| 1. |
- Adolfsson, Emelie, 1985-, et al.
(författare)
-
Response of Lithium Formate EPR Dosimeters at Photon Energies Relelvant to Brachytherapy
- 2009
-
Ingår i: IFMBE Proceedings. - Heidelberg : Springer Berlin Heidelberg. - 9783642034725 - 9783642034749 ; , s. 236-239
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- After development of sensitive dosimeter materials Electron Paramagnetic Resonance EPR dosimetry has been successfully used also in radiation therapy. The intensity of the EPR-signal is a measure of the amount of free radicals created by ionizing radiation which is proportional to the absorbed dose in the dosimeter. Lithium formate monohydrate is a dosimeter material with 2-6 times higher sensitivity than alanine, a linear dose response over a wide dose range and mass-energy absorption properties similar to water. These properties make lithium formate promising for verification of absorbed doses around high dose rate brachytherapy sources where the dose gradient is steep and the photon energy distribution changing with distance from the source. Calibration of the dosimeters is performed in 60Co or MV photon beams where high dosimetric accuracy is feasible. The use in brachytherapy field relies on the assumption that the production of free radicals per mean absorbed dose in the dosimeter is similar at the lower photon energies present there. The aim of this work was to test that assumption. The response of the dosimeters as a function of photon energy was determined by irradiations with four x-ray qualities in the range 100-250 kV and 137Cs, relative to the response when irradiated with 60Co, all photon beams with well-known air kerma rates at the Swedish Secondary Standards Dosimetry Laboratory. Monte Carlo simulations were used to convert air kerma free in air to mean absorbed dose to the dosimeter. The measured response relative 60Co as a function of photon energy was below unity for all qualities. The maximum deviation from unity was 2.5% (100 kV, 135 kV) with a relative standard deviation of 1.5% (k = 1).
|
|
| 2. |
|
|
| 3. |
- Nyholm, Tufve, et al.
(författare)
-
MR based workflow for external radiotherapy of prostate cancer.
- 2009
-
Ingår i: IFMBE Proceedings 25/1. - Berlin, Heidelberg : Springer Verlag. - 9783642034725 ; , s. 60-63
-
Bokkapitel (refereegranskat)abstract
- The requirements on spatial accuracy are high in external radiotherapy of prostate cancer. The technical achievements in image guided radiotherapy (IGRT) during the recent years have lead to a significant improvement of the daily positioning accuracy. Improved accuracy has made it possible to reduce the margin between the clinical target volume and the planning target volume, thus reduce the risk for normal tissue complications. MRI is to prefer to CT for delineation of the prostate target because of superior soft tissue contrast. Recent studies have shown that the dosimetric accuracy of dose calculations on MR material is acceptable. In the present work we analyze the spatial uncertainties that are connected to workflows where the CT has been excluded. We found that accuracy increase with a fully MR based workflow. The main reason is that the MR based workflow does not require any registration between MR and CT to enable target delineation on the MR series. Two different methodologies for patient positioning with MR as baseline were identified: A. Implanted fiducial markers with portal imaging at each treatment session. B. Imaging of the patient in the MR at every treatment session to localize the prostate. We found that the two positioning methods give equivalent spatial accuracy. The estimated required margins for the MR based workflows was around 8 mm, corresponding numbers for a CT based workflow using the same assessment methods was around 10 mm.
|
|
| 4. |
- Toma-Dasu, Iuliana, et al.
(författare)
-
Therapy optimization based on non-linear uptake of PET tracers versus “linear dose painting”
- 2009
-
Ingår i: IFMBE Proceedings. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1680-0737 .- 1433-9277. ; 25/I, s. 221-224
-
Tidskriftsartikel (refereegranskat)abstract
- Treatment optimization based on positron emission tomography (PET) images of tumor hypoxia has been proposed as a method to improve the cure rates in radiotherapy through the increased dose delivery to tumor regions with increased radioresistance. One of the major advantages of PET imaging of hypoxia is that it can provide information on both the extent and the spatial distribution of the resistant regions. One of the key issues for the practical implementation of this approach is the accurate conversion of the intensities in the recorded images into radiosensitivity maps that could then be used for dose escalation. The present paper explores the influence of the conversion from uptake to prescribed doses. Transformation functions derived from the uptake properties of the PET tracers were taken into consideration. The results have shown that the available tracers have different uptake properties and therefore they could interpret differently the gradients in the images which in turn would lead to different dose predictions. Best results in terms of dose prescription would therefore be achieved by carefully taking into account the uptake characteristics of the imaged tracers. Linear approximations could lead to unnecessary overestimations of the doses for cases of partial hypoxia in tumors. This highlights the need for more experimental studies of the uptake properties of PET tracers proposed to image tissue hypoxia. These would eventually provide more reliable methods for dose prescription that could be used with optimization algorithms for the successful individualization of radiation therapy.
|
|
| 5. |
- Toma-Dasu, Iuliana, et al.
(författare)
-
Therapy optimization based on non-linear uptake of PET tracers versus "linear dose painting"
- 2009
-
Ingår i: IFMBE Proceedings. - Berlin : Springer. - 9783642034725 ; , s. 221-224
-
Konferensbidrag (refereegranskat)abstract
- Treatment optimization based on positron emission tomography (PET) images of tumor hypoxia has been proposed as a method to improve the cure rates in radiotherapy through the increased dose delivery to tumor regions with increased radioresistance. One of the major advantages of PET imaging of hypoxia is that it can provide information on both the extent and the spatial distribution of the resistant regions. One of the key issues for the practical implementation of this approach is the accurate conversion of the intensities in the recorded images into radiosensitivity maps that could then be used for dose escalation. The present paper explores the influence of the conversion from uptake to prescribed doses. Transformation functions derived from the uptake properties of the PET tracers were taken into consideration. The results have shown that the available tracers have different uptake properties and therefore they could interpret differently the gradients in the images which in turn would lead to different dose predictions. Best results in terms of dose prescription would therefore be achieved by carefully taking into account the uptake characteristics of the imaged tracers. Linear approximations could lead to unnecessary over-estimations of the doses for cases of partial hypoxia in tumors. This highlights the need for more experimental studies of the uptake properties of PET tracers proposed to image tissue hypoxia. These would eventually provide more reliable methods for dose prescription that could be used with optimization algorithms for the successful individualization of radiation therapy.
|
|
