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OVERALL ACCURACY OF INTRACRANIAL STEREOTACTIC TREATMENTS USING A VARIAN TRILOGY AND THE CMS TPS
S 248
SRS-SBRT
677 poster (Physics Track) OPTIMIZING DOSE PRESCRIPTION IN SBRT FOR LUNG LESIONS USING MONTE CARLO CALCULATION J. Widder1 , M. Hollander1 , F. Ubbels1 , R. Bolt1 , H. Langendijk1 1 U NIVERSITY M EDICAL C ENTRE G RONINGEN (UMCG), Radiotherapy, Groningen, Netherlands
Purpose: To define a method of dose prescription employing Monte Carlo dose calculation in stereotactic body radiotherapy (SBRT) for lung tumors aiming at a low as possible dose outside of the PTV. Materials: The four-dimensional planning CT-scan acquired with free uncoached breathing of a patient treated with SBRT for a NSCLC stage T1N0M0 was entered into the planning software (iPlan RT dose 4.0, Brainlab, Feldkirchen, Germany). Gross tumour volumes (GTVs), internal target volumes (ITVs) and planning target volumes (PTVs) for two typical T1-tumors (small: diameter 15 mm, and large: diameter 30 mm, respectively) were constructed at three localizations: near the mediastinum, peripherally in the lung, and nearby the thoracic wall, respectively. Mimicking pronounced respiratory motion, a margin of 1 cm was added cranially and caudally to arrive at ITV, which was attributed a density of -122 HU in keeping with the actual mean density of the patient’s tumor. The ITV to PTV margin was 5 mm according to our institutional protocol.For each of these six virtual cases, five treatment plans were made in which the dose was prescribed to encompass the PTV with the prescription isodose level (PIL) set at 50%, 60%, 65%, 70%, and 80% of the isocenter dose, respectively. Each of these thirty treatment plans was planned and optimized using the MC dose calculation module. Beam arrangement consisted of four non-coplanar dynamic arcs, which has been our standard approach for more than 200 patients treated stereotactically at our institution for lung lesions within the last two years. Optimization aimed at achieving a minimal conformity index (CI, ratio of prescription isodose volume to PTV) with a coverage (percent of PTV covered by the prescribed dose) of at least 95%, and a preferred coverage of >99%. Three shells of respectively 10 mm thickness around the PTV were constructed to assess the dose in the tissues directly adjacent to the PTV.Dose-volume parameters were calculated for relevant structures and the ratio of the volume of the isodose comprising 50% of the prescribed dose to the PTV (R50) was calculated for all plans. Results: Table 1 displays the results. The PTV was nicely covered in all cases. All normal tissue parameters including integral dose were worst for the PIL 80%, but the relative amount of monitor units (MU) was considerably higher for lower PIL.
V30(40)%pi = Volume of lung (in%) receiving 30(40)% of prescribed doseS1, S2, S3: shell 1,2,3; mean dose (expressed as percentage of prescribed dose) Conclusions: Given that treatment time should not be prolonged out of proportion for reasons of precision and patient comfort, dose should be prescribed at the 65% PIL because of improved normal tissue sparing compared with prescribing at the 80% PIL. 678 poster (Physics Track) OVERALL ACCURACY OF INTRACRANIAL STEREOTACTIC TREATMENTS USING A VARIAN TRILOGY AND THE CMS TPS M. Essers1 , S. Hol2 , W. de Kruijf1 , J. Venselaar1 , R. Martens2 , E. Raaijmakers1 , D. Eekers2 , M. van de Pol2 1 I NSTITUTE V ERBEETEN, Medical Physics, Tilburg, Netherlands 2 I NSTITUTE V ERBEETEN, Radiation Oncology, Tilburg, Netherlands
Purpose: Using equipment also applied for non-stereotactic treatments, extra care is needed to assure acceptable geometric and dosimetric accuracy for stereotactic treatments. The purpose of this study was to investigate all uncertainties that occur in the treatment chain and to establish the overall uncertainty, resulting in a clinical CTV to PTV margin. Materials: A Varian Trilogy machine, a CIVCO individual head support and mask, CMS Focal for contouring and coregistration and CMS XiO for treatment planning were used. Several error sources contribute to the overall geometric uncertainty for stereotactic treatments: CT-MRI match, contouring, treatment planning, CT-CBCT match, intrafraction motion and agreement of MV and kV isocentre. The CT-MRI registration accuracy was tested in Focal both on patients with a stereotactic frame, and on a Radionics head phantom. Different angulations, sequences and slice thicknesses were analysed.The GTV is the summation of the volumes delineated by two radiation oncologists. No (extra) margin for contouring errors is applied.The GTV-CTV margin is 1mm. Treatment plan calculations (14 beams per lesion, 6 couch angles, a maximum of 3 lesions) were compared with dose measurements
using GafChromic films. The intrafraction motion was determined and corrected for using a CBCT halfway (and at the end of) the treatment. The CT-CBCT match accuracy was determined using phantom measurements. Finally, the agreement of MV and kV isocentre was checked by comparing the position of a marker in orthogonal MV images after couch repositioning as a result of a CBCT match. Results: CT-MRI registration accuracy is 1 mm for T1 or T2 sequences, slice thickness ≤ 2.5 mm, and angulation differences ≤ 10 degrees. For 14 (or more) beam stereotactic plans, the dose falloff position is overestimated up to 0.5 mm by the calculations. In the high dose region, the uncertainty in dose calculation is less than 1.5%. The CT-CBCT match uncertainty was 1 mm at maximum. The agreement between kV and MV isocentre, after couch repositioning is within 1 mm. The intrafraction motion is corrected for and less than 1 mm. A CTV-PTV margin of 2 mm is sufficient to take all these uncertainties into account for patients treated with at least 3 treatment fractions. Conclusions: For intracranial stereotactic patient treatments with at least 3 treatment fractions, all uncertainties in the treatment chain can be taken into account by applying a 2 mm CTV-PTV margin. 679 poster (Physics Track) PRELIMINARY RESULT OF FIDUCIAL-LESS TUMOR TRACKING STEREOTACTIC RADIOTHERAPY FOR TREATMENT OF LUNG TUMORS H. J. Kim1 , W. Kim1 1 I NHA U NIVERSITY H OSPITAL, Department of Radiation Oncology, Inchon, Korea Republic of
Purpose: Higher biological doses can be delivered with stereotactic radiotherapy. However, it requires the reduction of the margin used to expand the clinical target volume (CTV) to the planning target volume (PTV). A reduction in the setup margin can be achieved by immobilizing the patient with a noninvasive body frame and/or by using image guided positioning. The Cyberknife with the Synchrony system can reduce the set up margin. An extra margin for tumor motion is not necessary due to its tumor tracking system but Synchrony system required the insertion of markers in or close to the tumor as a reference during treatment delivery. Xsight Lung Tracking System, which is a fiducial-less direct soft tissue tracking method, recently became available and eliminated the risk of pneumothorax and other fiducial placement complications. We report preliminary outcomes from 10 consecutive patients with non-small cell lung cancer (NSCLC) or pulmonary metastasis and evaluated control rates and toxicity. Materials: From March to December in 2008, 7 patients with biopsy proven lung cancer and 3 pulmonary metastasis were treated with stereotactic radiotherapy using Xsight lung tracking system of 4th generation of Cyberknife. Gross tumor volumes (GTVs) were contoured using lung windows. The GTV margin was expanded to all directions by 5 mm to establish the CTV. A dose of 45-60 Gy was delivered to the CTV in 3-5 equal fractions in less than 2 weeks. We have made V20 of whole lung was less than 20%. Results: All patients completed treatment with few acute side effects and no procedure-related complication. Ten patients with an average maximum tumor diameter of 38 mm (range, 15 75 mm) and a mean CTV of 48 cc (range, 5.6-141.2 cc) were treated. The median follow-up time was 6 months (range, 3-9 months). There was no grade 3 to 4 radiation-related complication. Radiographic response was scored as complete in 5 patients, partial in 5. Two patients died of metastasis at 7 and 4 months after radiation. Conclusions: The stereotactic radiotherapy using fiducial-less tumor tracking system was safe and effective treatment option for patients with lung tumor without targeting error. Long term follow-up is needed to validate the local control and late complication. 680 poster (Physics Track) THE FEASIBILITY OF GOGGLE MONITOR TO REDUCE CORRELATION ERROR USING REAL-TIME TUMOR TRACKING CYBERKNIFE SYSTEM. H. D. Huh1 , S. H. Choi1 , W. Kim1 , H. J. Kim1 , S. H. Kim2 , Y. H. Ji3 , J. Choi4 , S. J. Cho5 , D. Shin6 1 I NHA U NIVERSITY H OSPITAL, Deparment of Radiation Oncology, Inchon, Korea Republic of 2 H ANYANG U NIVERSITY H OSPITAL, Deparment of Radiation Oncology, Seoul, Korea Republic of 3 KOREA I NSTITUTE OF R ADIOLOGICAL AND M EDICAL S CIENCE, Deparment of Radiation Oncology, Seoul, Korea Republic of 4 G ACHON M EDICAL SCHOOL , G IL M EDICAL C ENTER, Deparment of Radiation Oncology, Incheon, Korea Republic of 5 D ONGGUK U NIVERSITY I NTERNATIONAL H OSPITAL, Deparment of Radiation Oncology, Goyang, Korea Republic of 6 S CHOOL OF M EDICINE , K YUNG H EE U NIVERSITY, Deparment of Radiation Oncology, Seoul, Korea Republic of
Purpose: The aim of this study was to reduce correlation errors between
SRS-SBRT
677 poster (Physics Track) OPTIMIZING DOSE PRESCRIPTION IN SBRT FOR LUNG LESIONS USING MONTE CARLO CALCULATION J. Widder1 , M. Hollander1 , F. Ubbels1 , R. Bolt1 , H. Langendijk1 1 U NIVERSITY M EDICAL C ENTRE G RONINGEN (UMCG), Radiotherapy, Groningen, Netherlands
Purpose: To define a method of dose prescription employing Monte Carlo dose calculation in stereotactic body radiotherapy (SBRT) for lung tumors aiming at a low as possible dose outside of the PTV. Materials: The four-dimensional planning CT-scan acquired with free uncoached breathing of a patient treated with SBRT for a NSCLC stage T1N0M0 was entered into the planning software (iPlan RT dose 4.0, Brainlab, Feldkirchen, Germany). Gross tumour volumes (GTVs), internal target volumes (ITVs) and planning target volumes (PTVs) for two typical T1-tumors (small: diameter 15 mm, and large: diameter 30 mm, respectively) were constructed at three localizations: near the mediastinum, peripherally in the lung, and nearby the thoracic wall, respectively. Mimicking pronounced respiratory motion, a margin of 1 cm was added cranially and caudally to arrive at ITV, which was attributed a density of -122 HU in keeping with the actual mean density of the patient’s tumor. The ITV to PTV margin was 5 mm according to our institutional protocol.For each of these six virtual cases, five treatment plans were made in which the dose was prescribed to encompass the PTV with the prescription isodose level (PIL) set at 50%, 60%, 65%, 70%, and 80% of the isocenter dose, respectively. Each of these thirty treatment plans was planned and optimized using the MC dose calculation module. Beam arrangement consisted of four non-coplanar dynamic arcs, which has been our standard approach for more than 200 patients treated stereotactically at our institution for lung lesions within the last two years. Optimization aimed at achieving a minimal conformity index (CI, ratio of prescription isodose volume to PTV) with a coverage (percent of PTV covered by the prescribed dose) of at least 95%, and a preferred coverage of >99%. Three shells of respectively 10 mm thickness around the PTV were constructed to assess the dose in the tissues directly adjacent to the PTV.Dose-volume parameters were calculated for relevant structures and the ratio of the volume of the isodose comprising 50% of the prescribed dose to the PTV (R50) was calculated for all plans. Results: Table 1 displays the results. The PTV was nicely covered in all cases. All normal tissue parameters including integral dose were worst for the PIL 80%, but the relative amount of monitor units (MU) was considerably higher for lower PIL.
V30(40)%pi = Volume of lung (in%) receiving 30(40)% of prescribed doseS1, S2, S3: shell 1,2,3; mean dose (expressed as percentage of prescribed dose) Conclusions: Given that treatment time should not be prolonged out of proportion for reasons of precision and patient comfort, dose should be prescribed at the 65% PIL because of improved normal tissue sparing compared with prescribing at the 80% PIL. 678 poster (Physics Track) OVERALL ACCURACY OF INTRACRANIAL STEREOTACTIC TREATMENTS USING A VARIAN TRILOGY AND THE CMS TPS M. Essers1 , S. Hol2 , W. de Kruijf1 , J. Venselaar1 , R. Martens2 , E. Raaijmakers1 , D. Eekers2 , M. van de Pol2 1 I NSTITUTE V ERBEETEN, Medical Physics, Tilburg, Netherlands 2 I NSTITUTE V ERBEETEN, Radiation Oncology, Tilburg, Netherlands
Purpose: Using equipment also applied for non-stereotactic treatments, extra care is needed to assure acceptable geometric and dosimetric accuracy for stereotactic treatments. The purpose of this study was to investigate all uncertainties that occur in the treatment chain and to establish the overall uncertainty, resulting in a clinical CTV to PTV margin. Materials: A Varian Trilogy machine, a CIVCO individual head support and mask, CMS Focal for contouring and coregistration and CMS XiO for treatment planning were used. Several error sources contribute to the overall geometric uncertainty for stereotactic treatments: CT-MRI match, contouring, treatment planning, CT-CBCT match, intrafraction motion and agreement of MV and kV isocentre. The CT-MRI registration accuracy was tested in Focal both on patients with a stereotactic frame, and on a Radionics head phantom. Different angulations, sequences and slice thicknesses were analysed.The GTV is the summation of the volumes delineated by two radiation oncologists. No (extra) margin for contouring errors is applied.The GTV-CTV margin is 1mm. Treatment plan calculations (14 beams per lesion, 6 couch angles, a maximum of 3 lesions) were compared with dose measurements
using GafChromic films. The intrafraction motion was determined and corrected for using a CBCT halfway (and at the end of) the treatment. The CT-CBCT match accuracy was determined using phantom measurements. Finally, the agreement of MV and kV isocentre was checked by comparing the position of a marker in orthogonal MV images after couch repositioning as a result of a CBCT match. Results: CT-MRI registration accuracy is 1 mm for T1 or T2 sequences, slice thickness ≤ 2.5 mm, and angulation differences ≤ 10 degrees. For 14 (or more) beam stereotactic plans, the dose falloff position is overestimated up to 0.5 mm by the calculations. In the high dose region, the uncertainty in dose calculation is less than 1.5%. The CT-CBCT match uncertainty was 1 mm at maximum. The agreement between kV and MV isocentre, after couch repositioning is within 1 mm. The intrafraction motion is corrected for and less than 1 mm. A CTV-PTV margin of 2 mm is sufficient to take all these uncertainties into account for patients treated with at least 3 treatment fractions. Conclusions: For intracranial stereotactic patient treatments with at least 3 treatment fractions, all uncertainties in the treatment chain can be taken into account by applying a 2 mm CTV-PTV margin. 679 poster (Physics Track) PRELIMINARY RESULT OF FIDUCIAL-LESS TUMOR TRACKING STEREOTACTIC RADIOTHERAPY FOR TREATMENT OF LUNG TUMORS H. J. Kim1 , W. Kim1 1 I NHA U NIVERSITY H OSPITAL, Department of Radiation Oncology, Inchon, Korea Republic of
Purpose: Higher biological doses can be delivered with stereotactic radiotherapy. However, it requires the reduction of the margin used to expand the clinical target volume (CTV) to the planning target volume (PTV). A reduction in the setup margin can be achieved by immobilizing the patient with a noninvasive body frame and/or by using image guided positioning. The Cyberknife with the Synchrony system can reduce the set up margin. An extra margin for tumor motion is not necessary due to its tumor tracking system but Synchrony system required the insertion of markers in or close to the tumor as a reference during treatment delivery. Xsight Lung Tracking System, which is a fiducial-less direct soft tissue tracking method, recently became available and eliminated the risk of pneumothorax and other fiducial placement complications. We report preliminary outcomes from 10 consecutive patients with non-small cell lung cancer (NSCLC) or pulmonary metastasis and evaluated control rates and toxicity. Materials: From March to December in 2008, 7 patients with biopsy proven lung cancer and 3 pulmonary metastasis were treated with stereotactic radiotherapy using Xsight lung tracking system of 4th generation of Cyberknife. Gross tumor volumes (GTVs) were contoured using lung windows. The GTV margin was expanded to all directions by 5 mm to establish the CTV. A dose of 45-60 Gy was delivered to the CTV in 3-5 equal fractions in less than 2 weeks. We have made V20 of whole lung was less than 20%. Results: All patients completed treatment with few acute side effects and no procedure-related complication. Ten patients with an average maximum tumor diameter of 38 mm (range, 15 75 mm) and a mean CTV of 48 cc (range, 5.6-141.2 cc) were treated. The median follow-up time was 6 months (range, 3-9 months). There was no grade 3 to 4 radiation-related complication. Radiographic response was scored as complete in 5 patients, partial in 5. Two patients died of metastasis at 7 and 4 months after radiation. Conclusions: The stereotactic radiotherapy using fiducial-less tumor tracking system was safe and effective treatment option for patients with lung tumor without targeting error. Long term follow-up is needed to validate the local control and late complication. 680 poster (Physics Track) THE FEASIBILITY OF GOGGLE MONITOR TO REDUCE CORRELATION ERROR USING REAL-TIME TUMOR TRACKING CYBERKNIFE SYSTEM. H. D. Huh1 , S. H. Choi1 , W. Kim1 , H. J. Kim1 , S. H. Kim2 , Y. H. Ji3 , J. Choi4 , S. J. Cho5 , D. Shin6 1 I NHA U NIVERSITY H OSPITAL, Deparment of Radiation Oncology, Inchon, Korea Republic of 2 H ANYANG U NIVERSITY H OSPITAL, Deparment of Radiation Oncology, Seoul, Korea Republic of 3 KOREA I NSTITUTE OF R ADIOLOGICAL AND M EDICAL S CIENCE, Deparment of Radiation Oncology, Seoul, Korea Republic of 4 G ACHON M EDICAL SCHOOL , G IL M EDICAL C ENTER, Deparment of Radiation Oncology, Incheon, Korea Republic of 5 D ONGGUK U NIVERSITY I NTERNATIONAL H OSPITAL, Deparment of Radiation Oncology, Goyang, Korea Republic of 6 S CHOOL OF M EDICINE , K YUNG H EE U NIVERSITY, Deparment of Radiation Oncology, Seoul, Korea Republic of
Purpose: The aim of this study was to reduce correlation errors between