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What Is the Optimal Beam Margin for Extracranial Stereotactic Radiotherapy?
S552
I. J. Radiation Oncology
● Biology ● Physics
Volume 63, Number 2, Supplement, 2005
analyzed. The distribution of residence time, total effective dose equivalent (TEDE) for the public and the exposure rate measured from the patient immediately following dose administration is presented for all the patients. The variation of platelet counts following therapy on all the patients treated with I-131 labeled RIC is analyzed. Materials/Methods: We have been using the Bexxar sponsored I-131 labeled RIC called Tositumomab for treatment of non-Hodgkin’s lymphoma since 2001, we have treated 11 patients since then. Patient specific dosimetry was performed on all patients using 5 mCi dosimetric dose and whole body counts performed at 0, 48 and 168 hours following dose administration. The total body residence time (TBRT), for 37% of activity to remain in the patient was determined using the 3 whole body counts. A spread sheet program was used for calculating the net count rates and the data was fitted to a mathematical model, which was used for calculating the TBRT. Based on the study, a relationship between TBRT and the calculated therapeutic activity expressed as mCi/kgm body mass was established. All patients treated were discharged on the same day, after therapeutic dose was administered, with radiation safety precautions called “safe release”. The variation of TEDE was studied as a function of TBRT and exposure rate measured at 1 meter prior to discharge. The platelet counts taken before and after treatment at weekly and monthly intervals were used for estimating the recovery period following therapy. The clinical results of the patients treated at our center will be discussed. Results: A mathematical model was established for calculation of TBRT based on the total body counts. The calculated mCi/Kgm decreased with TBRT for both group of patients treated with total body dose of 65 cGy and 75 cGy. The mCi/Kgm was the same for 65cGy and 75 cGy for TBRT greater than 100 hours. From the results of 11 patients, the TEDE to public ranged from 175 to 325 cGy, while the exposure rate measured on the patient at 1 meter was less than the permissible value reported in the literature for safe release. The platelet counts decreased to a minimum after 2 weeks following therapy and then reached normal after 6 to 9 weeks in all 9 patients and for the remaining 2 patients, the recovery period was unusually longer due to prior chemotherapy regimen. The whole body scans taken on selected patients, during dosimetry will be presented for demonstrating the bio-distribution of the tracer. Conclusions: We have implemented the radioimmunotherapy technique for non-Hodgkin’s lymphoma as an out patient therapy procedure. All patients tolerated the treatment very well and there were no anaphylactic reaction observed during treatment and no elevation of TSH or HAMA after the treatment as evidenced from the follow up. One patient had up normal uptake in thyroid during dosimetry study and therefore, the patient was treated with Y-90 ibritumomab ( Zevalin). The clinical response; partial and complete response for 11 patients was in line with the published data.
2535
What Is the Optimal Beam Margin for Extracranial Stereotactic Radiotherapy?
S. Feigenberg,1 C.M. Morris,2 L. Wang,1 R. Price,1 K. Paskalev,1 A. Konski,1 C. Ma1 Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA, 2Radiation Oncology, Shands Cancer Center at the University of Florida, Gainesville, FL 1
Purpose/Objective: In conventional radiation treatment planning, the distance from the block edge to the planning target volume (PTV) needs to be 7–10 mm to account for penumbra, so a homogeneous plan can be achieved. In stereotactic radiosurgery treatment planning, no margin is used. Subsequently, a lower prescription isodose line (50 – 80%) is used to allow a steep dose fall-off. There is increased dose hetereogeneity over the target but there is minimal normal tissue in the PTV. In extracranial stereotactic radiation, organ motion is a significant challenge and significant volume of normal tissue will receive a significant dose. The purpose of this study is to determine the optimal block edge margin in extracranial stereotactic treatment planning. Materials/Methods: The first five patients treated on a prospective dose escalation stereotactic radiotherapy protocol for patients with malignant lung cancer made up the study cohort. Patients were treated to 40 Gy in four fractions with 6 MV photons using 5 to 8 coplanar/non-coplanar fixed fields. The PTV included a combination of the gross tumor volume based on a normal respiration scan, a maximum end inspiration scan and a maximum end expiration scan plus 5 mm in all directions. The plans were generated using the Xknife treatment planning system (Radionics) and were altered by changing each beam margin from 0 to 5 mm using a mMLC delivery. Hetereogeneity corrections were used for treatment planning. The prescription isodose volume (PIV) was chosen for each beam margin such that the lowest isodose volume covered 100% of the PTV. The volume of the ipsilateral lung (Vx) receiving 5, 10, 15, 20, 25 and 30 Gy was evaluated and compared for each beam margin. A standard regression using a quadratic transformation of margin size was implemented to predict Vx. Results: There is a linear relationship between the beam margin and the PIV (p⬍0.001). The median PIV was 74, 81, 86, 90, 93, 95 for beam margins of 0, 1, 2,3,4 and 5 mm, respectively. The Vx receiving 5, 10, 15, 20, 25 and 30 Gy fit a simple quadratic relationship such that the Vx was proportional to the beam margin squared. Based on the regression, there was a minimal improvement in reducing dose to the ipsilateral lung (Vx) when the margin was 2 mm or less for all plans (all p⬍0.10). Conclusions: Based on the current treatment algorithm, a 2 mm block edge margin maximizes the falloff of radiation dose to the ipsilateral lung while minimizing hetereogeneity across the PTV.
2536
Image Registration with Auto-Mapped Control Volumes 2
D. Paquin, E. Schreibmann,1 L. Xing1 Department of Radiation Oncology, Stanford Cancer Center, Stanford, CA, 2Department of Mathematics, Stanford University, Stanford, CA 1
Purpose/Objective: Many image registration algorithms rely on the use of homologous control points on the two input image sets to be registered. In reality, the interactive identification of the control points on both images is tedious, difficult and often a source of error. The purpose of this work is to automate the selection of control points for both rigid and deformable image registrations and to demonstrate the utility of the new approach by using a few examples. Materials/Methods: The registration of two images in our approach proceeds in two steps. First, a number of small control regions having distinct anatomical features are identified on the model image. Instead of attempting to find the correspondences
I. J. Radiation Oncology
● Biology ● Physics
Volume 63, Number 2, Supplement, 2005
analyzed. The distribution of residence time, total effective dose equivalent (TEDE) for the public and the exposure rate measured from the patient immediately following dose administration is presented for all the patients. The variation of platelet counts following therapy on all the patients treated with I-131 labeled RIC is analyzed. Materials/Methods: We have been using the Bexxar sponsored I-131 labeled RIC called Tositumomab for treatment of non-Hodgkin’s lymphoma since 2001, we have treated 11 patients since then. Patient specific dosimetry was performed on all patients using 5 mCi dosimetric dose and whole body counts performed at 0, 48 and 168 hours following dose administration. The total body residence time (TBRT), for 37% of activity to remain in the patient was determined using the 3 whole body counts. A spread sheet program was used for calculating the net count rates and the data was fitted to a mathematical model, which was used for calculating the TBRT. Based on the study, a relationship between TBRT and the calculated therapeutic activity expressed as mCi/kgm body mass was established. All patients treated were discharged on the same day, after therapeutic dose was administered, with radiation safety precautions called “safe release”. The variation of TEDE was studied as a function of TBRT and exposure rate measured at 1 meter prior to discharge. The platelet counts taken before and after treatment at weekly and monthly intervals were used for estimating the recovery period following therapy. The clinical results of the patients treated at our center will be discussed. Results: A mathematical model was established for calculation of TBRT based on the total body counts. The calculated mCi/Kgm decreased with TBRT for both group of patients treated with total body dose of 65 cGy and 75 cGy. The mCi/Kgm was the same for 65cGy and 75 cGy for TBRT greater than 100 hours. From the results of 11 patients, the TEDE to public ranged from 175 to 325 cGy, while the exposure rate measured on the patient at 1 meter was less than the permissible value reported in the literature for safe release. The platelet counts decreased to a minimum after 2 weeks following therapy and then reached normal after 6 to 9 weeks in all 9 patients and for the remaining 2 patients, the recovery period was unusually longer due to prior chemotherapy regimen. The whole body scans taken on selected patients, during dosimetry will be presented for demonstrating the bio-distribution of the tracer. Conclusions: We have implemented the radioimmunotherapy technique for non-Hodgkin’s lymphoma as an out patient therapy procedure. All patients tolerated the treatment very well and there were no anaphylactic reaction observed during treatment and no elevation of TSH or HAMA after the treatment as evidenced from the follow up. One patient had up normal uptake in thyroid during dosimetry study and therefore, the patient was treated with Y-90 ibritumomab ( Zevalin). The clinical response; partial and complete response for 11 patients was in line with the published data.
2535
What Is the Optimal Beam Margin for Extracranial Stereotactic Radiotherapy?
S. Feigenberg,1 C.M. Morris,2 L. Wang,1 R. Price,1 K. Paskalev,1 A. Konski,1 C. Ma1 Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA, 2Radiation Oncology, Shands Cancer Center at the University of Florida, Gainesville, FL 1
Purpose/Objective: In conventional radiation treatment planning, the distance from the block edge to the planning target volume (PTV) needs to be 7–10 mm to account for penumbra, so a homogeneous plan can be achieved. In stereotactic radiosurgery treatment planning, no margin is used. Subsequently, a lower prescription isodose line (50 – 80%) is used to allow a steep dose fall-off. There is increased dose hetereogeneity over the target but there is minimal normal tissue in the PTV. In extracranial stereotactic radiation, organ motion is a significant challenge and significant volume of normal tissue will receive a significant dose. The purpose of this study is to determine the optimal block edge margin in extracranial stereotactic treatment planning. Materials/Methods: The first five patients treated on a prospective dose escalation stereotactic radiotherapy protocol for patients with malignant lung cancer made up the study cohort. Patients were treated to 40 Gy in four fractions with 6 MV photons using 5 to 8 coplanar/non-coplanar fixed fields. The PTV included a combination of the gross tumor volume based on a normal respiration scan, a maximum end inspiration scan and a maximum end expiration scan plus 5 mm in all directions. The plans were generated using the Xknife treatment planning system (Radionics) and were altered by changing each beam margin from 0 to 5 mm using a mMLC delivery. Hetereogeneity corrections were used for treatment planning. The prescription isodose volume (PIV) was chosen for each beam margin such that the lowest isodose volume covered 100% of the PTV. The volume of the ipsilateral lung (Vx) receiving 5, 10, 15, 20, 25 and 30 Gy was evaluated and compared for each beam margin. A standard regression using a quadratic transformation of margin size was implemented to predict Vx. Results: There is a linear relationship between the beam margin and the PIV (p⬍0.001). The median PIV was 74, 81, 86, 90, 93, 95 for beam margins of 0, 1, 2,3,4 and 5 mm, respectively. The Vx receiving 5, 10, 15, 20, 25 and 30 Gy fit a simple quadratic relationship such that the Vx was proportional to the beam margin squared. Based on the regression, there was a minimal improvement in reducing dose to the ipsilateral lung (Vx) when the margin was 2 mm or less for all plans (all p⬍0.10). Conclusions: Based on the current treatment algorithm, a 2 mm block edge margin maximizes the falloff of radiation dose to the ipsilateral lung while minimizing hetereogeneity across the PTV.
2536
Image Registration with Auto-Mapped Control Volumes 2
D. Paquin, E. Schreibmann,1 L. Xing1 Department of Radiation Oncology, Stanford Cancer Center, Stanford, CA, 2Department of Mathematics, Stanford University, Stanford, CA 1
Purpose/Objective: Many image registration algorithms rely on the use of homologous control points on the two input image sets to be registered. In reality, the interactive identification of the control points on both images is tedious, difficult and often a source of error. The purpose of this work is to automate the selection of control points for both rigid and deformable image registrations and to demonstrate the utility of the new approach by using a few examples. Materials/Methods: The registration of two images in our approach proceeds in two steps. First, a number of small control regions having distinct anatomical features are identified on the model image. Instead of attempting to find the correspondences