- Email: [email protected]
Treatment Dose Evaluation of IGRT and Adaptive Plan Modifications for Head-and-Neck Cancer Radiation Therapy
Volume 87 Number 2S Supplement 2013
Poster Viewing Abstracts S695
For each case, two VMAT plans were created in a treatment planning system. The double arc (DA) plan was created using two arcs: first arc in an anti-clockwise direction (arc angle: 1 to 359 ; collimator angle: 170 ) and the second arc in a clockwise direction (arc angle: 359 to 1 ; collimator angle: 190 ). The single arc (SA) plan was created using a single arc in an anti-clockwise direction (arc angle: 1 to 359 ; collimator angle: 170 ), and the beam parameters in the SA plan were identical to that of first arc in the DA plan. In order to make fair comparisons between the DA and SA plans, same dose-volume constraints with appropriate weightings were used during the optimization processes for both sets of plans. Furthermore, the DA and SA plans were inversely optimized with an objective of achieving at least 95% of the planning target volume (PTV) receiving the prescription dose of 79.2 Gy in 44 fractions. All optimized plans were calculated with Anisotropic Analytical Algorithm. Radiobiological modeling response evaluation was done by calculating Niemierko’s Equivalent Uniform Dose (EUD)-based TCP and NTCP values. Results: For prostate tumor, the average EUD in the SA plans was slightly higher than in the DA plans (78.10 Gy vs 77.77 Gy; p Z 0.001), but the average TCP was comparable (98.30% vs 98.27%; p Z 0.001). In comparison to the DA plans, the SA plans produced higher average EUD to bladder (40.71 Gy vs 40.46 Gy; p Z 0.029) and femoral heads (10.39 Gy vs 9.40 Gy; p Z 0.029), whereas both techniques produced NTCP well below 0.1% for bladder (p Z 0.139) and femoral heads (p Z 0.263). In contrast, the SA plans produced higher average NTCP compared to the DA plans (2.21% vs 1.88%; p Z 0.001). Furthermore, the EUD to rectum was slightly higher in the SA plans (62.88 Gy vs 62.20 Gy; p Z 0.001). Conclusions: The SA and DA techniques produced similar TCP for lowrisk prostate cancer. The NTCP for femoral heads and bladder was comparable in the SA and DA plans; however, the SA technique resulted in higher NTCP for rectum in comparison with the DA technique. Author Disclosure: S. Rana: None. C. Cheng: None.
3290 Interobserver and Intraobserver Variability in Image Registration for Image Guided Radiation Therapy R. Murakami, Y. Fujita, N. Kai, Y. Nakaguchi, M. Maruyama, K. Kakei, T. Saito, and R. Toya; Kumamoto University, Kumamoto, Japan Purpose/Objective(s): Image-guided radiation therapy (IGRT) is widely used to determine and to correct the daily setup error. In high-precision radiation therapy (RT) with appropriate immobilization devices, 3D cone beam CT images acquired before each treatment are registered with the 3D planning CT images (3D-3D registration). Registration of orthogonal 2D planar images with digitally reconstructed radiographs from the planning CT data (2D-2D registration) on bony anatomy is widely available for daily setup in conventional RT with standard beam geometries, e.g., parallel opposed pair and four field box beams. Inter-fractional body changes including internal motions should be more frequent in conventional RT than high-precision RT. The purpose of this study was to assess the reproducibility of 2D-2D registration on bony anatomy in conventional RT. Materials/Methods: A total of 100 patients, each 20 patients treated with conventional RT at five anatomic sites consisting of the brain, head-andneck, chest, abdomen, and pelvis, were retrospectively chosen at random. The brain and head-and-neck were immobilized with thermoplastic masks,
Poster Viewing Abstract 3291; Table
Mean SD Pre-Tx Plan T1 (%) T2 (%) T3 (%)
(Gy) 70.7 0.4 0.8 0.8
Brain
Interobserver (O-R1) Mean SD 1.9 1.2 (mm) Range (mm) 0.0-4.2 Interobserver (O-R2) Mean SD 1.6 0.9 (mm) Range (mm) 0.0-3.2 Interobserver (R1-R2) Mean SD 1.6 0.8 (mm) Range (mm) 0.0-3.3
Head-andNeck
Chest
Abdomen
Pelvis
1.7 0.8
2.1 1.8
1.5 1.1
2.1 1.2
0.0-3.2
0.0-6.1
0.0-4.6
0.0-4.6
1.3 0.9
2.1 1.2
1.2 0.7
1.9 0.9
0.0-3.3
0.0-5.1
0.0-2.2
0.0-3.3
1.3 1.0
1.2 1.2
1.5 0.9
1.2 0.7
0.0-3.6
0.0-4.0
0.0-3.0
0.0-3.2
* Comparison among the 3 data sets of the original (O) and reviews (R1 and R2) in 2D-2D registration but the other sites were treated without specific immobilization devices. Daily 2D-2D registration had been achieved with a consensus by two radiation therapists. The original data set of image registration on Day 1 was selected for evaluation. A medical physicist reviewed the registration twice at a 1-month interval for each patient. Among three data sets of the original and reviews in 2D-2D registration, the differences were determined along the three directions and the 3D displacement vectors were calculated. Inter- and intra-observer differences were characterized by their mean values, standard deviations, and ranges. Results: The inter- and intra-observer displacement existed in each direction for each anatomic site. The mean 3D displacement vectors were approximately 1 - 2 mm in all setting for observers and anatomic sites. The shifts of larger magnitudes occurred more frequently for the chest; the maximum inter- and intra-observer shift was 6.1 mm and 4.0 mm, respectively (Table). Conclusions: The setup error derived from image registration should be recognized in the IGRT. The absence of precise immobilization should yield the uncertainty of image registration even on bony anatomy. Author Disclosure: R. Murakami: None. Y. Fujita: None. N. Kai: None. Y. Nakaguchi: None. M. Maruyama: None. K. Kakei: None. T. Saito: None. R. Toya: None.
3291 Treatment Dose Evaluation of IGRT and Adaptive Plan Modifications for Head-and-Neck Cancer Radiation Therapy K. Yang, J. Liang, L. Zhuang, and D. Yan; William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): To address how much extra improvement adaptive planning could provide on IGRT for H&N cancer. Materials/Methods: Pre-treatment planning CT and daily CBCT images of 19 H&N cancer patients were included in the study. For each patient, a pretreatment IMRT plan with 0 mm CTV to PTV margin, and prescription dose 70 Gy to the primary CTV1, while 59.85 Gy to nodal CTV2, in 35 fractions was created as the reference plan, and applied to each of the following 3 treatment techniques. T1. Daily CBCT guided position correction was mimicked using the rigid bony registration between the planning CT image and the daily CBCT images of each individual (without
CTV2 V110% (%)
0.5 0.8 0.5 0.5
Variability*
3D displacement vector in 2D-2D
Statistics of organ dose volume parameters
CTV1 D95%
Poster Viewing Abstract 3290; Table registration on bony anatomy
12.5 6.1 1.4 5.7
D95% (Gy)
1.4 5.6 3.6 2.5
60.3 0.9 0.1 0.1
0.4 1.1 0.4 0.2
Parotids V110%
Dmean
Dmean
(%)
Contralateral (Gy)
Ipsilateral (Gy)
23.4 7.3 0.7 0.0
5.5 5.3 3.7 3.0
15.3 5.3 2.4 8.9
1.3 4.3 3.6 6.2
27.2 10.2 4.3 0.7
6.2 5.3 5.4 4.8
Mandible
Cord
Brainstem
D1%
D1%
D1%
(Gy) 66.3 1.1 0.8 1.9
(Gy) 3.5 3.4 5.0 3.4
19.3 1.3 7.8 10
(Gy) 2.2 2.3 1.6 5.9
9.0 1.5 3.2 4.2
0.9 3.5 3.9 5.7
S696
International Journal of Radiation Oncology Biology Physics
considering correction residual and intra-treatment motion); T2. Daily IGRT (T1), and additional IMRT plans created with the same treatment prescription and target margin on a single daily CBCT image obtained at the plan modification days, the 11th and 21st treatment day respectively; T3. Daily IGRT (T1), and additional adaptive IMRT plans created with the same prescription using all daily CBCTs obtained, to date, at the plan modification days. Treatment cumulative dose was reconstructed for each technique using the deformable image registration on each daily CBCT image. For T1, the pre-treatment plan was applied to all treatment days of each patient, while for T2 and T3, the pre-treatment plan was applied on the first 10 days, and then the additional plans were applied for the remaining treatment days. Treatment dose/volume parameters to each organ of interest were used in the evaluation, which included the D95% and V110% for CTV1 and CTV2, Dmean for parotids, and D1% for cord, brain stem and mandible. Results: Study of 6 patients has been, so far, completed. Table shows the statistics of organ dose volume parameters of the reference plan, as well as the corresponding %discrepancy obtained from the treatment dose constructed using the daily CBCT images. The daily IGRT maintains the prescription dose, but the target dose heterogeneity, as well as parotids dose, were largely increased. Extra plan modification improved all criteria. Specifically, the adaptive technique (T3) had largest improvement in target dose heterogeneity, parotids and cord dose reduction. Conclusions: IGRT enables to minimize target margin, but with increasing target dose heterogeneity and parotids dose. With extra adaptive plan modification, all plan criteria can be largely improved or maintained as those in the pre-treatment planning. Acknowledgment: This study is partially supported by Elekta R & D Grant. Author Disclosure: K. Yang: None. J. Liang: None. L. Zhuang: None. D. Yan: None.
surfaces defined by the 2 dosimetrists. CT2 and associated ROIs were deformably registered to CT1. Registration accuracy was quantified by calculating the average separation between CT1 and deformed CT2 ROI surfaces. Registration accuracy was then compared to interobserver variation. Mean differences were tested for statistical significance using 1tailed paired t-tests (a Z 0.05). Results: Average separation for deformable image registration and interobserver variation are shown in the table. Deformable image registration yielded separation slightly larger than interobserver variation but was generally less than 2 mm and was not significantly greater than interobserver variation for 7 of 9 ROIs. The separation for the reference vertebral bodies was 0.5 0.3 mm. Conclusions: Average separation for deformable image registration and interobserver variation are shown in the Table. Deformable image registration yielded separation slightly larger than interobserver variation but was generally less than 2 mm and was not significantly greater than interobserver variation for 7 of 9 ROIs. The separation for the reference vertebral bodies was 0.5 0.3 mm. Author Disclosure: A.C. Riegel: None. J. Antone: None. H. Zhang: None. A. Bergamo: None. A. Kapur: None. L. Potters: None.
3292
Purpose/Objective(s): The hippocampus (HPC) is believed to play a considerable role in the neurocognitive functions such as learning, memory, and spatial processing. The HPC is at relatively high risk for injury associated with the use of radiation, even with intensity-modulated radiation therapy (IMRT) for patient with locally advanced (T3 or T4) nasopharyngeal carcinoma (NPC). Despite this risk, the HPC has not been routinely defined as an organ at risk (OAR) for IMRT treatment planning. The objective of this study was to demonstrate the dosimetric characteristics of IMRT plans with or without HPC sparing for NPC. Materials/Methods: Eight patients with either T3 or T4 NPC were selected for this study. The planning target volumes (PTVs) and OARs were contoured based on contrast enhanced T1 MRI that was fused to the planning CT. According to Radiation Therapy Oncology Group (RTOG) protocol (0615), 70.0 Gy and 59.4 Gy were prescribed to the planning target volumes, PTV-70 and PTV-59.4, respectively, in daily dose of 2.12 Gy and 1.8 Gy. The dose constraints for the OARs other than the HPC were applied according to RTOG 0615 for optimization. A treatment planning system was used to generate 9-field step-and-shoot IMRT plans with and without the HPC sparing. The dose-volume histograms were compared for the PTVs and OARs. Results: All plans achieved the protocol dose criteria. The homogeneity index, conformity index, and coverage index for the PTV-70 and PTV-59.4 were not significantly compromised by avoidance of the HPC. The doses to all OARs excluding the HPC were similar. The mean HPC volume ranged from 2.8 to 3.5 cm3 (median; 3.15 0.24 cm3). In the non-HPC sparing plans, the dose parameters of Dmax, D2%, Dmedian, D98% and Dmin for the HPC were 54.34 6.83 Gy, 47.62 8 Gy, 21.43 3.86 Gy, 7.85 2 Gy and 6.94 2.1 Gy, respectively as compared to those of 38.345 8.61 Gy, 31.85 4.98 Gy, 12.49 1.31 Gy, 6.79 1.25 Gy, and 5.74 1.34 Gy in the HPC sparing plans. The percentage volumes of HPC exposed to the radiation such as V10, V15, V20, V30, and V40 in non-HPC sparing plans were 89.45 8.36%, 65.5 3.02%, 52.81 6.87%, 32.39 12.4%, and 12.16 6.7% as compared to those of 71.43 12.15%, 33.73 16.2%, 15.56 6.79%, 3.9 2.43%, and 0.44 0.8% observed in the HPCsparing IMRT. Both the dose and volume parameters for the HPC were significantly higher in the non-HPC sparing plans (p < 0.05), expect for Dmin (p Z 0.06).
Deformable Image Registration Accuracy in the Head and Neck Region: Comparison With Interobserver Variation A.C. Riegel, J. Antone, H. Zhang, A. Bergamo, A. Kapur, and L. Potters; North Shore LIJ Health System, New Hyde Park, NY Purpose/Objective(s): Deformable image registration is a useful tool for aligning image sets with many degrees of freedom where rigid registration is inadequate. Deformation maps can be particularly useful when applied to linked data sets, such as PET imaging paired with attenuation correction CT or dose matrices paired with previous CT simulations. The current work compares the accuracy of a commercially available deformable image registration algorithm to interobserver variation for 30 head-andneck cancer patients. Materials/Methods: Thirty head-and-neck cancer patients who received CT simulation and diagnostic PET/CT were retrospectively included. Two dosimetrists contoured regions of interest (ROIs) including spinal cord, brainstem, larynx, parotids, eyes, and optic nerves on the simulation CT (CT1) and attenuation correction CT (CT2) from the PET/CT scan. Vertebral bodies C2-C4 were auto-contoured using a 100 HU threshold as an observer-independent reference. Interobserver variation was quantified by calculating the average separation in millimeters between the ROI
Poster Viewing Abstract 3292; Table interobserver variation Region of interest Brainstem Spinal cord Left eye Right eye Larynx Left optic nerve Right optic nerve Left parotid Right parotid
Interobserver variation (mm) 0.4 0.5 0.2 0.3 1.0 1.7 1.4 0.8 0.8
0.1 0.2 0.1 0.1 0.4 0.5 0.4 0.5 0.8
Deformable registration accuracy vs Deformable registration accuracy (mm)
P value (1-tailed)
< .01 .15 < .01 < .01 .49 .37 .07 .42 .67
0.8 0.6 0.4 0.4 1.0 1.7 1.7 0.8 0.7
0.5 0.2 0.2 0.2 1.0 0.8 0.9 0.6 0.2
3293 A Dosimetric Study of Intensity Modulated Radiation Therapy for Locally-Advanced Nasopharyngeal Carcinoma With or Without Hippocampal Sparing G. Han,1 D. Liu,1 H. Gan,2 S. Li,2 Z. Wang,1 W. Tan,1 W. Zhen,2 and D. Hu1; 1Department of Radiation Oncology, Hubei Cancer Hospital, Wuhan, China, 2Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE
Poster Viewing Abstracts S695
For each case, two VMAT plans were created in a treatment planning system. The double arc (DA) plan was created using two arcs: first arc in an anti-clockwise direction (arc angle: 1 to 359 ; collimator angle: 170 ) and the second arc in a clockwise direction (arc angle: 359 to 1 ; collimator angle: 190 ). The single arc (SA) plan was created using a single arc in an anti-clockwise direction (arc angle: 1 to 359 ; collimator angle: 170 ), and the beam parameters in the SA plan were identical to that of first arc in the DA plan. In order to make fair comparisons between the DA and SA plans, same dose-volume constraints with appropriate weightings were used during the optimization processes for both sets of plans. Furthermore, the DA and SA plans were inversely optimized with an objective of achieving at least 95% of the planning target volume (PTV) receiving the prescription dose of 79.2 Gy in 44 fractions. All optimized plans were calculated with Anisotropic Analytical Algorithm. Radiobiological modeling response evaluation was done by calculating Niemierko’s Equivalent Uniform Dose (EUD)-based TCP and NTCP values. Results: For prostate tumor, the average EUD in the SA plans was slightly higher than in the DA plans (78.10 Gy vs 77.77 Gy; p Z 0.001), but the average TCP was comparable (98.30% vs 98.27%; p Z 0.001). In comparison to the DA plans, the SA plans produced higher average EUD to bladder (40.71 Gy vs 40.46 Gy; p Z 0.029) and femoral heads (10.39 Gy vs 9.40 Gy; p Z 0.029), whereas both techniques produced NTCP well below 0.1% for bladder (p Z 0.139) and femoral heads (p Z 0.263). In contrast, the SA plans produced higher average NTCP compared to the DA plans (2.21% vs 1.88%; p Z 0.001). Furthermore, the EUD to rectum was slightly higher in the SA plans (62.88 Gy vs 62.20 Gy; p Z 0.001). Conclusions: The SA and DA techniques produced similar TCP for lowrisk prostate cancer. The NTCP for femoral heads and bladder was comparable in the SA and DA plans; however, the SA technique resulted in higher NTCP for rectum in comparison with the DA technique. Author Disclosure: S. Rana: None. C. Cheng: None.
3290 Interobserver and Intraobserver Variability in Image Registration for Image Guided Radiation Therapy R. Murakami, Y. Fujita, N. Kai, Y. Nakaguchi, M. Maruyama, K. Kakei, T. Saito, and R. Toya; Kumamoto University, Kumamoto, Japan Purpose/Objective(s): Image-guided radiation therapy (IGRT) is widely used to determine and to correct the daily setup error. In high-precision radiation therapy (RT) with appropriate immobilization devices, 3D cone beam CT images acquired before each treatment are registered with the 3D planning CT images (3D-3D registration). Registration of orthogonal 2D planar images with digitally reconstructed radiographs from the planning CT data (2D-2D registration) on bony anatomy is widely available for daily setup in conventional RT with standard beam geometries, e.g., parallel opposed pair and four field box beams. Inter-fractional body changes including internal motions should be more frequent in conventional RT than high-precision RT. The purpose of this study was to assess the reproducibility of 2D-2D registration on bony anatomy in conventional RT. Materials/Methods: A total of 100 patients, each 20 patients treated with conventional RT at five anatomic sites consisting of the brain, head-andneck, chest, abdomen, and pelvis, were retrospectively chosen at random. The brain and head-and-neck were immobilized with thermoplastic masks,
Poster Viewing Abstract 3291; Table
Mean SD Pre-Tx Plan T1 (%) T2 (%) T3 (%)
(Gy) 70.7 0.4 0.8 0.8
Brain
Interobserver (O-R1) Mean SD 1.9 1.2 (mm) Range (mm) 0.0-4.2 Interobserver (O-R2) Mean SD 1.6 0.9 (mm) Range (mm) 0.0-3.2 Interobserver (R1-R2) Mean SD 1.6 0.8 (mm) Range (mm) 0.0-3.3
Head-andNeck
Chest
Abdomen
Pelvis
1.7 0.8
2.1 1.8
1.5 1.1
2.1 1.2
0.0-3.2
0.0-6.1
0.0-4.6
0.0-4.6
1.3 0.9
2.1 1.2
1.2 0.7
1.9 0.9
0.0-3.3
0.0-5.1
0.0-2.2
0.0-3.3
1.3 1.0
1.2 1.2
1.5 0.9
1.2 0.7
0.0-3.6
0.0-4.0
0.0-3.0
0.0-3.2
* Comparison among the 3 data sets of the original (O) and reviews (R1 and R2) in 2D-2D registration but the other sites were treated without specific immobilization devices. Daily 2D-2D registration had been achieved with a consensus by two radiation therapists. The original data set of image registration on Day 1 was selected for evaluation. A medical physicist reviewed the registration twice at a 1-month interval for each patient. Among three data sets of the original and reviews in 2D-2D registration, the differences were determined along the three directions and the 3D displacement vectors were calculated. Inter- and intra-observer differences were characterized by their mean values, standard deviations, and ranges. Results: The inter- and intra-observer displacement existed in each direction for each anatomic site. The mean 3D displacement vectors were approximately 1 - 2 mm in all setting for observers and anatomic sites. The shifts of larger magnitudes occurred more frequently for the chest; the maximum inter- and intra-observer shift was 6.1 mm and 4.0 mm, respectively (Table). Conclusions: The setup error derived from image registration should be recognized in the IGRT. The absence of precise immobilization should yield the uncertainty of image registration even on bony anatomy. Author Disclosure: R. Murakami: None. Y. Fujita: None. N. Kai: None. Y. Nakaguchi: None. M. Maruyama: None. K. Kakei: None. T. Saito: None. R. Toya: None.
3291 Treatment Dose Evaluation of IGRT and Adaptive Plan Modifications for Head-and-Neck Cancer Radiation Therapy K. Yang, J. Liang, L. Zhuang, and D. Yan; William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): To address how much extra improvement adaptive planning could provide on IGRT for H&N cancer. Materials/Methods: Pre-treatment planning CT and daily CBCT images of 19 H&N cancer patients were included in the study. For each patient, a pretreatment IMRT plan with 0 mm CTV to PTV margin, and prescription dose 70 Gy to the primary CTV1, while 59.85 Gy to nodal CTV2, in 35 fractions was created as the reference plan, and applied to each of the following 3 treatment techniques. T1. Daily CBCT guided position correction was mimicked using the rigid bony registration between the planning CT image and the daily CBCT images of each individual (without
CTV2 V110% (%)
0.5 0.8 0.5 0.5
Variability*
3D displacement vector in 2D-2D
Statistics of organ dose volume parameters
CTV1 D95%
Poster Viewing Abstract 3290; Table registration on bony anatomy
12.5 6.1 1.4 5.7
D95% (Gy)
1.4 5.6 3.6 2.5
60.3 0.9 0.1 0.1
0.4 1.1 0.4 0.2
Parotids V110%
Dmean
Dmean
(%)
Contralateral (Gy)
Ipsilateral (Gy)
23.4 7.3 0.7 0.0
5.5 5.3 3.7 3.0
15.3 5.3 2.4 8.9
1.3 4.3 3.6 6.2
27.2 10.2 4.3 0.7
6.2 5.3 5.4 4.8
Mandible
Cord
Brainstem
D1%
D1%
D1%
(Gy) 66.3 1.1 0.8 1.9
(Gy) 3.5 3.4 5.0 3.4
19.3 1.3 7.8 10
(Gy) 2.2 2.3 1.6 5.9
9.0 1.5 3.2 4.2
0.9 3.5 3.9 5.7
S696
International Journal of Radiation Oncology Biology Physics
considering correction residual and intra-treatment motion); T2. Daily IGRT (T1), and additional IMRT plans created with the same treatment prescription and target margin on a single daily CBCT image obtained at the plan modification days, the 11th and 21st treatment day respectively; T3. Daily IGRT (T1), and additional adaptive IMRT plans created with the same prescription using all daily CBCTs obtained, to date, at the plan modification days. Treatment cumulative dose was reconstructed for each technique using the deformable image registration on each daily CBCT image. For T1, the pre-treatment plan was applied to all treatment days of each patient, while for T2 and T3, the pre-treatment plan was applied on the first 10 days, and then the additional plans were applied for the remaining treatment days. Treatment dose/volume parameters to each organ of interest were used in the evaluation, which included the D95% and V110% for CTV1 and CTV2, Dmean for parotids, and D1% for cord, brain stem and mandible. Results: Study of 6 patients has been, so far, completed. Table shows the statistics of organ dose volume parameters of the reference plan, as well as the corresponding %discrepancy obtained from the treatment dose constructed using the daily CBCT images. The daily IGRT maintains the prescription dose, but the target dose heterogeneity, as well as parotids dose, were largely increased. Extra plan modification improved all criteria. Specifically, the adaptive technique (T3) had largest improvement in target dose heterogeneity, parotids and cord dose reduction. Conclusions: IGRT enables to minimize target margin, but with increasing target dose heterogeneity and parotids dose. With extra adaptive plan modification, all plan criteria can be largely improved or maintained as those in the pre-treatment planning. Acknowledgment: This study is partially supported by Elekta R & D Grant. Author Disclosure: K. Yang: None. J. Liang: None. L. Zhuang: None. D. Yan: None.
surfaces defined by the 2 dosimetrists. CT2 and associated ROIs were deformably registered to CT1. Registration accuracy was quantified by calculating the average separation between CT1 and deformed CT2 ROI surfaces. Registration accuracy was then compared to interobserver variation. Mean differences were tested for statistical significance using 1tailed paired t-tests (a Z 0.05). Results: Average separation for deformable image registration and interobserver variation are shown in the table. Deformable image registration yielded separation slightly larger than interobserver variation but was generally less than 2 mm and was not significantly greater than interobserver variation for 7 of 9 ROIs. The separation for the reference vertebral bodies was 0.5 0.3 mm. Conclusions: Average separation for deformable image registration and interobserver variation are shown in the Table. Deformable image registration yielded separation slightly larger than interobserver variation but was generally less than 2 mm and was not significantly greater than interobserver variation for 7 of 9 ROIs. The separation for the reference vertebral bodies was 0.5 0.3 mm. Author Disclosure: A.C. Riegel: None. J. Antone: None. H. Zhang: None. A. Bergamo: None. A. Kapur: None. L. Potters: None.
3292
Purpose/Objective(s): The hippocampus (HPC) is believed to play a considerable role in the neurocognitive functions such as learning, memory, and spatial processing. The HPC is at relatively high risk for injury associated with the use of radiation, even with intensity-modulated radiation therapy (IMRT) for patient with locally advanced (T3 or T4) nasopharyngeal carcinoma (NPC). Despite this risk, the HPC has not been routinely defined as an organ at risk (OAR) for IMRT treatment planning. The objective of this study was to demonstrate the dosimetric characteristics of IMRT plans with or without HPC sparing for NPC. Materials/Methods: Eight patients with either T3 or T4 NPC were selected for this study. The planning target volumes (PTVs) and OARs were contoured based on contrast enhanced T1 MRI that was fused to the planning CT. According to Radiation Therapy Oncology Group (RTOG) protocol (0615), 70.0 Gy and 59.4 Gy were prescribed to the planning target volumes, PTV-70 and PTV-59.4, respectively, in daily dose of 2.12 Gy and 1.8 Gy. The dose constraints for the OARs other than the HPC were applied according to RTOG 0615 for optimization. A treatment planning system was used to generate 9-field step-and-shoot IMRT plans with and without the HPC sparing. The dose-volume histograms were compared for the PTVs and OARs. Results: All plans achieved the protocol dose criteria. The homogeneity index, conformity index, and coverage index for the PTV-70 and PTV-59.4 were not significantly compromised by avoidance of the HPC. The doses to all OARs excluding the HPC were similar. The mean HPC volume ranged from 2.8 to 3.5 cm3 (median; 3.15 0.24 cm3). In the non-HPC sparing plans, the dose parameters of Dmax, D2%, Dmedian, D98% and Dmin for the HPC were 54.34 6.83 Gy, 47.62 8 Gy, 21.43 3.86 Gy, 7.85 2 Gy and 6.94 2.1 Gy, respectively as compared to those of 38.345 8.61 Gy, 31.85 4.98 Gy, 12.49 1.31 Gy, 6.79 1.25 Gy, and 5.74 1.34 Gy in the HPC sparing plans. The percentage volumes of HPC exposed to the radiation such as V10, V15, V20, V30, and V40 in non-HPC sparing plans were 89.45 8.36%, 65.5 3.02%, 52.81 6.87%, 32.39 12.4%, and 12.16 6.7% as compared to those of 71.43 12.15%, 33.73 16.2%, 15.56 6.79%, 3.9 2.43%, and 0.44 0.8% observed in the HPCsparing IMRT. Both the dose and volume parameters for the HPC were significantly higher in the non-HPC sparing plans (p < 0.05), expect for Dmin (p Z 0.06).
Deformable Image Registration Accuracy in the Head and Neck Region: Comparison With Interobserver Variation A.C. Riegel, J. Antone, H. Zhang, A. Bergamo, A. Kapur, and L. Potters; North Shore LIJ Health System, New Hyde Park, NY Purpose/Objective(s): Deformable image registration is a useful tool for aligning image sets with many degrees of freedom where rigid registration is inadequate. Deformation maps can be particularly useful when applied to linked data sets, such as PET imaging paired with attenuation correction CT or dose matrices paired with previous CT simulations. The current work compares the accuracy of a commercially available deformable image registration algorithm to interobserver variation for 30 head-andneck cancer patients. Materials/Methods: Thirty head-and-neck cancer patients who received CT simulation and diagnostic PET/CT were retrospectively included. Two dosimetrists contoured regions of interest (ROIs) including spinal cord, brainstem, larynx, parotids, eyes, and optic nerves on the simulation CT (CT1) and attenuation correction CT (CT2) from the PET/CT scan. Vertebral bodies C2-C4 were auto-contoured using a 100 HU threshold as an observer-independent reference. Interobserver variation was quantified by calculating the average separation in millimeters between the ROI
Poster Viewing Abstract 3292; Table interobserver variation Region of interest Brainstem Spinal cord Left eye Right eye Larynx Left optic nerve Right optic nerve Left parotid Right parotid
Interobserver variation (mm) 0.4 0.5 0.2 0.3 1.0 1.7 1.4 0.8 0.8
0.1 0.2 0.1 0.1 0.4 0.5 0.4 0.5 0.8
Deformable registration accuracy vs Deformable registration accuracy (mm)
P value (1-tailed)
< .01 .15 < .01 < .01 .49 .37 .07 .42 .67
0.8 0.6 0.4 0.4 1.0 1.7 1.7 0.8 0.7
0.5 0.2 0.2 0.2 1.0 0.8 0.9 0.6 0.2
3293 A Dosimetric Study of Intensity Modulated Radiation Therapy for Locally-Advanced Nasopharyngeal Carcinoma With or Without Hippocampal Sparing G. Han,1 D. Liu,1 H. Gan,2 S. Li,2 Z. Wang,1 W. Tan,1 W. Zhen,2 and D. Hu1; 1Department of Radiation Oncology, Hubei Cancer Hospital, Wuhan, China, 2Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE