (<5cm). ACT was effective in improving registration between the CT and PET data in PET/CT for the colorectal and esophageal cancer patients. PACS number: 87.58.Fg

SU‐FF‐J‐01: A Comparision of 4DCT with Breath‐Hold CT for Determination of Tumor Motion with Respiration

Purpose: Internal target volumes (ITVs) have been determined using both breath‐hold CT scans (BHCTs) and four‐dimensional CT (4DCT) to assess the extent of tumor motion during normal respiration. The purpose of this work is to compare the differences in tumor excursion when measured with BHCT and 4DCT. Method and Materials: All 4DCT and BHCT datasets in this study were acquired as a part of the radiotherapy simulation process using a commercial 4DCT system (Discovery ST, GE Healthcare, Waukesha, WI). Respiratory tracking was accomplished using a commercial system (RPM, Varian Medical Systems, Palo Alto, CA). A visual prompt from this system was displayed to patients to assist them in holding their breath at the correct level during BHCTs. The locations of the tumors with respect to a reference dataset (4DCT end‐expiration) was determined using a rigid‐body cross‐correlation algorithm that found the location on each dataset that best matched the region of the physician‐determined gross tumor volume (GTV) on the reference dataset. The patient did not move between the 4DCT and BHCT scans, thus differences in tumor location were due to tumor motion rather than bulk patient motion. Results: For 20 patients, the average difference in displacement of the GTV between BHCT and 4DCT scans was 5 mm at end‐inspiration and 3 mm at end‐expiration with maximum differences of 12 mm and 10 mm respectively. GTV motion on BHCTs was always greater than or equal to the motion on the 4DCT. The direction of tumor motion was also found to be different between 4DCT and BHCT images with the average difference in the vector angles being 14°. Conclusion: The results of this work suggests that patients being treated during normal breathing should be simulated during normal breathing (4DCT) and those to be treated using a breath‐hold technique should be simulated using BHCT.

SU-F-T-453: Improved Head and Neck SBRT Treatment Planning Using PlanIQ

PURPOSE Treatment planning for Head and Neck(HN) re-irradiation is a challenge because of ablative doses to target volume and strict critical structure constraints. PlanIQ(Sun Nuclear Corporation) can assess the feasibility of clinical goals and quantitatively measure plan quality. Here, we assess whether incorporation of PlanIQ in our SBRT treatment planning process can improve plan quality and planning efficiency. METHODS From 2013-2015, 35 patients (29 retrospective, 6 prospective) with recurrent HN tumors were treated with SBRT using VMAT treatment plans. The median prescription dose was 45 Gy in 5 fractions. We retrospectively reviewed the treatment plans and physician directives of our first 29 patients and generated score functions of the dosimetric goals used in our practice and obtained a baseline histogram. We then re-optimized 12 plans that had potential to further reduce organs-at-risk (OAR) doses according to PlanIQ feasibility DVH and plan quality analysis and compared them to the original plans. We applied our new PlanIQ-assisted planning process for our 6 most recently treated patients and evaluated the plan quality and planning efficiency. RESULTS The mean plan quality metric(PQM) and feasibility adjusted PQM(APQM) scores of our initial 29 treatment plans were 77.1±13.1 and 88.7±11.9, respectively (0-100 scale). The PQM and APQM scores for the 12 optimized plans improved from 75.9±11.0 and 85.1±10.2 to 80.7±9.3 and 90.2±8.0, respectively (p<0.005). Using our newly developed PlanIQ-assisted planning process, the PQM and APQM scores for the 6 most recently treated patients were 93.6±6.5 and 99.1±0.6, respectively. The planning goals were more straightforward to minimize OAR doses during optimization, thus less planning and revision time were used than before. CONCLUSION PlanIQ has the potential to provide achievable planning goals and also improve plan quality and planning efficiency.

SU-F-T-406: Verification of Total Body Irradiation Commissioned MU Lookup Table Accuracy Using Treatment Planning System for Wide Range of Patient Sizes

PURPOSE To verify the accuracy of total body irradiation (TBI) measurement commissioning data using the treatment planning system (TPS) for a wide range of patient separations. METHODS Our institution conducts TBI treatments with an 18MV photon beam at 380cm extended SSD using an AP/PA technique. Currently, the monitor units (MU) per field for patient treatments are determined using a lookup table generated from TMR measurements in a water phantom (75 × 41 × 30.5 cm3). The dose prescribed to an umbilicus midline point at spine level is determined based on patient separation, dose/ field and dose rate/MU. One-dimensional heterogeneous dose calculations from Pinnacle TPS were validated with thermoluminescent dosimeters (TLD) placed in an average adult anthropomorphic phantom and also in-vivo on four patients with large separations. Subsequently, twelve patients with various separations (17-47cm) were retrospectively analyzed. Computed tomography (CT) scans were acquired in the left and right decubitus positions from vertex to knee. A treatment plan for each patient was generated. The ratio of the lookup table MU to the heterogeneous TPS MU was compared. RESULTS TLD Measurements in the anthropomorphic phantom and large TBI patients agreed with Pinnacle calculated dose within 2.8% and 2%, respectively. The heterogeneous calculation compared to the lookup table agreed within 8.1% (ratio range: 1.014-1.081). A trend of reduced accuracy was observed when patient separation increases. CONCLUSION The TPS dose calculation accuracy was confirmed by TLD measurements, showing that Pinnacle can model the extended SSD dose without commissioning a special beam model for the extended SSD geometry. The difference between the lookup table and TPS calculation potentially comes from lack of scatter during commissioning when compared to extreme patient sizes. The observed trend suggests the need for development of a correction factor between the lookup table and TPS dose calculations.

SU-F-E-18: Training Monthly QA of Medical Accelerators: Illustrated Instructions for Self-Learning

PURPOSE To develop and test clear illustrated instructions for training of monthly mechanical QA of medical linear accelerators. METHODS Illustrated instructions were created for monthly mechanical QA with tolerance tabulated, and underwent several steps of review and refinement. Testers with zero QA experience were then recruited from our radiotherapy department (1 student, 2 computational scientists and 8 dosimetrists). The following parameters were progressively de-calibrated on a Varian C-series linac: Group A = gantry angle, ceiling laser position, X1 jaw position, couch longitudinal position, physical graticule position (5 testers); Group B = Group A + wall laser position, couch lateral and vertical position, collimator angle (3 testers); Group C = Group B + couch angle, wall laser angle, and optical distance indicator (3 testers). Testers were taught how to use the linac, and then used the instructions to try to identify these errors. A physicist observed each session, giving support on machine operation, as necessary. The instructions were further tested with groups of therapists, graduate students and physics residents at multiple institutions. We have also changed the language of the instructions to simulate using the instructions with non-English speakers. RESULTS Testers were able to follow the instructions. They determined gantry, collimator and couch angle errors within 0.4, 0.3, and 0.9degrees of the actual changed values, respectively. Laser positions were determined within 1mm, and jaw positions within 2mm. Couch position errors were determined within 2 and 3mm for lateral/longitudinal and vertical errors, respectively. Accessory positioning errors were determined within 1mm. ODI errors were determined within 2mm when comparing with distance sticks, and 6mm when using blocks, indicating that distance sticks should be the preferred approach for inexperienced staff. CONCLUSION Inexperienced users were able to follow these instructions, and catch errors within the criteria suggested by AAPM TG142 for linacs used for IMRT.

SU-F-T-419: Evaluation of PlanIQ Feasibility DVH as Planning Objectives for Skull Base SBRT Patients

PURPOSE PlanIQ(Sun Nuclear Corporation) can provide feasibility measures on organs-at-risk(OARs) around the target based on depth, local anatomy density and energy of radiation beam used. This study is to test and evaluate PlanIQ feasibility DVHs as optimization objectives in the treatment planning process, and to investigate the potential to use them in routine clinical cases to improve planning efficiency. METHODS Two to three arcs VMAT Treatment plans were generated in Pinnacle based on PlanIQ feasibility DVH for six skull base patients who previously treated with SBRT. The PlanIQ feasibility DVH for each OAR consists of four zones - impossible (at 100% target coverage), difficult, challenging and probable. Constrains to achieve DVH in difficult zone were used to start plan optimization. Further adjustment was made to improve coverage. The plan DVHs were compared to PlanIQ feasibility DVH to assess the dose received by 0%(D0), 5%(D5), 10%(D10) and 50%(D50) of the OAR volumes. RESULTS A total of 90 OARs were evaluated for 6 patients (mean 15 OARs, range 11-18 OARs). We used >98% PTV coverage as planning goal since its difficult to achieve 100% target coverage. For the generated plans, 96.7% of the OARs achieved D0 or D5 within difficult zone or impossible zone (ipsilateral OARs 93.5%, contralateral OARs 100%), while 90% and 65.6% of the OARs achieved D10 and D50 within difficult zone, respectively. Seventeen of the contralateral and out of field OARs achieved DVHs in impossible zone. For OARs adjacent or overlapped with target volume, the D0 and D5 are challenging to be optimized into difficult zone. All plans were completed within 2-4 adjustments to improve target coverage and uniformity. CONCLUSION PlanIQ feasibility tool has the potential to provide difficult but achievable initial optimization objectives and therefore reduce the planning time to obtain a well optimized plan.