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2837 Target Volume Definition for Stereotactic Radiotherapy of Lung Metastases : A Comparison Between PET/CT and 4D-PET/CT N. Di Muzio 1 , V. Bettinardi 2 , C. Landoni 3 , S. Schipani 1 , M. Danna 2 , P. Mancosu 2 , F. Fazio 4 1 Department of Radiation Oncology , Scientific Institute San Raffaele, Milan, Italy, 2 Department of Nuclear Medicine, Scientific Institute San Raffaele, Milan, Italy, 3 Department of Nuclear Medicine, Scientific Institute San Raffaele; University Milano-Bicocca, Milan, Italy, 4 Department of Radiation Oncology , Department of Nuclear Medicine, Scientific Institute San Raffaele;IBFM-CNR University Milano-Bicocca, Milan, Italy Background: Positron Emission Tomography with 18F-FDG (PET) and Computed Tomography (CT) play an important role in the definition of target volume in Radiotherapy. PET/CT are usually acquired during normal breathing; respiratory and cardiac motions can induce artefacts resulting in inaccurate assessment of organ shape and location. Imprecise knowledge of target shifts during respiratory cycle imposes a strategy of larger safety margins resulting in suboptimal dose conformation with excessive irradiation of healthy tissues. Four-dimensional PET/CT data acquisition (4D-PET/CT) could offer the opportunity to improve image quality and to precisely define target shape and its motion during the entire respiratory cycle. Purpose/Objective(s): The aim of this study is to compare PET/CT and 4D-PET/CT for target volume definition in the stereotactic treatment of lung metastases. Materials/Methods: Five patients affected by a single lung metastasis were studied. They were immobilized in supine position by a vacuum pillow (BodyFix, MedTec). A PET/CT scanner (Discovery STe,GE) was used to acquire “standard” PET/CT images from the base of the skull to the pelvis and PET/CT images in 4D-mode with Varian RPM system on the region of interest in the thorax. All images where acquired during free breathing, digitally reconstructed and coregistered (Advantage Workstation,GE). Two image data sets were obtained on the target region: “standard” PET/CT; “integral” PET/CT representing the entire respiratory cycle. Treatment volumes were generated and compared on each data set. A “standard” Gross Target Volume (GTVst) was contoured on “standard” PET/CT; GTVst was expanded (2 mm) for microscopic infiltration to a “standard” Clinical Target Volume (CTVst), to a “standard” Internal Target Volume (ITVst, 10,10,15 mm) for organ motion and to a “standard” Planning Target Volume (PTVst, 3,3,5 mm) for set-up errors. An “Integral” GTV (GTVint) was contoured on “integral” PET/CT and expanded to a CTVint (2 mm) for microscopic infiltration and to a PTVint (3,3,5 mm) for set-up errors.The following volumes were calculated: GTVst 17.5130.83 cc; GTVint 19.6431.86 cc; PTVst 135.96121.61 cc; PTVint 42.4455.69 cc. To evaluate the grade of spatial reproducibility of these volumes, a reproducibility index ( R ), defined as the ratio between volume intersection and volume union, was calculated. A R 1 was expected in case of the highest reproducibility; a R 0 was expected in case of the lowest reproducibility between the volumes. Results: GTVst was smaller than GTVint (6424%) in accordance to organ motion; a R 0.450.32 was calculated between GTVst and GTVint. PTVint was smaller than PTVst (2312%); a R 0.230.12 was calculated between PTVint and PTVst. Conclusions: GTVst and PTVst inadequately reproduce GTVint and PTVint (R 0.450.32 and 0.230.12 respectively). PTVint could represent patient-specific target motion in order to ensure its correct dose coverage when free breathing is present during beam delivery. Further patients need to be studied to validate these data. Author Disclosure: N. Di Muzio, None; V. Bettinardi, None; C. Landoni, None; S. Schipani, None; M. Danna, None; P. Mancosu, None; F. Fazio, None. 2838 The Effect of IMRT Treatment on Second Malignancies S. Stathakis, J. Fan, J. Li, C. Ma Fox Chase Cancer Center, Philadelphia, PA Purpose/Objective(s): The application of intensity-modulated radiation therapy (IMRT) has provided a tool to accomplish superior target coverage and normal tissue sparing. With IMRT higher doses can be delivered to the target volume while maintaining low doses to the surrounding normal tissues. The drawback of intensity modulation, as implemented using a computer-controlled multileaf collimator (MLC), is the larger number of monitor units (MU) required compared to 3DCRT. This is because each IMRT field/port is divided into smaller segments with varying intensities (weights), each segment delivering a portion of the dose for that particular field/port. The increase in MU for IMRT has been reported to be 3 - 10 fold. Materials/Methods: EGS4/BEAM was used to simulate our clinical photon beams with different nominal energies ranging from 6 to 18MV. Good agreement between measurements and calculations was achieved using different set of simulation parameters for each accelerator simulated. IMRT treatment plans for prostate, head and neck, breast and lung cases were taken from our department’s database along with the corresponding CT data for each patient. IMRT and 3DCRTdose distributions were computed using the EGS4/MCSIM user code. Results: A total of 30 cases with an increase in MU from 2– 8 times due to IMRT were chosen. The dose to the organs at risk was then computed for different treatment modalities and treatment sites. Three scenarios were considered in order to estimate the percent likehood of second malignancies: a) no dose threshold for the average dose received by an organ, b) a threshold of 4 Gy and c) organs received doses above 4Gy were excluded. The estimated percent likelihood of a fatal secondary cancer due to 72Gy delivered to prostate using IMRT ranged from 2.4 to 4.2% depending on the plan complexity and modality, while for respective conventional treatments it was 0.9 to 3.5%. Conclusions: The excess in leakage radiation due to the application of IMRT causes an increase in the risk of secondary malignancies, which is proportional to the increase of number of MUs. From the computed risks of fatal cancer due to IMRT we can see that there is substantial increase when compared to 3DCRT. The percent likehood of second cancer is also increased with the complexity of the treatment plan. Plans with more complex intensity modulation will increase the risk of second malignancies because the leakage from the linac’s head will proportionally increase with MUs required. The risks estimated in this work are in agreement with previous works of Hall and Wuu and Followill et al. Accurate dose distributions using Monte Carlo simulations will allow investigating the total dose equivalent for various energies, and for various linac designs. S681 Proceedings of the 48th Annual ASTRO Meeting
Transcript of 2837
2837 Target Volume Definition for Stereotactic Radiotherapy of Lung Metastases : A Comparison BetweenPET/CT and 4D-PET/CT
N. Di Muzio1, V. Bettinardi2, C. Landoni3, S. Schipani1, M. Danna2, P. Mancosu2, F. Fazio4
1Department of Radiation Oncology , Scientific Institute San Raffaele, Milan, Italy, 2Department of Nuclear Medicine,Scientific Institute San Raffaele, Milan, Italy, 3Department of Nuclear Medicine, Scientific Institute San Raffaele; UniversityMilano-Bicocca, Milan, Italy, 4Department of Radiation Oncology , Department of Nuclear Medicine, Scientific InstituteSan Raffaele;IBFM-CNR University Milano-Bicocca, Milan, Italy
Background: Positron Emission Tomography with 18F-FDG (PET) and Computed Tomography (CT) play an important rolein the definition of target volume in Radiotherapy. PET/CT are usually acquired during normal breathing; respiratory andcardiac motions can induce artefacts resulting in inaccurate assessment of organ shape and location. Imprecise knowledge oftarget shifts during respiratory cycle imposes a strategy of larger safety margins resulting in suboptimal dose conformation withexcessive irradiation of healthy tissues. Four-dimensional PET/CT data acquisition (4D-PET/CT) could offer the opportunityto improve image quality and to precisely define target shape and its motion during the entire respiratory cycle.
Purpose/Objective(s): The aim of this study is to compare PET/CT and 4D-PET/CT for target volume definition in thestereotactic treatment of lung metastases.
Materials/Methods: Five patients affected by a single lung metastasis were studied. They were immobilized in supine positionby a vacuum pillow (BodyFix, MedTec). A PET/CT scanner (Discovery STe,GE) was used to acquire “standard” PET/CTimages from the base of the skull to the pelvis and PET/CT images in 4D-mode with Varian RPM system on the region ofinterest in the thorax. All images where acquired during free breathing, digitally reconstructed and coregistered (AdvantageWorkstation,GE). Two image data sets were obtained on the target region: “standard” PET/CT; “integral” PET/CT representingthe entire respiratory cycle. Treatment volumes were generated and compared on each data set. A “standard” Gross TargetVolume (GTVst) was contoured on “standard” PET/CT; GTVst was expanded (�2 mm) for microscopic infiltration to a“standard” Clinical Target Volume (CTVst), to a “standard” Internal Target Volume (ITVst, �10,10,15 mm) for organ motionand to a “standard” Planning Target Volume (PTVst, �3,3,5 mm) for set-up errors. An “Integral” GTV (GTVint) was contouredon “integral” PET/CT and expanded to a CTVint (�2 mm) for microscopic infiltration and to a PTVint (�3,3,5 mm) for set-uperrors.The following volumes were calculated: GTVst � 17.51�30.83 cc; GTVint � 19.64�31.86 cc; PTVst �135.96�121.61 cc; PTVint � 42.44�55.69 cc. To evaluate the grade of spatial reproducibility of these volumes, areproducibility index ( R ), defined as the ratio between volume intersection and volume union, was calculated. A R � 1 wasexpected in case of the highest reproducibility; a R � 0 was expected in case of the lowest reproducibility between the volumes.
Results: GTVst was smaller than GTVint (64�24%) in accordance to organ motion; a R � 0.45�0.32 was calculated betweenGTVst and GTVint. PTVint was smaller than PTVst (23�12%); a R � 0.23�0.12 was calculated between PTVint and PTVst.
Conclusions: GTVst and PTVst inadequately reproduce GTVint and PTVint (R 0.45�0.32 and 0.23�0.12 respectively).PTVint could represent patient-specific target motion in order to ensure its correct dose coverage when free breathing is presentduring beam delivery. Further patients need to be studied to validate these data.
Author Disclosure: N. Di Muzio, None; V. Bettinardi, None; C. Landoni, None; S. Schipani, None; M. Danna, None; P.Mancosu, None; F. Fazio, None.
2838 The Effect of IMRT Treatment on Second Malignancies
S. Stathakis, J. Fan, J. Li, C. Ma
Fox Chase Cancer Center, Philadelphia, PA
Purpose/Objective(s): The application of intensity-modulated radiation therapy (IMRT) has provided a tool to accomplishsuperior target coverage and normal tissue sparing. With IMRT higher doses can be delivered to the target volume whilemaintaining low doses to the surrounding normal tissues. The drawback of intensity modulation, as implemented using acomputer-controlled multileaf collimator (MLC), is the larger number of monitor units (MU) required compared to 3DCRT.This is because each IMRT field/port is divided into smaller segments with varying intensities (weights), each segmentdelivering a portion of the dose for that particular field/port. The increase in MU for IMRT has been reported to be 3 - 10 fold.
Materials/Methods: EGS4/BEAM was used to simulate our clinical photon beams with different nominal energies rangingfrom 6 to 18MV. Good agreement between measurements and calculations was achieved using different set of simulationparameters for each accelerator simulated. IMRT treatment plans for prostate, head and neck, breast and lung cases were takenfrom our department’s database along with the corresponding CT data for each patient. IMRT and 3DCRTdose distributionswere computed using the EGS4/MCSIM user code.
Results: A total of 30 cases with an increase in MU from 2–8 times due to IMRT were chosen. The dose to the organs at riskwas then computed for different treatment modalities and treatment sites. Three scenarios were considered in order to estimatethe percent likehood of second malignancies: a) no dose threshold for the average dose received by an organ, b) a threshold of4 Gy and c) organs received doses above 4Gy were excluded. The estimated percent likelihood of a fatal secondary cancer dueto 72Gy delivered to prostate using IMRT ranged from 2.4 to 4.2% depending on the plan complexity and modality, while forrespective conventional treatments it was 0.9 to 3.5%.
Conclusions: The excess in leakage radiation due to the application of IMRT causes an increase in the risk of secondarymalignancies, which is proportional to the increase of number of MUs. From the computed risks of fatal cancer due toIMRT we can see that there is substantial increase when compared to 3DCRT. The percent likehood of second cancer isalso increased with the complexity of the treatment plan. Plans with more complex intensity modulation will increase therisk of second malignancies because the leakage from the linac’s head will proportionally increase with MUs required.The risks estimated in this work are in agreement with previous works of Hall and Wuu and Followill et al. Accurate dosedistributions using Monte Carlo simulations will allow investigating the total dose equivalent for various energies, andfor various linac designs.
S681Proceedings of the 48th Annual ASTRO Meeting
