Physical Principles of Intensity Modulated Electronic Brachytherapy (IMEB)
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Results: Although the CT numbers of local tomography reconstruction were different from the open field reconstruction, no sig- nificant artifacts were observed and soft tissue contrast between the tumor bed and normal breast tissue was well preserved after proper adjustment of the window and leveling. The slice location of each fiducial marker was the same for both reconstructions. The shift of the fiducial markers was 0.0 ± 0.5 mm (mean ± standard deviation) and -0.1 ± 0.3 mm in the anterior-posterior and right-left directions, respectively. The heart was included in the open field reconstructions but mostly excluded in the local tomography re- constructions. The mean imaging dose to the heart for the local tomography reconstruction was estimated 50% lower. The con- tralateral breast was totally excluded from both reconstructions but the maximum dose for the local tomography reconstruction was 57% lower. Conclusions: Local tomography provides similar image quality for CBCT-guided partial breast irradiation but significantly re- duces the imaging dose to the contra lateral breast and heart. This technique can be applied to CBCT-guided radiotherapy of other sites if the target anatomy (e.g., lung) or it surrogates (e.g., implanted fiducial markers for liver, prostate) can be directly imaged with CBCT. Author Disclosure: J. Chang, None. 2803 Physical Principles of Intensity Modulated Electronic Brachytherapy (IMEB) J. R. Hiatt, J. Hepel, M. Carol, G. Cardarelli, D. E. Wazer, E. S. Sternick Rhode Island Hospital, Providence, RI Purpose/Objective(s): To describe a new collimator design and methodology for controlling the effective intensity of radiation produced by a microminiature X-ray source which allows the modulation of dose distributions for electronic brachytherapy appli- cations. Materials/Methods: A close fitting, two-section adjustable lead collimator consisting of a lower cylindrical band and an upper cap was constructed for a 50kV electronic brachytherapy source (Xoft Inc., Fremont, CA) to provide controllable beam hardening and attenuation. Micro-indexing the source through an indwelling catheter using variable step sizes, in combination with adjustment of the collimator gap width, modifies the resulting depth-doses. Results: Depth dose increases with wider collimator aperture settings and for decreasing step sizes. For a multi-stepped plan using step sizes of 1 mm, the relative dose at 1 cm depth was 0.50 for a gap of 0.5 mm and 0.74 for a gap of 2.0 mm. For a multi-stepped plan with a constant 2 mm aperture size, the relative dose at 1 cm depth increased from 0.70 for a step size of 2.5 mm to 0.74 for a step size of 1.0 mm. Conclusions: Appropriate depth dose adjustment by microindexing and variable collimation of an electronic brachytherapy source facilitates the development of IMEB plans that can be made to conform to irregularly shaped tumor cavities or those closely bounded by critical structures. Author Disclosure: J.R. Hiatt, None; J. Hepel, None; M. Carol, Consultant, F. Consultant/Advisory Board; G. Cardarelli, None; D.E. Wazer, None; E.S. Sternick, None. 2804 Evaluation of Patient Positioning Accuracy during Stereotactic Spinal Radiosurgery using Cone Beam CT M. A. Quader 1 , J. Novotny 1 , J. C. Flickinger 1 , M. S. Huq 1 , P. C. Gerszten 2 1 University of Pittsburgh Cancer Institute, Pittsburgh, PA, 2 University of Pittsburgh, Pittsburgh, PA Purpose/Objective(s): The purpose of this study was to evaluate patient positioning accuracy during spinal stereotactic radiosur- gery (SRS) treatment using a cone beam CT (CBCT) image-guided localization system. Materials/Methods: The positioning deviation of 23 patients during single fraction spinal SRS treatments using the ELEKTA Synergy S 6 MV linear accelerator with a beam modulator and CBCT image guidance system was quantitatively evaluated. The accelerator was equipped with a HexaPOD couch (Medical Intelligence) that allowed patient positioning correction in three translational and three rotational directions. Patients were immobilized with the BodyFix (total body bag, Medical Intelligence) when treatment sites were below T6. Otherwise, a head and shoulder mask with S-board from CIVCO Medical Solutions and BodyFix (hip plus bag) were employed. The treatment volume ranged from 1.6-152.7 cc. The median treatment time including initial setup, for the SRS procedures using 7-14 IMRT fields to deliver 11-18 Gy per fraction was 58 minutes. To verify initial set up and assure accuracy of patient positioning, three quality assurance (QA) CBCTs were performed during the procedure. The first QA CBCT was performed immediately before the treatment started, the second in the middle, and the last immediately after completing the treatment. This strategy was employed to monitor patient movement after the initial set up and to correct po- sitional deviation if needed. The positioning data and fused images of planning CT and CBCT from the treatments were analyzed to determine intra-fraction patient movements. From each of three QA CBCTs, three translational and three rotational coordinates were obtained. Results: The translational variations for the 23 patients in the middle of the treatment were -0.1 ± 0.8, -0.1 ± 0.6, and -0.4 ± 1.3 mm in the lateral (X), longitudinal (Y) and AP (Z) directions, respectively. The magnitude of the 3D vector was 1.3 ± 1.0 mm. Similarly, the variations at the end of the treatment were 0.3 ± 0.7, -0.1 ± 1.0, and -0.6 ± 0.8 mm along X, Y and Z directions, respectively; the 3D vector was 1.5 ± 0.7 mm. The average and standard deviation of rotational angles were -0.1 ± 0.6, -0.1 ± 0.8, and 0.1 ± 0.5 degrees in the middle of the treatment, and -0.2 ± 0.9, -0.2 ± 0.9, 0.1 ± 0.5 degrees at the end of the treatment, along yaw, roll and pitch respectively. Conclusions: The average variations along three translational and three rotational coordinates were \1.0 mm and \0.3 degree respectively. The magnitude of the mean 3D vector was 1.5 mm. Hence, a CBCT image guided system coupled with the Hexa- POD couch and non-invasive immobilization provides for a positioning accuracy of less than 2.5 mm when expressed as a 3D vector. Author Disclosure: M.A. Quader, None; J. Novotny, None; J.C. Flickinger, None; M.S. Huq, None; P.C. Gerszten, None. S528 I. J. Radiation Oncology d Biology d Physics Volume 72, Number 1, Supplement, 2008
Transcript of Physical Principles of Intensity Modulated Electronic Brachytherapy (IMEB)
S528 I. J. Radiation Oncology d Biology d Physics Volume 72, Number 1, Supplement, 2008
Results: Although the CT numbers of local tomography reconstruction were different from the open field reconstruction, no sig-nificant artifacts were observed and soft tissue contrast between the tumor bed and normal breast tissue was well preserved afterproper adjustment of the window and leveling. The slice location of each fiducial marker was the same for both reconstructions. Theshift of the fiducial markers was 0.0 ± 0.5 mm (mean ± standard deviation) and -0.1 ± 0.3 mm in the anterior-posterior and right-leftdirections, respectively. The heart was included in the open field reconstructions but mostly excluded in the local tomography re-constructions. The mean imaging dose to the heart for the local tomography reconstruction was estimated �50% lower. The con-tralateral breast was totally excluded from both reconstructions but the maximum dose for the local tomography reconstruction was�57% lower.
Conclusions: Local tomography provides similar image quality for CBCT-guided partial breast irradiation but significantly re-duces the imaging dose to the contra lateral breast and heart. This technique can be applied to CBCT-guided radiotherapy of othersites if the target anatomy (e.g., lung) or it surrogates (e.g., implanted fiducial markers for liver, prostate) can be directly imagedwith CBCT.
Author Disclosure: J. Chang, None.
2803 Physical Principles of Intensity Modulated Electronic Brachytherapy (IMEB)
J. R. Hiatt, J. Hepel, M. Carol, G. Cardarelli, D. E. Wazer, E. S. Sternick
Rhode Island Hospital, Providence, RI
Purpose/Objective(s): To describe a new collimator design and methodology for controlling the effective intensity of radiationproduced by a microminiature X-ray source which allows the modulation of dose distributions for electronic brachytherapy appli-cations.
Materials/Methods: A close fitting, two-section adjustable lead collimator consisting of a lower cylindrical band and an upper capwas constructed for a 50kV electronic brachytherapy source (Xoft Inc., Fremont, CA) to provide controllable beam hardening andattenuation. Micro-indexing the source through an indwelling catheter using variable step sizes, in combination with adjustment ofthe collimator gap width, modifies the resulting depth-doses.
Results: Depth dose increases with wider collimator aperture settings and for decreasing step sizes. For a multi-stepped plan usingstep sizes of 1 mm, the relative dose at 1 cm depth was 0.50 for a gap of 0.5 mm and 0.74 for a gap of 2.0 mm. For a multi-steppedplan with a constant 2 mm aperture size, the relative dose at 1 cm depth increased from 0.70 for a step size of 2.5 mm to 0.74 fora step size of 1.0 mm.
Conclusions: Appropriate depth dose adjustment by microindexing and variable collimation of an electronic brachytherapy sourcefacilitates the development of IMEB plans that can be made to conform to irregularly shaped tumor cavities or those closelybounded by critical structures.
Author Disclosure: J.R. Hiatt, None; J. Hepel, None; M. Carol, Consultant, F. Consultant/Advisory Board; G. Cardarelli, None;D.E. Wazer, None; E.S. Sternick, None.
2804 Evaluation of Patient Positioning Accuracy during Stereotactic Spinal Radiosurgery using Cone Beam CT
M. A. Quader1, J. Novotny1, J. C. Flickinger1, M. S. Huq1, P. C. Gerszten2
1University of Pittsburgh Cancer Institute, Pittsburgh, PA, 2University of Pittsburgh, Pittsburgh, PA
Purpose/Objective(s): The purpose of this study was to evaluate patient positioning accuracy during spinal stereotactic radiosur-gery (SRS) treatment using a cone beam CT (CBCT) image-guided localization system.
Materials/Methods: The positioning deviation of 23 patients during single fraction spinal SRS treatments using the ELEKTASynergy S 6 MV linear accelerator with a beam modulator and CBCT image guidance system was quantitatively evaluated.The accelerator was equipped with a HexaPOD couch (Medical Intelligence) that allowed patient positioning correction in threetranslational and three rotational directions. Patients were immobilized with the BodyFix (total body bag, Medical Intelligence)when treatment sites were below T6. Otherwise, a head and shoulder mask with S-board from CIVCO Medical Solutions andBodyFix (hip plus bag) were employed. The treatment volume ranged from 1.6-152.7 cc. The median treatment time includinginitial setup, for the SRS procedures using 7-14 IMRT fields to deliver 11-18 Gy per fraction was 58 minutes. To verify initialset up and assure accuracy of patient positioning, three quality assurance (QA) CBCTs were performed during the procedure.The first QA CBCT was performed immediately before the treatment started, the second in the middle, and the last immediatelyafter completing the treatment. This strategy was employed to monitor patient movement after the initial set up and to correct po-sitional deviation if needed. The positioning data and fused images of planning CT and CBCT from the treatments were analyzed todetermine intra-fraction patient movements. From each of three QA CBCTs, three translational and three rotational coordinateswere obtained.
Results: The translational variations for the 23 patients in the middle of the treatment were -0.1 ± 0.8, -0.1 ± 0.6, and -0.4 ± 1.3 mmin the lateral (X), longitudinal (Y) and AP (Z) directions, respectively. The magnitude of the 3D vector was 1.3 ± 1.0 mm. Similarly,the variations at the end of the treatment were 0.3 ± 0.7, -0.1 ± 1.0, and -0.6 ± 0.8 mm along X, Y and Z directions, respectively; the3D vector was 1.5 ± 0.7 mm. The average and standard deviation of rotational angles were -0.1 ± 0.6, -0.1 ± 0.8, and 0.1 ± 0.5degrees in the middle of the treatment, and -0.2 ± 0.9, -0.2 ± 0.9, 0.1 ± 0.5 degrees at the end of the treatment, along yaw, roll andpitch respectively.
Conclusions: The average variations along three translational and three rotational coordinates were \1.0 mm and \0.3 degreerespectively. The magnitude of the mean 3D vector was 1.5 mm. Hence, a CBCT image guided system coupled with the Hexa-POD couch and non-invasive immobilization provides for a positioning accuracy of less than 2.5 mm when expressed as a 3Dvector.
Author Disclosure: M.A. Quader, None; J. Novotny, None; J.C. Flickinger, None; M.S. Huq, None; P.C. Gerszten, None.
