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348 oral Evaluation of two methods for soft tissue registration in online CT guided prostate cancer radiotherapy
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rotated to a new, average position. To estimate the remaining systematic error for this new prostate position, the geometric overlap between this new position and the average treatment position was considered as a function of the margins used to expand the prostate in the new position. To determine an average rectum shape, a method was used which unfolds the rectal wall at the dorsal side and maps the coordinates of the rectal wall onto a 2D map. A new, average rectum was obtained by averaging the coordinate maps of the pCT and the first four repeat scans.
Results: With ART a uniform margin of 4 mm was needed to obtain an average overlap of 99.8%. If the margin in the AP direction, i.e. in the direction of the rectum, was decreased to 2 ram, the average overlap dropped to 99.2%. Without ART a margin of 7 mm was required to obtain an average overlap of 99.0%. Average rectums were successfully generated. For a few patients a very small overlap was observed with the average prostate. This small overlap does not hamper IMRT treatment planning. Conclusions: We have successfully developed methods to obtain an average position of the prostate and an average shape of the rectum for the ART process. Towards the rectum, the CTV-to-PTV margin for organ motion can be reduced by 3-5 mm when using ART. In the near future dose distributions will be considered. 348 oral
Evaluation of two methods for soft tissue registration in online CT guided prostate cancer radiotherapy
G. Ivaldi 1'2, Q. WJ, J. Liang ~, D. Lockman ~, D. Yan ~ ~William Beaumont Hospital, Radiation Oncology, Royal Oak, Michigan, U.S.A. 2European Institute of Oncology, Radiation Oncology, Milan, Italy Purpose/Objectives: We are conducting a feasibility study for CT-guided hypo-fractionation of prostate cancer. Patients will receive a CT scan on our cone-beam CT-equipped linac prior to each treatment, the displacement of the prostate gland will be determined, and corrected accordingly. Our purpose is to evaluate two methods of soft tissue registration. Materials/Method: We used serial CT data from 28 prostate cancer patients. After the planning CT, each patient had a pelvic CT scan bi-weekly for a total of 15 CTs. To eliminate the setup error component of the inter-fraction geometric variation, each daily CT was fused with the planning CT based on bony anatomy. The prostate was contoured on each scan by a single physician. Subsequently prostatic image-based registration (IBR) was performed by translating and rotating each daily prostate CT image to match the reference CT prostate contour. The registration was then repeated, by the same investigator, using only the contour of the daily prostate (CBR). Each registration induces a transformation (dx,dy,dz,®) that characterizes the motion of the daily prostate per a rigid body formalism, where ® is the rotation in the sagittal plane. We assessed the IBR accuracy and stability by comparing each parameter of the 420 transformations between IBR and CBR. Results: While both CBR and IBR have uncertainties, we adopted the CBR as a reference both because there is a greater body of experience in contour definition than in soft tissue registration, and because there is less ambiguity in performing the CBR. The average shift magnitude from the AP perspective was 0.18+0.18cm for CBR and 0.12+0.13cm for IBR. From the lateral perspective, the results for the CBR and IBR are 0.37_+0.30cm and 0.25_+0.25cm, respectively. The analysis shows a larger shift in the AP direction then in any other direction, both in the IBR and CBR, a result consistent
Wednesday, October 27, 2004
$157
with previous studies. The average rotation angle ®, was 0.7+2.0 ° for the CBR and 1.3+2.4 ° for the IBR. The per-patient average discrepancy (A dx,A dy,A dz) between the CBR and IBR is within l m m for all but 6 patients in the LAT direction, but have more variation for the AP and SI directions. In particular, the IBR indicates a larger posterior shift of the patient than does the CBR. The composite result over all patients reiterates these observations: (A dx, A dy, A d z ) = (0.04+0.12, 0.16_+0.22, 0.0_+0.26) cm. The CBR-IBR agreement was within 2mm for 69% of results, between 2 and 4mm for 23%, and greater than 4mm for 8%.
Conclusions: The IBR showed an overestimation of the sagittal rotation angle and an underestimation of the shift magnitude for both the AP and LAT perspectives respect to the CBR. The discrepancy between IBR and CBR is a measure of the intraobserver uncertainty and require offline CBR verification of an online IBR-based correction. The use of on board KV conebeam device, capable of good quality images, will possibly narrow the gap observed between IBR and CBR. 349 oral Three-dimensional l h MR spectroscopic imaging of prostate: time-course assessment of metabolic response post-brachytherapy
A. Damyanovich 1, J. Jezioranskf, D. Taussky2, A. Amirabadi 1, A. Kirilova 1, J. Crook2 ~Princess Margaret Hospital, Radiation Physics, Toronto, Canada 2princess Margaret Hospital, Radiation Oncology, Toronto, Canada Introduction: Proton magnetic resonance spectroscopic imaging (MRSI) of prostate, is able to distinguish between areas of cancer and normal prostatic epithelium through differences in the endogenous [choline+creatine]/citrate ratios Post treatment, MRSI can provide a non invasive measure of intracellular metabolic conditions that may accurately identify viable malignant tissue within the treated prostate. The purpose of this study was to establish and monitor the metabolic profile of prostate pre- and post treatment following brachytherapy with 1251 seeds for the treatment of prostate cancer, using proton MRSI. Results obtained thus far are presented and discussed. Materials and Methods: Twenty patients undergoing prostate brachytherapy using the modified peripheral loading technique ( > 100 1251seeds) were invited to take part in this study, following a histologic diagnosis of prostate cancer. A baseline MRSI scan was performed prior to the TRUS prostate mapping to plan the implant; subsequent MRSI scans followed in 6-month intervals over a two-year period following implant. MRSI scans were performed using an endorectal coil and a GE 1.5T MRI scanner. 3D-MRSI data were acquired using the Prostate Spectroscopy Examination pulse sequence with a TE/TR = 130/1500 ms in order to most clearly resolve the choline/creatine and citrate resonances, while reducing the intensity of fat. Magnetic field homogeneity at the site of the prostate was optimized through the use of higher-order gradient shimming. The whole volume of the prostate was covered by the spectroscopic imaging grid consisting of 8 (S/I) x 16 (R/L) x 8 (A/P) voxels (total: 1024). Analysis was done voxel-by-voxet for each MRSI acquisition by assigning to each voxel a score based on the [choline+creatine]/citrate ratio, and the choline signal-tonoise. :Results: We were able to characterize the metabolic profile of patients pre-implant and monitor changes therein over time post-implant, using MRSI. Although individual metabolic response over time was variable, the overall MRSI-based score demonstrated a decreasing trend over time, consistent with PSA measurements acquired concurrently
rotated to a new, average position. To estimate the remaining systematic error for this new prostate position, the geometric overlap between this new position and the average treatment position was considered as a function of the margins used to expand the prostate in the new position. To determine an average rectum shape, a method was used which unfolds the rectal wall at the dorsal side and maps the coordinates of the rectal wall onto a 2D map. A new, average rectum was obtained by averaging the coordinate maps of the pCT and the first four repeat scans.
Results: With ART a uniform margin of 4 mm was needed to obtain an average overlap of 99.8%. If the margin in the AP direction, i.e. in the direction of the rectum, was decreased to 2 ram, the average overlap dropped to 99.2%. Without ART a margin of 7 mm was required to obtain an average overlap of 99.0%. Average rectums were successfully generated. For a few patients a very small overlap was observed with the average prostate. This small overlap does not hamper IMRT treatment planning. Conclusions: We have successfully developed methods to obtain an average position of the prostate and an average shape of the rectum for the ART process. Towards the rectum, the CTV-to-PTV margin for organ motion can be reduced by 3-5 mm when using ART. In the near future dose distributions will be considered. 348 oral
Evaluation of two methods for soft tissue registration in online CT guided prostate cancer radiotherapy
G. Ivaldi 1'2, Q. WJ, J. Liang ~, D. Lockman ~, D. Yan ~ ~William Beaumont Hospital, Radiation Oncology, Royal Oak, Michigan, U.S.A. 2European Institute of Oncology, Radiation Oncology, Milan, Italy Purpose/Objectives: We are conducting a feasibility study for CT-guided hypo-fractionation of prostate cancer. Patients will receive a CT scan on our cone-beam CT-equipped linac prior to each treatment, the displacement of the prostate gland will be determined, and corrected accordingly. Our purpose is to evaluate two methods of soft tissue registration. Materials/Method: We used serial CT data from 28 prostate cancer patients. After the planning CT, each patient had a pelvic CT scan bi-weekly for a total of 15 CTs. To eliminate the setup error component of the inter-fraction geometric variation, each daily CT was fused with the planning CT based on bony anatomy. The prostate was contoured on each scan by a single physician. Subsequently prostatic image-based registration (IBR) was performed by translating and rotating each daily prostate CT image to match the reference CT prostate contour. The registration was then repeated, by the same investigator, using only the contour of the daily prostate (CBR). Each registration induces a transformation (dx,dy,dz,®) that characterizes the motion of the daily prostate per a rigid body formalism, where ® is the rotation in the sagittal plane. We assessed the IBR accuracy and stability by comparing each parameter of the 420 transformations between IBR and CBR. Results: While both CBR and IBR have uncertainties, we adopted the CBR as a reference both because there is a greater body of experience in contour definition than in soft tissue registration, and because there is less ambiguity in performing the CBR. The average shift magnitude from the AP perspective was 0.18+0.18cm for CBR and 0.12+0.13cm for IBR. From the lateral perspective, the results for the CBR and IBR are 0.37_+0.30cm and 0.25_+0.25cm, respectively. The analysis shows a larger shift in the AP direction then in any other direction, both in the IBR and CBR, a result consistent
Wednesday, October 27, 2004
$157
with previous studies. The average rotation angle ®, was 0.7+2.0 ° for the CBR and 1.3+2.4 ° for the IBR. The per-patient average discrepancy (A dx,A dy,A dz) between the CBR and IBR is within l m m for all but 6 patients in the LAT direction, but have more variation for the AP and SI directions. In particular, the IBR indicates a larger posterior shift of the patient than does the CBR. The composite result over all patients reiterates these observations: (A dx, A dy, A d z ) = (0.04+0.12, 0.16_+0.22, 0.0_+0.26) cm. The CBR-IBR agreement was within 2mm for 69% of results, between 2 and 4mm for 23%, and greater than 4mm for 8%.
Conclusions: The IBR showed an overestimation of the sagittal rotation angle and an underestimation of the shift magnitude for both the AP and LAT perspectives respect to the CBR. The discrepancy between IBR and CBR is a measure of the intraobserver uncertainty and require offline CBR verification of an online IBR-based correction. The use of on board KV conebeam device, capable of good quality images, will possibly narrow the gap observed between IBR and CBR. 349 oral Three-dimensional l h MR spectroscopic imaging of prostate: time-course assessment of metabolic response post-brachytherapy
A. Damyanovich 1, J. Jezioranskf, D. Taussky2, A. Amirabadi 1, A. Kirilova 1, J. Crook2 ~Princess Margaret Hospital, Radiation Physics, Toronto, Canada 2princess Margaret Hospital, Radiation Oncology, Toronto, Canada Introduction: Proton magnetic resonance spectroscopic imaging (MRSI) of prostate, is able to distinguish between areas of cancer and normal prostatic epithelium through differences in the endogenous [choline+creatine]/citrate ratios Post treatment, MRSI can provide a non invasive measure of intracellular metabolic conditions that may accurately identify viable malignant tissue within the treated prostate. The purpose of this study was to establish and monitor the metabolic profile of prostate pre- and post treatment following brachytherapy with 1251 seeds for the treatment of prostate cancer, using proton MRSI. Results obtained thus far are presented and discussed. Materials and Methods: Twenty patients undergoing prostate brachytherapy using the modified peripheral loading technique ( > 100 1251seeds) were invited to take part in this study, following a histologic diagnosis of prostate cancer. A baseline MRSI scan was performed prior to the TRUS prostate mapping to plan the implant; subsequent MRSI scans followed in 6-month intervals over a two-year period following implant. MRSI scans were performed using an endorectal coil and a GE 1.5T MRI scanner. 3D-MRSI data were acquired using the Prostate Spectroscopy Examination pulse sequence with a TE/TR = 130/1500 ms in order to most clearly resolve the choline/creatine and citrate resonances, while reducing the intensity of fat. Magnetic field homogeneity at the site of the prostate was optimized through the use of higher-order gradient shimming. The whole volume of the prostate was covered by the spectroscopic imaging grid consisting of 8 (S/I) x 16 (R/L) x 8 (A/P) voxels (total: 1024). Analysis was done voxel-by-voxet for each MRSI acquisition by assigning to each voxel a score based on the [choline+creatine]/citrate ratio, and the choline signal-tonoise. :Results: We were able to characterize the metabolic profile of patients pre-implant and monitor changes therein over time post-implant, using MRSI. Although individual metabolic response over time was variable, the overall MRSI-based score demonstrated a decreasing trend over time, consistent with PSA measurements acquired concurrently