Learning Objectives: •Define first ionisation energy and successive ionisation energy.
1068 poster Correction factor for a pinpoint ionisation chamber in no reference conditions
1
$448 Posters Whereby the position of the patient is marked by laser systems which are available at both modalities. The accuracy of the registration is examined with external markers attached at the patient. The registration of the CT and PET data takes place by means of automatic or manual methods of a Hermes® workstation. Outlining the GTV is done in the CT data set (GTVcT), as well as in the fused CT-PET image (GTVcT÷PET). A comparative evaluation of the CT and CT+PET radiation plans is made using both physical as well as biological criteria. Result: By adequate and reproducible positioning techniques it is possible to fuse CT and PET imaging and to integrate CT-PET fused images into irradiation planning. For 20 patients with NSCLC outlining the GTVcT and GTVcT+PET w a s carried out. Here significant differences in the CT and CT+PET based volumes were found in 30% of the patients. For these cases a comparison was made for the dose- volume histograms based criteria: ICRU conformity, average value and homogeneity in the planning volume, tumor control probability (TCP) and normal tissue complication probability (NTCP). 1068 poster Correction factor for a pinpoint ionisation chamber in no reference conditions E. Hermoso-Espinosa 1'2,R. Capote-Noj '2, F. S~nchez- Doblado ~'2, R. Linares ~'2,J.I. Lagares ~'2, F.J. Sal.quero ~'2, A. Leaf '2, T.P. Boul61'2, R. Arrans 1'2, J.V. Rosell(53 l Universidad de Sevilla, Fisiologia M6dica y Biofisica, Seviile, Spain 2Hospital Universitario Virgen Macarena, Servicio de Radioterapia, Seville, Spain 3Hospital General ERESA, Radioterapia, Valencia, Spain Introduction: Dosimetry of beamlets used in Intensity Modulated Radio Therapy (IMRT) and radiosurgery, is associated with of non-standard conditions due to the lack of electronic equilibrium. Protocols for absolute dosimetry consider reference fields (10xl0cm 2) and assume electronic equilibrium conditions. For this reason, the use of ionisation chambers in situations of non-reference is not trustworthy. The objective of this study is to determine the limits for which a field has to be considered small, for a given energy and for a concrete ionization chamber, by means of the Monte Carlo method. In relation with these limits, we present a calibration factor (fwater -air---- Dwater/D~,) that should be used in these extreme situations. This study was performed by placing the chamber both in vertical position (especially useful in radiosurgery) and horizontal, for standard measurements. Materials and Methods: The accelerator used is a Siemens Primus with 6MV nominal photon energy. The ionisation chamber selected was the IBA-WellhSfer NAC 007 with an active volume of 0.007cm 2, and it was located at 100cm from the source and 10cm depth in water. The dose (Da~r) was measured in the chamber as the energy deposited in the active volume. The dose in water (Owater) was measured as the dose deposited in the same volume, filled and surrounded by water, i.e. in the absence of the chamber. Both were calculated using the simulation code CAVRZnrc of the EGSnrc package. The cut-off energies considered in the transport of electrons and photons were 521keV and lkeV, respectively. Results and Conclusions: The result obtained from Dwater/Dair provided the global factor (fw~ter -~r) necessary to convert the absorbed dose in air in dose to water. When the chamber was situated vertically, we obtained fwate~ -air ---- 1.07_+1.0% for a 10x10 cm 2 field. Analogously, when the chamber was in horizontal position, this value became 1.10_+1.0%. Thus, when the chamber was placed horizontally (10x10 cm 2 field) the factor was 3% higher than when situated in vertical position. For a 0.3x0.3 cm 2 field, the calculated fw~t .... ir was 1.12_+1.1%, when the chamber laid vertically. This value was 4.5% higher than that obtained for the 10x10 cm 2 field and this value decreased when the field size increased. For a 2x2cm 2 field, the increase was around 2%, for example. Analogously, for a 0.3x0.3 cm ~ field, we expect fwater -air to be higher when the chamber is situated horizontally. Lastly, we also checked that fwater -air increased while decreasing the field size. 1069 poster Comparison of 2 dose calculation algorithms in tangential field irradiation for breast cancer D. Eekers, M. Kunze, M. Velders, J. Jager, B. Nijsten, A. Minken, L. Boersma, P. Lambin U.H. Maastricht, Radiation Oncology (MAASTRO clinic), Maastricht, The Netherlands Background and aim: In the treatment planning for lung cancer, it has been shown that simple dose-calculation algorithms yield significantly different dose distributions than more advanced algorithms like the convolution superposition (CS-A) algorithm (De Jaeger et al. R&O 2003, 69, 2-10). To study the potential relevance of this finding in the optimization of techniques for breast irradiation, we investigated thls difference in algorithms for tangential field irradiation in breast cancer, with respect to the dose in the lung, heart, and breast tissue. Material & Methods: In 8 left sided breast cancer patients, standard tangential fields were virtually simulated, using a radio-opaque wire around the palpable breast tissue. The medial field border was placed in midline and the lateral field border close to mid-axillary. The cranial and caudal field borders were chosen 1.5 cm outside the radio-opaque wire. To allow analysis of the calculated dose in the breast tissue and in the organs at risk, we delineated the CTV using the radio-opaque wire, and extended the CTV with 1 cm in craniocaudal direction, and with 0.5 cm in the direction of the thoracic wall to obtain the PTV. Finally, the heart and the lungs were delineated. The dose distribution was optimized in the central plane using wedges, with the clinically used convolution algorithm (C-A). The resulting beam parameters of this optimization were used to calculate the 3D dose distributions for both algorithms. In one patient, the CS-A was also compared with a Monte Carlo calculation. Results: Although the V95, V107, Dmax, and Dmean of the PTV were significantly higher (p< 0.01, < 0.05, <0.01, < 0.05, respectively) when calculated with the CS-A than with the C- A, these differences were very small (on average 1.3%, 0.6%, 0.4 Gy, and 0.1 Gy, respectively). For the heart no significant differences were seen between the 2 calculation algorithms in V20, V30, V40, and V45. Although the V45 for lung was significantly lower when calculated with the CS-A (1.5%, p < 0.01), this small difference did not translate into significant differences in clinically relevant parameters as V20 and Mean Lung Dose. The Monte Carlo calculation performed in 1 patient yielded very similar results to the results of the CS-A, which confirmed the higher accuracy of the CS-A in case of inhomogeneities. Conclusion: Although the differences in the dose to PTV and lung, calculated by the CS-A and CS, were significant, these differences were very small, and not clinically relevant for tangential field irradiation in breast cancer.
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Transcript of 1068 poster Correction factor for a pinpoint ionisation chamber in no reference conditions
$448 Posters
Whereby the position of the patient is marked by laser systems which are available at both modalities. The accuracy of the registration is examined with external markers attached at the patient. The registration of the CT and PET data takes place by means of automatic or manual methods of a Hermes® workstation. Outlining the GTV is done in the CT data set (GTVcT), as well as in the fused CT-PET image (GTVcT÷PET). A comparative evaluation of the CT and CT+PET radiation plans is made using both physical as well as biological criteria.
Result: By adequate and reproducible positioning techniques it is possible to fuse CT and PET imaging and to integrate CT-PET fused images into irradiation planning. For 20 patients with NSCLC outlining the GTVcT and GTVcT+PET was carried out. Here significant differences in the CT and CT+PET based volumes were found in 30% of the patients. For these cases a comparison was made for the dose- volume histograms based criteria: ICRU conformity, average value and homogeneity in the planning volume, tumor control probability (TCP) and normal tissue complication probability (NTCP).
1068 poster
Correction factor for a pinpoint ionisation chamber in no reference conditions E. Hermoso-Espinosa 1'2, R. Capote-Noj '2, F. S~nchez- Doblado ~'2, R. Linares ~'2, J.I. Lagares ~'2, F.J. Sal.quero ~'2, A. Leaf '2, T.P. Boul61'2, R. Arrans 1'2, J.V. Rosell(53
l Universidad de Sevilla, Fisiologia M6dica y Biofisica, Seviile, Spain 2Hospital Universitario Virgen Macarena, Servicio de Radioterapia, Seville, Spain 3Hospital General ERESA, Radioterapia, Valencia, Spain Introduction: Dosimetry of beamlets used in Intensity Modulated Radio Therapy (IMRT) and radiosurgery, is associated with of non-standard conditions due to the lack of electronic equilibrium. Protocols for absolute dosimetry consider reference fields (10xl0cm 2) and assume electronic equilibrium conditions. For this reason, the use of ionisation chambers in situations of non-reference is not trustworthy.
The objective of this study is to determine the limits for which a field has to be considered small, for a given energy and for a concrete ionization chamber, by means of the Monte Carlo method. In relation with these limits, we present a calibration factor (fwater -air---- Dwater/D~,) that should be used in these extreme situations. This study was performed by placing the chamber both in vertical position (especially useful in radiosurgery) and horizontal, for standard measurements.
Materials and Methods: The accelerator used is a Siemens Primus with 6MV nominal photon energy. The ionisation chamber selected was the IBA-WellhSfer NAC 007 with an active volume of 0.007cm 2, and it was located at 100cm from the source and 10cm depth in water. The dose (Da~r) was measured in the chamber as the energy deposited in the active volume. The dose in water (Owater) w a s measured as the dose deposited in the same volume, filled and surrounded by water, i.e. in the absence of the chamber. Both were calculated using the simulation code CAVRZnrc of the EGSnrc package. The cut-off energies considered in the transport of electrons and photons were 521keV and lkeV, respectively.
Results and Conclusions: The result obtained from Dwater/Dair provided the global factor (fw~ter -~r) necessary to convert the absorbed dose in air in dose to water. When the chamber was situated vertically, we obtained fwate~ -air ---- 1.07_+1.0% for a 10x10 cm 2 field. Analogously, when the chamber was in horizontal position, this value became
1.10_+1.0%. Thus, when the chamber was placed horizontally (10x10 cm 2 field) the factor was 3% higher than when situated in vertical position. For a 0.3x0.3 cm 2 field, the calculated fw~t . . . . ir was 1.12_+1.1%, when the chamber laid vert ical ly. This value was 4.5% higher than that obtained for the 10x10 cm 2 field and this value decreased when the field size increased. For a 2x2cm 2 field, the increase was around 2%, for example.
Analogously, for a 0.3x0.3 cm ~ field, we expect fwater -air to be higher when the chamber is situated horizontally. Lastly, we also checked that fwater -a i r increased while decreasing the field size.
1069 poster
Comparison of 2 dose calculation algorithms in tangential field irradiation for breast cancer
D. Eekers, M. Kunze, M. Velders, J. Jager, B. Nijsten, A. Minken, L. Boersma, P. Lambin U.H. Maastricht, Radiation Oncology (MAASTRO clinic), Maastricht, The Netherlands Background and aim: In the treatment planning for lung cancer, it has been shown that simple dose-calculation algorithms yield significantly different dose distributions than more advanced algorithms like the convolution superposition (CS-A) algorithm (De Jaeger et al. R&O 2003, 69, 2-10). To study the potential relevance of this finding in the optimization of techniques for breast irradiation, we investigated thls difference in algorithms for tangential field irradiation in breast cancer, with respect to the dose in the lung, heart, and breast tissue.
Material & Methods: In 8 left sided breast cancer patients, standard tangential fields were virtually simulated, using a radio-opaque wire around the palpable breast tissue. The medial field border was placed in midline and the lateral field border close to mid-axillary. The cranial and caudal field borders were chosen 1.5 cm outside the radio-opaque wire. To allow analysis of the calculated dose in the breast tissue and in the organs at risk, we delineated the CTV using the radio-opaque wire, and extended the CTV with 1 cm in craniocaudal direction, and with 0.5 cm in the direction of the thoracic wall to obtain the PTV. Finally, the heart and the lungs were delineated. The dose distribution was optimized in the central plane using wedges, with the clinically used convolution algorithm (C-A). The resulting beam parameters of this optimization were used to calculate the 3D dose distributions for both algorithms. In one patient, the CS-A was also compared with a Monte Carlo calculation.
Results: Although the V95, V107, Dmax, and Dmean of the PTV were significantly higher (p< 0.01, < 0.05, <0.01, < 0.05, respectively) when calculated with the CS-A than with the C- A, these differences were very small (on average 1.3%, 0.6%, 0.4 Gy, and 0.1 Gy, respectively). For the heart no significant differences were seen between the 2 calculation algorithms in V20, V30, V40, and V45. Although the V45 for lung was significantly lower when calculated with the CS-A (1.5%, p < 0.01), this small difference did not translate into significant differences in clinically relevant parameters as V20 and Mean Lung Dose. The Monte Carlo calculation performed in 1 patient yielded very similar results to the results of the CS-A, which confirmed the higher accuracy of the CS-A in case of inhomogeneities.
Conclusion: Although the differences in the dose to PTV and lung, calculated by the CS-A and CS, were significant, these differences were very small, and not clinically relevant for tangential field irradiation in breast cancer.
