Comparing Optical Localization to kV On-Board Imaging During Image-Guided Stereotaxy
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2932 Multi-Adaptive-Plan (MAP) IMRT to Accommodate the Independent Movement of the Prostate and Pelvic Lymph Nodes: A Proof of Principle Study Driven by Clinical Necessity P. Xia, A. Hwang, G. Mu, E. Ludlum, M. Aubin, J. Pouliot, M. Roach III University of California, San Francisco, CA Purpose/Objective(s): Concurrent irradiation of the prostate and pelvic lymph nodes with IMRT is indicated in patients with high- risk prostate cancer. One of the challenges to using IMRT in this concurrent treatment is the independent movement of the prostate relative to the lymph nodes, rendering the conventional iso-center shifting method inadequate. This proof of principle study dem- onstrates clinical feasibility of a novel solution to this challenge: MAP IMRT. Materials/Methods: A 70 year old patient with high risk prostate cancer and a ‘‘Horse-shoe’’ abdominal kidney was treated with MAP IMRT in two phases. Phase I involved concurrently treating the prostate to 50 Gy and pelvic nodes to 45 Gy in 25 fractions. A major dosimetric consideration for this patient was to keep the dose to the kidney as low as possible. In addition to the original IMRT plan, three MAP strategies were explored, each consisting of a set of 8 potential IMRT plans. The first strategy involved individual re-optimization of the MAP (reopt-MAP) plans to accommodate eight potential prostate positions. These prostate po- sitions were created by shifting the contour of the prostate 0.5 cm and 1 cm along the longitudinal and anterior-posterior directions inside the planning CT. Two additional strategies involved: (1) shifting the positions of selected MLC leaves (mlc-MAP); (2) mov- ing the iso-center (iso-MAP). Using the reopt-MAP strategy as a standard, we compared to the other two strategies to determine whether there could be an alternative based on selected dosimetric endpoints including D95% (dose to 95% of the prostate and nodes); D10% and D20% of the rectum, bladder, and the kidney; and the volume of the small bowel receiving more than 45 Gy (V45). Results: The patient was treated with reopt-MAP plans. Prior to each treatment, a megavoltage cone-beam CT (MVCB-CT) was acquired and aligned with the planning CT based on the bony structures and the implanted markers. The magnitude of the prostate movement was obtained by the differences between these two alignments. Based on data of MVCB-CT, the prostate of this patient moved 0.4–0.7 cm superior in 38% of treatment days, .0.8 cm superior in 19%, 0.4–0.7 cm posterior in 12%, and less than 0.3 cm in all directions in 31%. Among the three strategies, there is no difference for the D95 of the prostate. The averaged D95 are 50.4, 50.4, and 50.2 Gy, for the reopt-MAP, mlc-MAP, and iso-MAP plans, respectively. However, iso-MAP strategy resulted in sig- nificant under-dose to the pelvic lymph nodes. The averaged D95 of the pelvic nodes are 45.2, 44.7 and 42.8 Gy, respectively. The V10s of the kidney in all reopt-MAP and mlc-MAP plans were \25%, but iso-MAP plans with iso-center moved .0.5 cm in superior direction could resulting the V10 .25%. Conclusions: Although online re-planning may be the ideal strategy to accommodate independent movement of the prostate and pelvic lymph nodes during concurrent treatment, re-optimizing a set of IMRT plans with multiple prostate positions is clinically feasible and practical. The mlc-MAP strategy is a potential alternative to the reopt-MAP strategy. The conventional iso-center shift- ing method is inadequate, especially for this particular patient. Author Disclosure: P. Xia, Siemens research grant, C. Other Research Support; A. Hwang, None; G. Mu, None; E. Ludlum, None; M. Aubin, Research support from Siemens, C. Other Research Support; J. Pouliot, Research grant from Siemens, C. Other Research Support; M. Roach III, None. 2933 Comparing Optical Localization to kV On-Board Imaging During Image-Guided Stereotaxy R. E. Drzymala, D. B. Mansur Dept of Rad Oncol, Washington University Schl Med, Saint Louis, MO Purpose/Objective(s): Our intent is to determine the accuracy of on-board kV imaging (kVi) by comparing its suggested patient positioning to that obtained using an infrared optical camera (SOC), having an established accuracy of \0.6 mm and 0.1 . Materials/Methods: Twenty cranial stereotactic treatments were reviewed. A CT-based (spiral-axial, 512 512 pixels, 350 mm FOV, 1.5 mm spacing, 0.6 pitch) radiotherapy treatment plan was created for frameless SOC localization for each patient in a ther- moplastic mask. Two orthogonal digitally reconstructed radiographs (DRRs) were computed in the anterior-posterior (A/P) and the right-left (R/L) directions. Patients were initially positioned using the daily calibrated SOC system and were optically monitored for the duration of the treatment. Localization with SOC was the benchmark for position comparisons. A/P and R/L kVi immediately followed without moving the patient. The kVi transferred to a record and verify (RNV) system for registration to corresponding DRRs. Registration of the image pairs was achieved using 5 corresponding anatomical landmarks in paired images. The following were noted: shift to agreement in orthogonal directions; rotation in coronal and sagittal planes; the landmark residual errors (REs); mean, standard deviation, minimum and maximum distance to agreement for landmarks. Results: Analysis resulted in the following [key: mean (median)]: superior/inferior (S/I) shift: 0.3 (0.3) mm; A/P shift: 0.4 (0.4) mm; R/L shift: 1.0 (0.9) mm; coronal plane rotation: 0.3 (0.1) ; sagittal plane rotation: 1.2 (1.1) . Maximum in-plane rotation was 2.7 for one patient. Shifts were found up to 2.8 mm. Differences ranged from 0.1 to 1.9 mm for S/I shifts between the AP and the lateral views for the same treatment. Three of these exceeded 1.0 mm. REs for the 5 landmark correlation were 1 mm but ranged from 0.53 to 1.76 mm. Plots of the shift to agreement vs. REs showed little correlation. Errors in quadrature had values with mean (median) of 1.46 (1.33) mm, ranging up to 3 mm. Conclusions: Positional errors in any one direction or rotation were typically small (1 mm or less), below clinical and technical tolerances on the linac, and confirmed accurate agreement of kVi with SOC. Outliers existed, however, implying that errors can accumulate during the course of data manipulation to generate DRRs, registration of kVi with DRRs and transfer of the data through the various computer workstations: and small patient motions. DRR resolution is a probable source of error even though the scans were spaced 1.5 mm. Slice spacing of 1 mm would be desirable, but some treatment planning systems preclude the re- sulting large number of axial scans. REs of 1 mm corroborate good correlation technique Although patient position was continu- ously monitored with the SOC, small motions before taking the kVi could account for some observed error. This was evident since three S/I shifts were significantly different between successive A/P and R/L kVi, ranging from 1.2 to 1.9 mm. Patient data exhibited small rotations typically \1 with one sagittal rotation of 2.7 . Of note is that sagittal plane rotational error is 4.6 times greater compared to coronal. The implication is that the immobilization head mask restricts yaw rotation better than pitch. Author Disclosure: R.E. Drzymala, None; D.B. Mansur, None. S724 I. J. Radiation Oncology d Biology d Physics Volume 69, Number 3, Supplement, 2007
Transcript of Comparing Optical Localization to kV On-Board Imaging During Image-Guided Stereotaxy
S724 I. J. Radiation Oncology d Biology d Physics Volume 69, Number 3, Supplement, 2007
2932 Multi-Adaptive-Plan (MAP) IMRT to Accommodate the Independent Movement of the Prostate and Pelvic
Lymph Nodes: A Proof of Principle Study Driven by Clinical NecessityP. Xia, A. Hwang, G. Mu, E. Ludlum, M. Aubin, J. Pouliot, M. Roach III
University of California, San Francisco, CA
Purpose/Objective(s): Concurrent irradiation of the prostate and pelvic lymph nodes with IMRT is indicated in patients with high-risk prostate cancer. One of the challenges to using IMRT in this concurrent treatment is the independent movement of the prostaterelative to the lymph nodes, rendering the conventional iso-center shifting method inadequate. This proof of principle study dem-onstrates clinical feasibility of a novel solution to this challenge: MAP IMRT.
Materials/Methods: A 70 year old patient with high risk prostate cancer and a ‘‘Horse-shoe’’ abdominal kidney was treated withMAP IMRT in two phases. Phase I involved concurrently treating the prostate to 50 Gy and pelvic nodes to 45 Gy in 25 fractions. Amajor dosimetric consideration for this patient was to keep the dose to the kidney as low as possible. In addition to the originalIMRT plan, three MAP strategies were explored, each consisting of a set of 8 potential IMRT plans. The first strategy involvedindividual re-optimization of the MAP (reopt-MAP) plans to accommodate eight potential prostate positions. These prostate po-sitions were created by shifting the contour of the prostate 0.5 cm and 1 cm along the longitudinal and anterior-posterior directionsinside the planning CT. Two additional strategies involved: (1) shifting the positions of selected MLC leaves (mlc-MAP); (2) mov-ing the iso-center (iso-MAP). Using the reopt-MAP strategy as a standard, we compared to the other two strategies to determinewhether there could be an alternative based on selected dosimetric endpoints including D95% (dose to 95% of the prostate andnodes); D10% and D20% of the rectum, bladder, and the kidney; and the volume of the small bowel receiving more than 45Gy (V45).
Results: The patient was treated with reopt-MAP plans. Prior to each treatment, a megavoltage cone-beam CT (MVCB-CT) wasacquired and aligned with the planning CT based on the bony structures and the implanted markers. The magnitude of the prostatemovement was obtained by the differences between these two alignments. Based on data of MVCB-CT, the prostate of this patientmoved 0.4–0.7 cm superior in 38% of treatment days, .0.8 cm superior in 19%, 0.4–0.7 cm posterior in 12%, and less than 0.3 cmin all directions in 31%. Among the three strategies, there is no difference for the D95 of the prostate. The averaged D95 are 50.4,50.4, and 50.2 Gy, for the reopt-MAP, mlc-MAP, and iso-MAP plans, respectively. However, iso-MAP strategy resulted in sig-nificant under-dose to the pelvic lymph nodes. The averaged D95 of the pelvic nodes are 45.2, 44.7 and 42.8 Gy, respectively. TheV10s of the kidney in all reopt-MAP and mlc-MAP plans were \25%, but iso-MAP plans with iso-center moved .0.5 cm insuperior direction could resulting the V10 .25%.
Conclusions: Although online re-planning may be the ideal strategy to accommodate independent movement of the prostate andpelvic lymph nodes during concurrent treatment, re-optimizing a set of IMRT plans with multiple prostate positions is clinicallyfeasible and practical. The mlc-MAP strategy is a potential alternative to the reopt-MAP strategy. The conventional iso-center shift-ing method is inadequate, especially for this particular patient.
Author Disclosure: P. Xia, Siemens research grant, C. Other Research Support; A. Hwang, None; G. Mu, None; E. Ludlum, None;M. Aubin, Research support from Siemens, C. Other Research Support; J. Pouliot, Research grant from Siemens, C. OtherResearch Support; M. Roach III, None.
2933 Comparing Optical Localization to kV On-Board Imaging During Image-Guided Stereotaxy
R. E. Drzymala, D. B. Mansur
Dept of Rad Oncol, Washington University Schl Med, Saint Louis, MO
Purpose/Objective(s): Our intent is to determine the accuracy of on-board kV imaging (kVi) by comparing its suggested patientpositioning to that obtained using an infrared optical camera (SOC), having an established accuracy of \0.6 mm and 0.1�.
Materials/Methods: Twenty cranial stereotactic treatments were reviewed. A CT-based (spiral-axial, 512 � 512 pixels, 350 mmFOV, 1.5 mm spacing, 0.6 pitch) radiotherapy treatment plan was created for frameless SOC localization for each patient in a ther-moplastic mask. Two orthogonal digitally reconstructed radiographs (DRRs) were computed in the anterior-posterior (A/P) and theright-left (R/L) directions. Patients were initially positioned using the daily calibrated SOC system and were optically monitored forthe duration of the treatment. Localization with SOC was the benchmark for position comparisons. A/P and R/L kVi immediatelyfollowed without moving the patient. The kVi transferred to a record and verify (RNV) system for registration to correspondingDRRs. Registration of the image pairs was achieved using 5 corresponding anatomical landmarks in paired images. The followingwere noted: shift to agreement in orthogonal directions; rotation in coronal and sagittal planes; the landmark residual errors (REs);mean, standard deviation, minimum and maximum distance to agreement for landmarks.
Results: Analysis resulted in the following [key: mean (median)]: superior/inferior (S/I) shift: 0.3 (0.3) mm; A/P shift: 0.4 (0.4)mm; R/L shift: 1.0 (0.9) mm; coronal plane rotation: 0.3 (0.1)�; sagittal plane rotation: 1.2 (1.1)�. Maximum in-plane rotation was2.7� for one patient. Shifts were found up to 2.8 mm. Differences ranged from 0.1 to 1.9 mm for S/I shifts between the AP and thelateral views for the same treatment. Three of these exceeded 1.0 mm. REs for the 5 landmark correlation were�1 mm but rangedfrom 0.53 to 1.76 mm. Plots of the shift to agreement vs. REs showed little correlation. Errors in quadrature had values with mean(median) of 1.46 (1.33) mm, ranging up to 3 mm.
Conclusions: Positional errors in any one direction or rotation were typically small (�1 mm or less), below clinical and technicaltolerances on the linac, and confirmed accurate agreement of kVi with SOC. Outliers existed, however, implying that errors canaccumulate during the course of data manipulation to generate DRRs, registration of kVi with DRRs and transfer of the datathrough the various computer workstations: and small patient motions. DRR resolution is a probable source of error even thoughthe scans were spaced 1.5 mm. Slice spacing of 1 mm would be desirable, but some treatment planning systems preclude the re-sulting large number of axial scans. REs of 1 mm corroborate good correlation technique Although patient position was continu-ously monitored with the SOC, small motions before taking the kVi could account for some observed error. This was evident sincethree S/I shifts were significantly different between successive A/P and R/L kVi, ranging from 1.2 to 1.9 mm. Patient data exhibitedsmall rotations typically \1� with one sagittal rotation of 2.7�. Of note is that sagittal plane rotational error is 4.6 times greatercompared to coronal. The implication is that the immobilization head mask restricts yaw rotation better than pitch.
Author Disclosure: R.E. Drzymala, None; D.B. Mansur, None.
