- Email: [email protected]
167 invited Extracranial stereotactic radiotherapy
$60
166
Wednesday, 19 September 2001
invited
A new mobile electron accelerator for IORT A.S. Beddar1,2 1University Hospitals of Cleveland, Department of Radiation Oncology, Cleveland, U.S.A., 2Case Western Reserve University, Medical School, Cleveland, U.S.A. The delivery of a single fraction of high dose of radiation to surgically exposed tumor or tumor beds has been proven to reduce the side effects and complications of fractionated external beam radiotherapy (EBRT), reduce the need for the high postoperative doses using EBRT, shorten the overall patient treatment duration, and, prevent local relapse. A new approach to intraoperative radiotherapy (IORT) lead to the development of a mobile linear electron accelerator allowing the delivery of therapeutic treatments within the surgical suite. This mobile electron accelerator provides electron beams with nominal energies of 4, 6, 9 and 12 MeV capable of penetrating to a depth of 1, 2, 3, and 4 cm to the 80% isodose line. Traditionally, delivery of IORT required either heavily shielded operating rooms or the transport of the anesthetized patient to Radiation Oncology. The availability of a self-shielded mobile linear accelerator has made this modality accessible to institutions that otherwise would not consider performing IORT. The physical characteristics of this new type of accelerator will be presented with emphasis on new features that differ from conventional linear accelerators. The clinical implementation of the unit and our clinical experience to review the patient site characteristics and sites of treatment since we started the IORT program. The short and long-term stability data of this mobile accelerator after one year of usage will be presented. 167
invited
Extracranial stereotactic radiotherapy I. Lax, Karolinska Hospital, Dept. of Hospital Physics, Stockholm, Sweden Due to lack of local control often seen after radiotherapy of macroscopic tumors in the body, there is a trend in radiotherapy of dose escalation to improve the results locally. This requires an improved geometrical accuracy in the dose delivery to the target, while increased complications from higher dose to normal tissue is generally not acceptable. In conventional radiotherapy the total geometrical uncertainty is generally not quantified, often resulting in large margins between the CTV and PTV. Due to the non-rigid nature of the human body, internal target motions are generally unknown, as the sensitivity of the portal imaging method cannot detect the position of soft tissue tumors. The stereotactic methodology uses an external reference system throughout the treatment procedure. All geometrical aspects are related to this reference system (etereotactic space). Verification is made from CT examinations by quantitative measurements of the position of the tumor in repeated examinations. This gives a quantitative measure of the total geometrical uncertainty of the position of the tumor. Set-up at the accelerator is made in the stereotactic space. In that way the set-up uncertainty can be reduced to be insignificant. Extracranial stereotactic radiotherapy, using the Stereotactic Body Frame, has been used in clinical practice since 1991 at the Karolinska Hospital. The dose is generally prescribed at the 65% isodose at the periphery of the PTV. Thus the dose distribution is by purpose very heterogeneous in the PTV. This is believed to increase the therapeutic ratio. The fractionation schemes ranges from 8 Gy x 4 up to 15 Gy x 3, given every second day. Reproducibility of the tumor, in stereotactic space, is 3 mm (mean) in transversal plane and 5 mm (mean) in longitudinal direction. Other solutions for accurate delivery of very high target doses have been proposed, among which CT guided stereotactic radiotherapy (a dedicated CT scanner together with a linacc in the treatment room) is used at several institutions today. Long term clinical results indicates significant improvements in cure rate for patients with stage I lung cancer, treated with extracranial stereotactic radiotherapy using the Stereotactic Body Frame as well as treated with CT guided stereotactic radiotherapy compared to conventional radiotherapy.
Symposia/Proffered papers
168
oral
Artificial neural networks to design compensators for tangential breast irradiation S. Gulliford1, D.W. Come2, C.G. Rowbottom 3, S. Webb 1 1Institute of Cancer Research, Joint Department of Physics, Sutton, UK, 2University of Reading, Department of Computer Science, Reading, UK, 3William Beaumont Hospital, Radiation Oncology Department, Royal Oak, MI, USA Purpose. Production of intensity-modulated fields is a computer and often time intensive procedure. This preliminary study takes a 2-D portal image acquired in the treatment position for tangential breast radiotherapy and shows how an Artificial Neural Network (ANN) can 'learn' the required relationship between the image and the corresponding intensity-modulated field used to compensate for varying breast thickness. Method. A number of individual networks based on the standard multilayer perceptron with back propagation were developed. Portal image pixels were used exclusively as inputs into each ANN. The ANN was trained by learning from 22 examples. In each case the outputs were corresponding compensator intensity maps produced by an in-house algorithm taken to be the gold standard in this study (Method1). Two distinct groups of networks were developed. Method 2 was based on groups of 9 pixels in a 3 by 3 arrangement being connected to a node in the hidden layer. All pixel groups in the portal image were included and spatial integrity between the portal image and corresponding compensator intensity map was preserved. Method 3 linked spatially corresponding input and output pixels. Results.The aim of compensation is to improve dose homogeneity. In order to compare the ANN produced compensators with those produced using the algorithm, dose distributions and corresponding dose-volume histograms were calculated. As scaling factors are inherent to the process compensators created using Method 1 were passed otherwise unchanged through the ANN and subsequent calculation processes to provide a benchmark. A group of 10 test cases were calculated for each of Methods 1-3. An average of 5% of the planning target volume was outside the dose range 95-105% for Method 1. This increased to 8% for Method 2 and 16% for Method 3. Conclusion. It is encouraging that using a small number of cases it was possible to train the ANN to generate compensators from portal image data, although it is not intended that a neural network would replace an algorithm. In the majority of applications of IMRT there are no analytic algorithms and solutions are produced using time consuming iterative methods. ANN's may be an alternative. 169
oral
A dosimetric study on laser-accelerated proton beam radiation therapy C.-M. Ma, B. Shahine, J.S. Li, M.C. Lee, M. Ding Stanford Univ School of Medicine, Radiation Oncology, Stanford, USA In this work, we investigate the feasibility of energy- and intensity-modulated proton therapy (EIMPT) using laser-accelerated proton beams. Recent results have shown that high-energy protons can be accelerated through laser-induced plasmas. Up to 58 MeV protons have been generated using the Petawatt Laser at the Lawrence Livermore National Laboratory, Livermore, CA. Laser-accelerated protons have broad energy ad angular distributions, which are not suitable for radiotherapy applications directly. We have been investigating a compact particle selection and beam collimating system for EIMPT using laser-accelerated proton beams. We have implemented the GEANT3 Monte Carlo system for accurate dose calculations for EIMPT treatment planning. The GEANT3 code was also used to study the beam characteristics of the protons from the proposed treatment head design. A fast dose calculation algorithm has been developed for pre- and post-optimization dose calculation. We have compared dose distributions for prostate and head and neck treatments using photon IMRT, conventional proton beams and EIMPT. EIMPT provided superior target coverage and significantly reduced dose to the critical structures compared to other treatment modalitiee. For a prostate case, for example, the dose was reduced by 80% for the rectum and 60% for the femoral head in a 8-field EIMPT plan compared to a 8-field photon IMRT plan.
166
Wednesday, 19 September 2001
invited
A new mobile electron accelerator for IORT A.S. Beddar1,2 1University Hospitals of Cleveland, Department of Radiation Oncology, Cleveland, U.S.A., 2Case Western Reserve University, Medical School, Cleveland, U.S.A. The delivery of a single fraction of high dose of radiation to surgically exposed tumor or tumor beds has been proven to reduce the side effects and complications of fractionated external beam radiotherapy (EBRT), reduce the need for the high postoperative doses using EBRT, shorten the overall patient treatment duration, and, prevent local relapse. A new approach to intraoperative radiotherapy (IORT) lead to the development of a mobile linear electron accelerator allowing the delivery of therapeutic treatments within the surgical suite. This mobile electron accelerator provides electron beams with nominal energies of 4, 6, 9 and 12 MeV capable of penetrating to a depth of 1, 2, 3, and 4 cm to the 80% isodose line. Traditionally, delivery of IORT required either heavily shielded operating rooms or the transport of the anesthetized patient to Radiation Oncology. The availability of a self-shielded mobile linear accelerator has made this modality accessible to institutions that otherwise would not consider performing IORT. The physical characteristics of this new type of accelerator will be presented with emphasis on new features that differ from conventional linear accelerators. The clinical implementation of the unit and our clinical experience to review the patient site characteristics and sites of treatment since we started the IORT program. The short and long-term stability data of this mobile accelerator after one year of usage will be presented. 167
invited
Extracranial stereotactic radiotherapy I. Lax, Karolinska Hospital, Dept. of Hospital Physics, Stockholm, Sweden Due to lack of local control often seen after radiotherapy of macroscopic tumors in the body, there is a trend in radiotherapy of dose escalation to improve the results locally. This requires an improved geometrical accuracy in the dose delivery to the target, while increased complications from higher dose to normal tissue is generally not acceptable. In conventional radiotherapy the total geometrical uncertainty is generally not quantified, often resulting in large margins between the CTV and PTV. Due to the non-rigid nature of the human body, internal target motions are generally unknown, as the sensitivity of the portal imaging method cannot detect the position of soft tissue tumors. The stereotactic methodology uses an external reference system throughout the treatment procedure. All geometrical aspects are related to this reference system (etereotactic space). Verification is made from CT examinations by quantitative measurements of the position of the tumor in repeated examinations. This gives a quantitative measure of the total geometrical uncertainty of the position of the tumor. Set-up at the accelerator is made in the stereotactic space. In that way the set-up uncertainty can be reduced to be insignificant. Extracranial stereotactic radiotherapy, using the Stereotactic Body Frame, has been used in clinical practice since 1991 at the Karolinska Hospital. The dose is generally prescribed at the 65% isodose at the periphery of the PTV. Thus the dose distribution is by purpose very heterogeneous in the PTV. This is believed to increase the therapeutic ratio. The fractionation schemes ranges from 8 Gy x 4 up to 15 Gy x 3, given every second day. Reproducibility of the tumor, in stereotactic space, is 3 mm (mean) in transversal plane and 5 mm (mean) in longitudinal direction. Other solutions for accurate delivery of very high target doses have been proposed, among which CT guided stereotactic radiotherapy (a dedicated CT scanner together with a linacc in the treatment room) is used at several institutions today. Long term clinical results indicates significant improvements in cure rate for patients with stage I lung cancer, treated with extracranial stereotactic radiotherapy using the Stereotactic Body Frame as well as treated with CT guided stereotactic radiotherapy compared to conventional radiotherapy.
Symposia/Proffered papers
168
oral
Artificial neural networks to design compensators for tangential breast irradiation S. Gulliford1, D.W. Come2, C.G. Rowbottom 3, S. Webb 1 1Institute of Cancer Research, Joint Department of Physics, Sutton, UK, 2University of Reading, Department of Computer Science, Reading, UK, 3William Beaumont Hospital, Radiation Oncology Department, Royal Oak, MI, USA Purpose. Production of intensity-modulated fields is a computer and often time intensive procedure. This preliminary study takes a 2-D portal image acquired in the treatment position for tangential breast radiotherapy and shows how an Artificial Neural Network (ANN) can 'learn' the required relationship between the image and the corresponding intensity-modulated field used to compensate for varying breast thickness. Method. A number of individual networks based on the standard multilayer perceptron with back propagation were developed. Portal image pixels were used exclusively as inputs into each ANN. The ANN was trained by learning from 22 examples. In each case the outputs were corresponding compensator intensity maps produced by an in-house algorithm taken to be the gold standard in this study (Method1). Two distinct groups of networks were developed. Method 2 was based on groups of 9 pixels in a 3 by 3 arrangement being connected to a node in the hidden layer. All pixel groups in the portal image were included and spatial integrity between the portal image and corresponding compensator intensity map was preserved. Method 3 linked spatially corresponding input and output pixels. Results.The aim of compensation is to improve dose homogeneity. In order to compare the ANN produced compensators with those produced using the algorithm, dose distributions and corresponding dose-volume histograms were calculated. As scaling factors are inherent to the process compensators created using Method 1 were passed otherwise unchanged through the ANN and subsequent calculation processes to provide a benchmark. A group of 10 test cases were calculated for each of Methods 1-3. An average of 5% of the planning target volume was outside the dose range 95-105% for Method 1. This increased to 8% for Method 2 and 16% for Method 3. Conclusion. It is encouraging that using a small number of cases it was possible to train the ANN to generate compensators from portal image data, although it is not intended that a neural network would replace an algorithm. In the majority of applications of IMRT there are no analytic algorithms and solutions are produced using time consuming iterative methods. ANN's may be an alternative. 169
oral
A dosimetric study on laser-accelerated proton beam radiation therapy C.-M. Ma, B. Shahine, J.S. Li, M.C. Lee, M. Ding Stanford Univ School of Medicine, Radiation Oncology, Stanford, USA In this work, we investigate the feasibility of energy- and intensity-modulated proton therapy (EIMPT) using laser-accelerated proton beams. Recent results have shown that high-energy protons can be accelerated through laser-induced plasmas. Up to 58 MeV protons have been generated using the Petawatt Laser at the Lawrence Livermore National Laboratory, Livermore, CA. Laser-accelerated protons have broad energy ad angular distributions, which are not suitable for radiotherapy applications directly. We have been investigating a compact particle selection and beam collimating system for EIMPT using laser-accelerated proton beams. We have implemented the GEANT3 Monte Carlo system for accurate dose calculations for EIMPT treatment planning. The GEANT3 code was also used to study the beam characteristics of the protons from the proposed treatment head design. A fast dose calculation algorithm has been developed for pre- and post-optimization dose calculation. We have compared dose distributions for prostate and head and neck treatments using photon IMRT, conventional proton beams and EIMPT. EIMPT provided superior target coverage and significantly reduced dose to the critical structures compared to other treatment modalitiee. For a prostate case, for example, the dose was reduced by 80% for the rectum and 60% for the femoral head in a 8-field EIMPT plan compared to a 8-field photon IMRT plan.