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119 invited Quality assurance of a treatment planning system for IMRT applications
Symposia/Proffered papers
119
invited
Quality assurance of a treatment planning system for IMRT applications A.D. Bruinvis 1, E.M.F. Damen 2 1Radiation Therapy Institute Limburg (Academic Hospital Maastricht), Clinical Physics, Heerlen, The Netherlands, 2The Netherlands Cancer Institute (Antoni van Leeuwenhoekhuis), Radiation Therapy, Amsterdam, The Netherlands
Introduction: Quality assurance (QA) of a modern Treatment Planning System (TPS) concerns many aspects of 3-D patient anatomy, beam setup and dose calculations. Methods of optimisation and output ofthe treatment plan should also be considered. Intensity Modulated Radiation Therapy (IMRT) applications make specific use of a TPS. The aim of this presentation is to identify these specific IMRT aspects, discuss the associated QA items and propose QA procedures. Methods and results: One should distinguish delivery using dynamic multileaf collimation and step-and-shoot techniques using many beam segments. Step-and-shoot IMRT requires correct bookkeeping of many segments and grouping of segments with the same beam direction. Printed and electronic output of the treatment plan involves a large amount of data. A printed overview of the BEVs of all segments is required. For segments produced automatically by optimization software, all limitations of the specific type of MLC should be taken into account. Dosimetric verification should include fields down to 1 cm x 1 cm and elongated fields like 1 cm x 10 cm, both symmetrical and with a large offset. A large fraction of the total dose may consist of doses in blocked parts of fields. Therefore the accuracy of calculated dose at such points is important. Since optimization methods often use approximate dose calculations, the resulting segments should be recalculated with the proper algorithm of the TPS. Dynamic IMRT requires verification of leaf trajectories transfer from the TPS to the linac, e.g. by measurement with photographic film or electronic portal imager. 2-D dose verification is required for each beam. In the dynamic case recalculation of dose with the proper algorithm is often not possible; the approximate algorithm should then be tested carefully. Conclusion: In order to assure an adequately designed IMRT plan and corresponding treatment of the patient, a great effort in QA is needed. This comprises verification of TPS software, data transfer and actual geometric and dosimetric parameters of the individual treatment delivery. At present various reports offer guidelines and practical tests, but more work is required. 120
oral
Implementation and verification of a photon beam multisource model A. Ahnesjo 1, M. Saxner 1, L. Weber2, H. Hansson 2, A. Murman 1, E. Traneus 1, I. Thorslund 1 1MDS Nordion, Uppsala, Sweden, 2MDS Nordion, Lund, Sweden Accurate dose and monitor units calculations for photon beam treatment planning requires modelling of the beam phase space exiting the treatment machine. In this work, a multisource model with different source representations for the direct, unscattered radiation and the indirect, scattered radiation from the flattening filter, collimators and modulators is used. The spatial variation of the energy fiuence components are modelled based on geometrical and interaction properties of the beam, considering first scatter physics with measurement based corrections for higher order effects. Spectral variations of the direct radiation are considered from lateral variation of attenuation properties. The model is locally adapted for users of the Helax-TMS and DCM (MDS Nordion) dose calculation modules by means of parameter fitting of locally measured data. Calculated beam data are compared to measured beam data for approx. 200 clinics and deviations scored in different beam regions. The result is analysed with respect to dose calculation model, energy range and wedge angle. Results show mean deviations on the order of 1% for open beams, with slightly better results for the collapsed cone point kernel model as compared to the pencil kernel model. This is expected since the pencil kernel model neglects off axis softening effects. The result for wedged beam show larger deviations, essentially proportional to the wedge angle with deviations exceeding 2% for 60 degree wedges reflecting the dosimetry problems associated with high dose gradients. In conclusion, the multisource model
Tuesday, 18 September 2001
$45
is shown to provide a high degree of accuracy for a broad variety of clinical beam data. 121
oral
Sparing parotids and increasing conformity in laryngopharyngeal treatments: development of a new conformal technique, description of a complete quality assurance program and first results M. Tomsei, V. Gregoire, S. Vynckier, P. Scalliet U.C.L. St-Luc University Hospital, Radiation oncology, Brussels, Belgium To deliver a homogeneous dose to the head and neck region and specifically for laryngopharyngeal tumors, a complex treatment plan is often required with wedges oriented along both anterior-posterior and/or caudalcranial directions due to the well known complicated curvature of the patient skin in this region. Previously, a dose of 70 Gy is delivered on target volumes (shoe-horse shaped PTV) with large lateral fields, boosted at the dose of about 40 Gy and finally complemented with two lateral spinal electron fields. We propose in this study a new possibility to achieve this treatment in order to avoid the junction of the electron fields with the boost photon fields creating very cold and very hot spots, for a better sparing of parotids and to increase a lot the conformity index. A combination of five conformal segmented photon fields are applied to deliver to the PTV 55 Gy and then 15 Gy are delivered to a new boosted target volume. Those five photon beams are distributed as an anterior segmented field blocking partially parotids, two anterior oblique segmented fields and two posterior oblique fields blocking the spinal cord and partially parotids. A complete quality assurance program has been set up, beginning with the study of immobilization (1), the development of delineation of volumes (2) and to finally achieve a dosimetry quality control of the resulting intensity modulated beams in terms of output and relative dose distributions. This has been performed in a water phantom to check the output and the dose distributions for each field. Moreover, we then performed verifications on an Alderson phantom with thermoluminescence detectors and films. Relative and absolute dosimetry measurements were then compared to calculated results from our treatment planning system, HELAX-TMS v 5.1. The results of this study will be compared and discussed. (1): L. Gilbeau et al., Comparison of set-up accuracy of three different thermoplastic masks for the treatment of brain and head and neck tumors. Radiother. Oncol. 2001 Feb 1; 58(2): 155-162 (2): V. Gregoire et al., Selection and CT-based delineation of the lymph node target volumes in head and neck conformal radiotherapy. Standardizing the procedure based on the surgical terminology. Radiother. Oncol. 2000 Aug; 56(2): 135-150
IMRT - P L A N N I N G A N D O P T I M I S A T I O N II 122
oral
The effect of air cavities on dose homogeneity for multiplebeam arrangements in stereotactic radiosurgery and radiotherapy J.L. Robar BC Cancer Agency, Department of Medical Physics, Vancouver, Canada
Purpose: For a single, narrow photon beam passing though an air cavity, the perturbation of the dose at the distal air/tissue interface is significant due to both longitudinal and lateral electronic disequilibrium. However, it is difficult to predict whether this effect is important for realistic beam arrangements, in which a minority of the beams pass through the air cavity before reaching the target. We present a Monte Carlo study of an arrangement of radiosurgical beams incident on a head-and neck phantom. The degradation of tumour dose homogeneity has been quantified systematically as a function of i) the fraction of the total beam arrangement passing through an air cavity and ii) the air cavity thickness. The goal of this work is to provide guidelines for SRS/SRT treatment planning. Methods: The Monte Carlo BEAM system was used to model a 6 MV stereotactic treatment unit. The resultant beam phase-space record was validated against measured depth dose and beam profiles for both homogeneous and inhomogeneous geometries. The phase-space record was then used to simulate a regular arrangement of radiosurgical beams inci-
119
invited
Quality assurance of a treatment planning system for IMRT applications A.D. Bruinvis 1, E.M.F. Damen 2 1Radiation Therapy Institute Limburg (Academic Hospital Maastricht), Clinical Physics, Heerlen, The Netherlands, 2The Netherlands Cancer Institute (Antoni van Leeuwenhoekhuis), Radiation Therapy, Amsterdam, The Netherlands
Introduction: Quality assurance (QA) of a modern Treatment Planning System (TPS) concerns many aspects of 3-D patient anatomy, beam setup and dose calculations. Methods of optimisation and output ofthe treatment plan should also be considered. Intensity Modulated Radiation Therapy (IMRT) applications make specific use of a TPS. The aim of this presentation is to identify these specific IMRT aspects, discuss the associated QA items and propose QA procedures. Methods and results: One should distinguish delivery using dynamic multileaf collimation and step-and-shoot techniques using many beam segments. Step-and-shoot IMRT requires correct bookkeeping of many segments and grouping of segments with the same beam direction. Printed and electronic output of the treatment plan involves a large amount of data. A printed overview of the BEVs of all segments is required. For segments produced automatically by optimization software, all limitations of the specific type of MLC should be taken into account. Dosimetric verification should include fields down to 1 cm x 1 cm and elongated fields like 1 cm x 10 cm, both symmetrical and with a large offset. A large fraction of the total dose may consist of doses in blocked parts of fields. Therefore the accuracy of calculated dose at such points is important. Since optimization methods often use approximate dose calculations, the resulting segments should be recalculated with the proper algorithm of the TPS. Dynamic IMRT requires verification of leaf trajectories transfer from the TPS to the linac, e.g. by measurement with photographic film or electronic portal imager. 2-D dose verification is required for each beam. In the dynamic case recalculation of dose with the proper algorithm is often not possible; the approximate algorithm should then be tested carefully. Conclusion: In order to assure an adequately designed IMRT plan and corresponding treatment of the patient, a great effort in QA is needed. This comprises verification of TPS software, data transfer and actual geometric and dosimetric parameters of the individual treatment delivery. At present various reports offer guidelines and practical tests, but more work is required. 120
oral
Implementation and verification of a photon beam multisource model A. Ahnesjo 1, M. Saxner 1, L. Weber2, H. Hansson 2, A. Murman 1, E. Traneus 1, I. Thorslund 1 1MDS Nordion, Uppsala, Sweden, 2MDS Nordion, Lund, Sweden Accurate dose and monitor units calculations for photon beam treatment planning requires modelling of the beam phase space exiting the treatment machine. In this work, a multisource model with different source representations for the direct, unscattered radiation and the indirect, scattered radiation from the flattening filter, collimators and modulators is used. The spatial variation of the energy fiuence components are modelled based on geometrical and interaction properties of the beam, considering first scatter physics with measurement based corrections for higher order effects. Spectral variations of the direct radiation are considered from lateral variation of attenuation properties. The model is locally adapted for users of the Helax-TMS and DCM (MDS Nordion) dose calculation modules by means of parameter fitting of locally measured data. Calculated beam data are compared to measured beam data for approx. 200 clinics and deviations scored in different beam regions. The result is analysed with respect to dose calculation model, energy range and wedge angle. Results show mean deviations on the order of 1% for open beams, with slightly better results for the collapsed cone point kernel model as compared to the pencil kernel model. This is expected since the pencil kernel model neglects off axis softening effects. The result for wedged beam show larger deviations, essentially proportional to the wedge angle with deviations exceeding 2% for 60 degree wedges reflecting the dosimetry problems associated with high dose gradients. In conclusion, the multisource model
Tuesday, 18 September 2001
$45
is shown to provide a high degree of accuracy for a broad variety of clinical beam data. 121
oral
Sparing parotids and increasing conformity in laryngopharyngeal treatments: development of a new conformal technique, description of a complete quality assurance program and first results M. Tomsei, V. Gregoire, S. Vynckier, P. Scalliet U.C.L. St-Luc University Hospital, Radiation oncology, Brussels, Belgium To deliver a homogeneous dose to the head and neck region and specifically for laryngopharyngeal tumors, a complex treatment plan is often required with wedges oriented along both anterior-posterior and/or caudalcranial directions due to the well known complicated curvature of the patient skin in this region. Previously, a dose of 70 Gy is delivered on target volumes (shoe-horse shaped PTV) with large lateral fields, boosted at the dose of about 40 Gy and finally complemented with two lateral spinal electron fields. We propose in this study a new possibility to achieve this treatment in order to avoid the junction of the electron fields with the boost photon fields creating very cold and very hot spots, for a better sparing of parotids and to increase a lot the conformity index. A combination of five conformal segmented photon fields are applied to deliver to the PTV 55 Gy and then 15 Gy are delivered to a new boosted target volume. Those five photon beams are distributed as an anterior segmented field blocking partially parotids, two anterior oblique segmented fields and two posterior oblique fields blocking the spinal cord and partially parotids. A complete quality assurance program has been set up, beginning with the study of immobilization (1), the development of delineation of volumes (2) and to finally achieve a dosimetry quality control of the resulting intensity modulated beams in terms of output and relative dose distributions. This has been performed in a water phantom to check the output and the dose distributions for each field. Moreover, we then performed verifications on an Alderson phantom with thermoluminescence detectors and films. Relative and absolute dosimetry measurements were then compared to calculated results from our treatment planning system, HELAX-TMS v 5.1. The results of this study will be compared and discussed. (1): L. Gilbeau et al., Comparison of set-up accuracy of three different thermoplastic masks for the treatment of brain and head and neck tumors. Radiother. Oncol. 2001 Feb 1; 58(2): 155-162 (2): V. Gregoire et al., Selection and CT-based delineation of the lymph node target volumes in head and neck conformal radiotherapy. Standardizing the procedure based on the surgical terminology. Radiother. Oncol. 2000 Aug; 56(2): 135-150
IMRT - P L A N N I N G A N D O P T I M I S A T I O N II 122
oral
The effect of air cavities on dose homogeneity for multiplebeam arrangements in stereotactic radiosurgery and radiotherapy J.L. Robar BC Cancer Agency, Department of Medical Physics, Vancouver, Canada
Purpose: For a single, narrow photon beam passing though an air cavity, the perturbation of the dose at the distal air/tissue interface is significant due to both longitudinal and lateral electronic disequilibrium. However, it is difficult to predict whether this effect is important for realistic beam arrangements, in which a minority of the beams pass through the air cavity before reaching the target. We present a Monte Carlo study of an arrangement of radiosurgical beams incident on a head-and neck phantom. The degradation of tumour dose homogeneity has been quantified systematically as a function of i) the fraction of the total beam arrangement passing through an air cavity and ii) the air cavity thickness. The goal of this work is to provide guidelines for SRS/SRT treatment planning. Methods: The Monte Carlo BEAM system was used to model a 6 MV stereotactic treatment unit. The resultant beam phase-space record was validated against measured depth dose and beam profiles for both homogeneous and inhomogeneous geometries. The phase-space record was then used to simulate a regular arrangement of radiosurgical beams inci-