J Wu

Do Tumors in the Lung Deform During Normal Respiration? An Image Registration Investigation

PURPOSEnThe purpose of this study was to investigate whether lung tumors may be described adequately using a rigid body assumption or whether they deform during normal respiration.nnnMETHODS AND MATERIALSnThirty patients with early stage non-small-cell lung cancer underwent four-dimensional (4D) computed tomography (CT) simulation. The gross tumor volume (GTV) was delineated on the 4D CT images. Image registration was performed in the vicinity of the GTV. The volume of interest for registration was the GTV and minimal volume of surrounding non-GTV tissue. Three types of registration were performed: translation only, translation + rotation, and deformable. The GTV contour from end-inhale was mapped to end-exhale using the registration-derived transformation field. The results were evaluated using three metrics: overlap index (OI), root-mean-squared distance (RMS), and Hausdorff distance (HD).nnnRESULTSnAfter translation only image registration, on average OI increased by 21.3%, RMS and HD reduced by 1.2 mm and 2.0 mm, respectively. The succeeding increases in OI after translation + rotation and deformable registration were 1.1% and 1.4% respectively. The succeeding reductions in RMS were 0.1 mm and 0.2 mm respectively. No reduction in HD was observed after translation + rotation and deformable image registration compared with translation only registration. The difference in the results from the three registration scenarios was independent of GTV size and motion amplitude.nnnCONCLUSIONSnThe primary effect of normal respiration on lung tumors was the translation of tumors. Rotation and deformation of lung tumors was determined to be minimal.

An evaluation of planning techniques for stereotactic body radiation therapy in lung tumors

PURPOSEnTo evaluate four planning techniques for stereotactic body radiation therapy (SBRT) in lung tumors.nnnMETHODS AND MATERIALSnFour SBRT plans were performed for 12 patients with stage I/II non-small-cell lung cancer under the following conditions: (1) conventional margins on free-breathing CT (plan 1), (2) generation of an internal target volume (ITV) using 4DCT with beam delivery under free-breathing conditions (plan 2), (3) gating at end-exhale (plan 3), and (4) gating at end-inhale (plan 4). Planning was performed following the RTOG 0236 protocol with a prescription dose of 54 Gy (3 fractions). For each plan 4D dose was calculated using deformable-image registration.nnnRESULTSnThere was no significant difference in tumor dose delivered by the 4 plans. However, compared with plan 1, plans 2-4 reduced total lung BED by 1.9+/-1.2, 3.1+/-1.6 and 3.5+/-2.1 Gy, reduced mean lung dose by 0.8+/-0.5, 1.5+/-0.8, and 1.6+/-1.0 Gy, reduced V20 by 1.5+/-1.0%, 2.7+/-1.4%, and 2.8+/-1.8%, respectively, with p<0.01. Compared with plan 2, plans 3-4 reduced lung BED by 1.2+/-1.0 and 1.6+/-1.5 Gy, reduced mean lung dose by 0.6+/-0.5 and 0.8+/-0.7 Gy, reduced V20 by 1.2+/-1.1% and 1.3+/-1.5%, respectively, with p<0.01. The differences in lung BED, mean dose and V20 of plan 4 compared with plan 3 were insignificant.nnnCONCLUSIONSnTumor dose coverage was statistically insignificant between all plans. However, compared with plan 1, plans 2-4 significantly reduced lung doses. Compared with plan 2, plan 3-4 also reduced lung toxicity. The difference in lung doses between plan 3 and plan 4 was not significant.

Effect of Ultrasound Probe on Dose Delivery During Real-time Ultrasound-Guided Tumor Tracking

Ultrasound is a noninvasive and less costly modality for real-time imaging of soft tissues. It has the capability of tracking soft tissue at levels of submillimeter precision even in the presence of radiation beams. The effect of a transducer on radiation dose is not fully known. The best imaging location for an ultrasound transducer happens to coincide with the path of an anterior-posterior beam in intensity modulated radiation therapy (IMRT). This study indicates a significant change in dose when this juxtaposition occurs. If the anterior-posterior beam is avoided in IMRT planning, however, the effect of the transducer on radiotherapy is found to be negligible

Dosimetric comparison of patient setup strategies in stereotactic body radiation therapy for lung cancer

PURPOSEnIn this work, the authors retrospectively compared the accumulated dose over the treatment course for stereotactic body radiation therapy (SBRT) of lung cancer for three patient setup strategies.nnnMETHODSnTen patients who underwent lung SBRT were selected for this study. At each fraction, patients were immobilized using a vacuum cushion and were CT scanned. Treatment plans were performed on the simulation CT. The planning target volume (PTV) was created by adding a 5-mm uniform margin to the internal target volume derived from the 4DCT. All plans were normalized such that 99% of the PTV received 60 Gy. The plan parameters were copied onto the daily CT images for dose recalculation under three setup scenarios: skin marker, bony structure, and soft tissue based alignments. The accumulated dose was calculated by summing the dose at each fraction along the trajectory of a voxel over the treatment course through deformable image registration of each CT with the planning CT. The accumulated doses were analyzed for the comparison of setup accuracy.nnnRESULTSnThe tumor volume receiving 60 Gy was 91.7 ± 17.9%, 74.1 ± 39.1%, and 99.6 ± 1.3% for setup using skin marks, bony structures, and soft tissue, respectively. The isodose line covering 100% of the GTV was 55.5 ± 7.1, 42.1 ± 16.0, and 64.3 ± 7.1 Gy, respectively. The corresponding average biologically effective dose of the tumor was 237.3 ± 29.4, 207.4 ± 61.2, and 258.3 ± 17.7 Gy, respectively. The differences in lung biologically effective dose, mean dose, and V20 between the setup scenarios were insignificant.nnnCONCLUSIONSnThe authors results suggest that skin marks and bony structure are insufficient for aligning patients in lung SBRT. Soft tissue based alignment is needed to match the prescribed dose delivered to the tumors.

TU‐E‐BRB‐05: Impact of Interfractional Tumor Motion on Respiratory Gated Stereotactic Body Radiation Therapy for Lung Tumors

PURPOSEnTo investigate the impact of interfractional tumor motion on dose delivery of gated lung SBRT.nnnMETHODSn4DCT scan for five lung patient was performed without breathing control at simulation and prior to each treatment. Gated treatment plans were performed on the end-exhale (50% phase) simulation CT with a 30% duty cycle. ITV was created by combining the GTVs at 40%, 50% and 60% phases. PTV was created by adding a 5 mm uniform margin to the ITV. All plans were normalized such that 60 Gy (3 fractions) was prescribed to the 85% isodose line. To calculate the accumulated dose over the treatment course, the original plan parameters were copied to the 40%, 50% and 60% CTs obtained prior to each treatment. In order to eliminate the effect of setup error to dose delivery, treatment isocenters at each fraction were determined by aligning the tumors on the slow CTs obtained prior to each treatment to that on the slow simulation CT. Doses recalculated on the 40% and 60% CTs at each fraction were warped through deformable CT image registration to their corresponding 50% CT to compose the 4D dose at that fraction. Those fractional 4D doses were warped to the 50% simulation CT to compose the accumulated 4D dose over the treatment course.nnnRESULTSnThe minimum tumor doses over the treatment course were 59.9, 45.1, 68.9, 41.9 and 47.8 Gy respectively. Tumor V60s were 99.7, 92.2, 100, 97.2 and 93.0% respectively. The corresponding mean lung doses were 3.8, 6.4, 3.7, 4.4 and 3.7 Gy respectively.nnnCONCLUSIONSnChange in tumor motion pattern over the treatment course results in tumor underdosing. Tight margins are normally used in lung SBRT. Therefore monitoring of the reproducibility of interfractional tumor motion is critical to the success of dose delivery.

Automatic prostate localization using elastic registration of planning CT and daily 3D ultrasound images

The prostate is known to move between daily fractions during the course of radiation therapy using external beams. This movement causes problem with 3D conformal or intensity-modulated radiation therapy, in which tight margins are used for treatment planning. To minimize the adverse effect of this motion on dose delivery, daily localization of the prostate with respect to the planning CT is necessary. Current ultrasound-based localization systems require manual alignment of ultrasound images with the planning CT. The resulting localization is subjective and has high interobserver variability. To reduce the alignment uncertainty and increase the setup efficiency, we proposed an automatic prostate alignment method using a volume subdivision-based elastic image registration algorithm. The algorithm uses normalized mutual information as the measure of image similarity between the daily 3D ultrasound images and the planning CT. The prostate contours on the CT are mapped to the ultrasound space by applying the transformation fields from image registration. The displacement of the center-of-mass of the mapped contours is calculated for automatic patient setup. For validation purposes, six experts independently and manually aligned the archived CT and 3D ultrasound images using the SonArray system and reported their readings as shifts along the three principal axes. The mean shift and standard deviation of the readings along each axis were calculated. We regarded the automatic alignment as being acceptable if the difference between the mean shift and the automatic shift is within two times the standard deviation. Three out of five patients were successfully aligned with two failures.

SU‐GG‐T‐570: Selection of CT Image Sets for Treatment Planning in Stereotactic Body Radiation Therapy of Lung Cancer

Purpose: This study investigated the impact on tumordose coverage of treatment plans performed on various CTimage sets in stereotactic body radiation therapy(SBRT) of lungcancer.Methods and Materials: Five patients underwent SBRT for lungcancers were retrospectively investigated. For each patient, a free breathing (FB) CT and a four‐dimensional (4D) CT were acquired. Based on the 4DCT scans, two post‐processing CTimages were reconstructed: average intensity projection CT (AIP) and a low pitch, slow‐scan CT (SCT). The gross target volumes (GTVs) were delineated on the 4DCT images and combined to create the internal target volume (ITV). The planning target volume (PTV) was created by adding a 5 mm margin to the ITV. Treatment plans were performed on the FB CT, ATP CT, and SCT. Plan quality was evaluated by calculating and comparing the 4D dose for each plan using deformable‐image registration. Results: No matter which CTimage sets were used in treatment planning,lungtumors always receive at least the prescribed dose. The average difference in tumor D100 (minimum dose received by 100% of the tumor) is 0.28±0.61Gy (p=0.363) between the plans performed on AIP CT and those on FB CT, 0.62±1.35Gy (p=0.379) between the plans performed on AIP CT and those on SCT, and 0.34±0.77Gy (p=0.379) between the plans performed on FB CT and those on SCT. As for the mean lungdose, the average difference is −0.07±0.15Gy (p=0.390), 0.07±0.11Gy (p=0.221), and 0.14±0.15Gy (p=0.107) respectively. For the total lung V20, the average difference is −0.19±0.26% (p=0.186), 0.13±0.25% (p=0.321), and 0.31±0.27% (p=0.06) respectively. Conclusions: The differences in tumor and lungdose coverage for treatment plans performed on ATP CT, FB CT and SCT are indistinguishable. Those three CTdata sets are equally well in term of dose coverage for treatment planning in lungSBRT.

SU-FF-J-173: Tumor Volume Delineation: A Comparison of Imaging Protocols for Lung Tumors

Purpose: To investigate differences in tumor contours delineated on free breathing (FB), four dimensional (4D), maximum intensity projection (MIP), average intensity projection (AIP), and slow‐CT (SCT) images.Methods and Materials: Data from ten patients who underwent stereotactic body radiation therapy of lungcancers were retrospectively investigated. For each patient, a FB CT and a 4D CT scan was acquired during simulation. Following the scan, MIP and AIP images were reconstructed. Since the 4D CT scan was acquired at a low pitch, a slow‐CT scan was also reconstructed. These scans were repeated prior to each treatment, resulting in 48 CTdata sets. The GTV was delineated on FB, MIP, AIP, and SCT images and was compared with the internal target volume (ITV), which comprised the union of GTVs delineated on each phase of the 4DCT. Three evaluation metrics were used for contour comparison: GTV volume, overlap index (OI), and root‐mean‐squared (RMS) distance. Results: On average GTVFB, GTVMIP, GTVAIP, and GTVSCT volumes were 0.66±0.13 (p<0.01), 0.88±0.08 (p<0.01), 0.64±0.13 (p<0.01), and 0.64±0.12 (p<0.01) times smaller than the ITV, respectively. The OI of the ITV with GTVFB, GTVMIP, GTVAIP, and GTVSCT was 60%, 81%, 61% and 64%, respectively. The average RMS distances of the GTVFB, GTVMIP, GTVAIP, and GTVSCT relative to the ITV were 0.44cm, 0.30cm, 0.45cm and 0.40cm, respectively. The OI of GTVFB with GTVMIP, GTVAIP, and GTVSCT was 62% 68% and 71%, respectively and the corresponding RMS distances were 0.34cm, 0.36cm and 0.35cm, respectively. Conclusions: ITV is statistically larger than GTVMIP, and they are both larger than GTVFB, GTVAIP and GTVSCT. Even though GTVFB, GTVAIP and GTVSCT have similar size, the surface mismatch among them is still distinguishable.

SU‐FF‐T‐563: Dosimetric Impact of Five Tumor Delineation Strategies in Stereotactic Body Radiation Therapy for Lung Cancer

Purpose: The goal of this study was to examine the dosimetric impact of five different tumor delineation strategies in stereotactic body radiation therapy(SBRT) for lungcancer patients. Methods and Materials: Seven patients who had previously undergone SBRT for lungcancer were retrospectively investigated. For each patient, a free breathing (FB) CT and a 4DCT were acquired. Based on the 4DCT scans, three post‐processing CTimages were reconstructed: maximum intensity projection (MIP), average intensity projection (AIP), and slow‐CT (SCT) images. The gross target volumes (GTVs) were delineated on the following CTimagedata sets: GTVFB on FB CT, GTV0%, GTV10%, …, GTV90% on 4DCT, GTVMIP on MIP CT, GTVAIP on AIP CT, and GTVSCT on SCT. The GTVs delineated on the 4DCT were combined to create the internal target volume (ITV). Five SBRT treatment plans were created on the FB CTimage based on the tumor delineated above: GTVFB, ITV, GTVMIP, GTVAIP, and GTVSCT. For each plan, the 4D dose was calculated using deformable‐image registration. Results: On average the tumor D100 (minimum dose received by 100% of the tumor) of the ITV and GTVMIP based plans is 3.0±4.0Gy (p=0.09) and 0.9±4.5Gy (p=0.61) respectively above that of the GTVFB based plan. While the tumor D100 of the GTVAIP and GTVSCT based plans is 2.8±6.0Gy (p=0.26) and 0.8±4.1Gy (p=0.61) below that of the GTVFB based plan respectively. Compared with the GTVFB based plan, total lung V20 of the ITV based plan is 0.4±1.0% (p=0.36) absolute higher, while that of the GTVMIP, GTVAIP, and GTVSCT based plans is 0.5±0.7% (p=0.09), 0.4±0.7% (p=0.17), and 0.2±0.5% (p=0.44) absolutely lower respectively. Conclusions: All plans can deliver equally well dose coverage to the tumor. The difference in lungdose among the five plans is also significantly small.

TH‐D‐AUD‐06: Investigation of Planning and Delivery Techniques for Stereotactic Body Radiation Therapy in Lung Tumors

Purpose:SBRTdose calculation must consider deforming and moving anatomy and heterogeneous tissues. The goal of this work is to calculate the delivereddose resulting from SBRTdelivery in lungcancer using elastic registration of 4DCT images.Materials and Methods: Five patients with lungcancer were selected for this retrospective study. Patients underwent a 4DCT simulation. Treatment plans were performed according to the RTOG 0236 with 54Gy delivered in 18Gy fractions. Four treatment plans were performed for each case: 1 free‐breathing using composite tumor volume determined from 4DCT 2 free‐breathing using a standard margin 3 gating at end‐exhale 4 gating at end‐inhale. Plans were normalized such that 54Gy was prescribed to 85% isodose line. The plan parameters were superimposed onto each 3DCT, the dose was recalculated using a convolution/superposition algorithm with consideration of heterogeneities. Using elastic image registration, each 3DCT was registered to the reference CT. The transformation fields were used to warp the recalculated dose to the reference CT. Warped doses were weighted by temporal probability to calculate the composite 4D dose.Results: After heterogeneity correction, dose received by 100% of the ITV increased by 8.57±3.04%. V10 and V20 of the lung increased by 3.71±0.57% and 11.12±4.09% respectively. For all of the plans, the PTV was large enough to compensate for tumor motion. Treatment plans created with the knowledge of tumor motion information used smaller margins, which reduced the ipsilateral lungdose. The EUD of the ipsilateral lung gated at end‐exhale was the lowest, which was 21.76±4.53% of the EUD of plan 2. Conclusions: Treatment plans using homogeneity assumption underestimated tumordose by about 9%. For all of the plans, PTV was large enough to compensate tumor motion. Plans including tumor motion information used smaller PTV, which could reduce the ipsilateral lungdose by about 22%.