- Email: [email protected]
255 poster workshop Dosimetric pre-treatment IMRT verification with a new CCD camera based fluoroscopic EPID
Poster workshop
scattered dose must be separated before the primary dose can be back-projected inside the patient. We used a method based on measurements to perform this separation. Because primary dose is only related to radiological thickness, the accuracy of this method is assessed by evaluating radiological thickness. Methods: We estimated a scattered dose distribution using a physical model which is based on a pencil beam approximation and phantom measurements; this model is only dependent of phantom thickness for a fixed source-detector distance. Portal images were acquired with and without objects in the radiation beam by using a video-based electronic portal imaging device. A two-dimensional primary portal dose distribution could be calculated iteratively from the portal images using the model without any assumptions about radiological thickness. To asses the validity of the model, we used both homogenous and inhomogeneous phantoms. Finally, the method is tested on patient data. All measurements were done using 6 MV photons at an Elekta SL15 linear accelerator. Results: In case of homogeneous phantoms, the reconstructed radiological thickness is within 3-4% from the actual thickness; differences are related to primary dose differences of 2-3%. Conclusion: The method that was developed looks very promising for the application in patient treatments and is an important first step towards three-dimensional dose reconstruction inside the patient. In case of inhomogeneous phantoms and patients, analysis of the preliminary results is work in progress.
254 poster workshop EPID in vivo dosimetry and changes in anatomy
L.N. McDermott, M. Wendling, R. Tielenburg, M.B. van Herk, B.J. Mijnheer NKI-A VL, Radiotherapy Department, Amsterdam, The Netherlands ~ Purpose: Changes in anatomy throughout the course of radiotherapy can be expected to alter the day-to-day patient dose delivery. Even if the patient set-up is within tolerance, significant dosimetric changes may still occur due to tissue density or volume variation. Portal images, normally acquired for set-up verification, may also provide information regarding internal anatomical changes. The aim of this study was to relate the anatomical changes observed in EPID Iocalisation images of prostate patients to variations in the in vivo, 2D dose distribution, determined at the patient mid-plane, using EPID treatment images. Materials and Methods: Internal anatomical changes were analysed by studying series of a-Si EPID 'difference images' for 34 randomly selected prostate patients. Such a series was created by subtracting EPID Iocalisation images of each patient (after bony anatomy registration) from the corresponding images of the 1st fraction. Localisation images were acquired with 5 MUs, from AP and lateral directions, at the 1st 3 fractions and weekly thereafter (unless corrections were required). Changes in patient density directly resulted in variation in EPID signal, since the signal is linear with dose at this dose range. Next, an in-house back-projection algorithm was used to calculate the dose in the patient mid-plane (intersecting the isocentre, parallel to the EPID plane) based on EPID treatment images. The relation, between changes in Iocalisation images and treatment dose images from the same treatment session was evaluated. Results: Twenty-eight (82%) prostate patients showed regions of significant variation in difference images, i.e. >5% EPID signal difference from the 1st fraction. This was interpreted as a variation in either gas or liquid-filling of the bowel, bladder or rectum. Preliminary results show the EPID calculated dose to
Tuesday, October 26, 2004 $115
the mid-plane varied by up to 8% between treatment days. The largest variation in dose was found for those patients with significant changes in bowel filling. For difference images with little or no significant variation, treatment dose images were quite reproducible, with variation in dose profiles within 3% (relative to the isocentre dose of the 1st fraction). Conclusions: The use of 'difference portal images' is an efficient, simple method of detecting anatomical changes. If setup images are normally acquired throughout the course of radiotherapy, information about significant density changes in the irradiated volume is readily available and only requires simple analysis. Using EPID in vivo dosimetry, this information has been applied to relate the effect of changes in anatomy on the dose distribution throughout treatment.
255 poster workshop Dosimetric pre-treatment IMRT verification with a new CCD camera based fluoroscopic EPID
K.L. Pasma 1'4, R.A. Bolt2, L.H.M. Mestrom ~, H. Huizenga 2, M. Bal3, L. Spies3, A.G. Visser2 ~lnstitute for Radiation Oncology Arnhem, Physics, Arnhem, The Netherlands 2UMC Nijmegen, 341-Department for Radiation Oncology, Nijmegen, The Netherlands 3Philips Research Laboratories, Aachen, Germany [email protected] Introduction: We performed a dosimetric calibration of a new CCD camera based EPID. Dose profiles of intensity modulated beams produced with the step and shoot technique were measured prior to a treatment and compared with profiles predicted with a treatment planning system (TPS). Material and Methods: The applied EPID is a mid elbow Theraview NT (Cablon, Leusden, NL) using a Peltier cooled CCD camera, which is more resistant to radiation damage than cameras previously used (replacement interval >3 years). In addition to the standard 2 mm Cu plate, a 1 mm steel slab is mounted on top of the EPID to reduce the detection of high energy electrons. The focus to fluorescent screen distance is 150 cm. Megavolt images of all IMRT beams are acquired. These images are then converted into 2D dose distributions (Pasma et aL, Phys. Med. Biol. 2047-60, 1998) and compared with predicted dose distributions. Predictions are ~)erformed using a commercially available TPS (Pinnacle °, Philips Radiation Oncology Systems, Madison, USA). Calculations were preformed using the planar dose module at 2.2 cm depth, which is equal to the water equivalent measuring depth of the EPID.
The method was tested for intensity modulated 10 MV photon beams of an Elekta Precise Linac (Crawley, UK). Asymmetric fields have been used to test the accuracy of the conversion of the EPID images into 2D dose distributions. Dose profiles measured with the EPID were also compared with ionization chamber (IC) measurements at a depth of 2.2 cm in a 7x7x4.8 cm 3 polystyrene mini phantom. Results: IC measurements at the center of a leaf pair were compared with TPS predictions and EPID measurements. Outside narrow peaks and steep gradients the difference between EPID and IC dose measurements was -1.5+1.8% (1(~). For predictions and IC measurements the difference was 1.2+1.4% (lo). Discussion and conclusion: The method allows an overall and quick pre-treatment verification (~2 minutes/beam) of the beam profiles delivered by the treatment unit versus those predicted by the TPS.
scattered dose must be separated before the primary dose can be back-projected inside the patient. We used a method based on measurements to perform this separation. Because primary dose is only related to radiological thickness, the accuracy of this method is assessed by evaluating radiological thickness. Methods: We estimated a scattered dose distribution using a physical model which is based on a pencil beam approximation and phantom measurements; this model is only dependent of phantom thickness for a fixed source-detector distance. Portal images were acquired with and without objects in the radiation beam by using a video-based electronic portal imaging device. A two-dimensional primary portal dose distribution could be calculated iteratively from the portal images using the model without any assumptions about radiological thickness. To asses the validity of the model, we used both homogenous and inhomogeneous phantoms. Finally, the method is tested on patient data. All measurements were done using 6 MV photons at an Elekta SL15 linear accelerator. Results: In case of homogeneous phantoms, the reconstructed radiological thickness is within 3-4% from the actual thickness; differences are related to primary dose differences of 2-3%. Conclusion: The method that was developed looks very promising for the application in patient treatments and is an important first step towards three-dimensional dose reconstruction inside the patient. In case of inhomogeneous phantoms and patients, analysis of the preliminary results is work in progress.
254 poster workshop EPID in vivo dosimetry and changes in anatomy
L.N. McDermott, M. Wendling, R. Tielenburg, M.B. van Herk, B.J. Mijnheer NKI-A VL, Radiotherapy Department, Amsterdam, The Netherlands ~ Purpose: Changes in anatomy throughout the course of radiotherapy can be expected to alter the day-to-day patient dose delivery. Even if the patient set-up is within tolerance, significant dosimetric changes may still occur due to tissue density or volume variation. Portal images, normally acquired for set-up verification, may also provide information regarding internal anatomical changes. The aim of this study was to relate the anatomical changes observed in EPID Iocalisation images of prostate patients to variations in the in vivo, 2D dose distribution, determined at the patient mid-plane, using EPID treatment images. Materials and Methods: Internal anatomical changes were analysed by studying series of a-Si EPID 'difference images' for 34 randomly selected prostate patients. Such a series was created by subtracting EPID Iocalisation images of each patient (after bony anatomy registration) from the corresponding images of the 1st fraction. Localisation images were acquired with 5 MUs, from AP and lateral directions, at the 1st 3 fractions and weekly thereafter (unless corrections were required). Changes in patient density directly resulted in variation in EPID signal, since the signal is linear with dose at this dose range. Next, an in-house back-projection algorithm was used to calculate the dose in the patient mid-plane (intersecting the isocentre, parallel to the EPID plane) based on EPID treatment images. The relation, between changes in Iocalisation images and treatment dose images from the same treatment session was evaluated. Results: Twenty-eight (82%) prostate patients showed regions of significant variation in difference images, i.e. >5% EPID signal difference from the 1st fraction. This was interpreted as a variation in either gas or liquid-filling of the bowel, bladder or rectum. Preliminary results show the EPID calculated dose to
Tuesday, October 26, 2004 $115
the mid-plane varied by up to 8% between treatment days. The largest variation in dose was found for those patients with significant changes in bowel filling. For difference images with little or no significant variation, treatment dose images were quite reproducible, with variation in dose profiles within 3% (relative to the isocentre dose of the 1st fraction). Conclusions: The use of 'difference portal images' is an efficient, simple method of detecting anatomical changes. If setup images are normally acquired throughout the course of radiotherapy, information about significant density changes in the irradiated volume is readily available and only requires simple analysis. Using EPID in vivo dosimetry, this information has been applied to relate the effect of changes in anatomy on the dose distribution throughout treatment.
255 poster workshop Dosimetric pre-treatment IMRT verification with a new CCD camera based fluoroscopic EPID
K.L. Pasma 1'4, R.A. Bolt2, L.H.M. Mestrom ~, H. Huizenga 2, M. Bal3, L. Spies3, A.G. Visser2 ~lnstitute for Radiation Oncology Arnhem, Physics, Arnhem, The Netherlands 2UMC Nijmegen, 341-Department for Radiation Oncology, Nijmegen, The Netherlands 3Philips Research Laboratories, Aachen, Germany [email protected] Introduction: We performed a dosimetric calibration of a new CCD camera based EPID. Dose profiles of intensity modulated beams produced with the step and shoot technique were measured prior to a treatment and compared with profiles predicted with a treatment planning system (TPS). Material and Methods: The applied EPID is a mid elbow Theraview NT (Cablon, Leusden, NL) using a Peltier cooled CCD camera, which is more resistant to radiation damage than cameras previously used (replacement interval >3 years). In addition to the standard 2 mm Cu plate, a 1 mm steel slab is mounted on top of the EPID to reduce the detection of high energy electrons. The focus to fluorescent screen distance is 150 cm. Megavolt images of all IMRT beams are acquired. These images are then converted into 2D dose distributions (Pasma et aL, Phys. Med. Biol. 2047-60, 1998) and compared with predicted dose distributions. Predictions are ~)erformed using a commercially available TPS (Pinnacle °, Philips Radiation Oncology Systems, Madison, USA). Calculations were preformed using the planar dose module at 2.2 cm depth, which is equal to the water equivalent measuring depth of the EPID.
The method was tested for intensity modulated 10 MV photon beams of an Elekta Precise Linac (Crawley, UK). Asymmetric fields have been used to test the accuracy of the conversion of the EPID images into 2D dose distributions. Dose profiles measured with the EPID were also compared with ionization chamber (IC) measurements at a depth of 2.2 cm in a 7x7x4.8 cm 3 polystyrene mini phantom. Results: IC measurements at the center of a leaf pair were compared with TPS predictions and EPID measurements. Outside narrow peaks and steep gradients the difference between EPID and IC dose measurements was -1.5+1.8% (1(~). For predictions and IC measurements the difference was 1.2+1.4% (lo). Discussion and conclusion: The method allows an overall and quick pre-treatment verification (~2 minutes/beam) of the beam profiles delivered by the treatment unit versus those predicted by the TPS.