SP-0017: Reference dosimetry in low and medium energy x-rays from air kerma to dose to water standards

P. Mayles
2013 Radiotherapy and Oncology  
S6 2 nd ESTRO Forum 2013 images but also underline the need for harmonizing image resolution and data analysis methodology to ensure harmonized quantitative performance across multiple sites. This lecture will focus on the impact and interplay of image resolution, noise and data collection and analysis methodology on the various ways of PET tracer uptake quantification. Moreover, the impact of relatively new PET technologies, such as time of flight technology and new image reconstruction
more » ... construction algorithm, on the accuracy and precision of tracer uptake quantification will be addressed. Finally, based on the first results obtained from the EARL accreditation program, the feasibility and need for a central QA/QC accreditation program will be discussed. The role of positron emission tomography (PET) scanning in radiation oncology has evolved as it is the most specific and sensitive means of imaging fundamental aspects of tumour biology. PET has been applied in different RT procedures: correct staging and optimal treatment strategy, accurate delineation of biological target volume (BTV) for treatment planning, prediction of tumor response, evaluation of healthy tissue function after radiotherapy. In the era of high-precision radiotherapy, accurate tumor volume delineation regarding tumor boundaries, shape and volume is crucial. Quite different approaches have been used for target volume delineation on PET images: the anatomic sites of the pathologic zones on PET scan were delineated on CT scan, absolute/relative thresholding algorithms, complex algorithms (i.e. gradient/statistical based methods). The introduction of the combined PET/CT imaging modalities into routine clinical RT practice has promise to be of great clinical significance in the accurate delineation of RT target volume in the treatment of cervix, head and neck, and lung cancers. The overall sensitivity, specificity and accuracy of FDG-PET for detection of lung cancers are very high for primary, residual and recurrent disease; contour guided by FDG-PET/CT led to significant modification of treatment strategy and radiotherapy planning in NSCLC patients. A limiting factor to the accuracy of target volume definition by PET/CT is organ and tumor motion, which is mainly due to the patient respiration. Motion management is thus becoming an important issue in both diagnostic and RT applications, particularly when PET/CT images are used for tumor delineation; within this contest 4D techniques provide information which can be used to improve/personalize volume definition and treatment planning strategy. Careful comparison of FDG-PET, MRI and CT scans with the histopathology of resented tumor specimens shows is the most accurate of the three for the detection of head and neck cancer. PET images are used for tumor detection and delineation, however such images may also contain information on the spatial distribution of factors influencing tumor radiosensitivity (hypoxia, proliferation) and a few studies have used PET in biologic image-guided dose escalation to the radioresistant BTV using IMRT. This use of PET imaging in combination with dose escalation is of great interest in tumor sites such as head-and-neck and the prostate. Kilovoltage x-rays are something of a Cinderella subject in radiotherapy physics although accurate absolute dosimetry is perhaps more challenging than for megavoltage beams. The first step is to determine the energy/quality of the beam. This is usually done in terms of the first half value layer (HVL) measured under narrow beam conditions. In order to fully characterise the energy spectrum the generating kV is also needed and TG-61 shows that there is wide variation in the HVL for the same generating potential. The IAEA defines low energy as up to a generating potential of 100 kV or 3 mm of Al HVL and medium energy as starting at 80kV or 2mm of Al. The IPEM defined an additional very low energy band from 0.035 mm of Al up to 1 mm (8-50kV) with low energy considered as up to 160kV. Most standards laboratories offer only an air kerma calibration based on ionometry, but PTB also has a calorimetric standard. The IAEA TRS398 code of practice offers a formalism for the use of a calibration factor defined in terms of absorbed dose to water and suggests that standards laboratories that offer only an air kerma calibration could also provide calibrations in terms of absorbed dose to water by applying an appropriate air kerma code of practice. In IAEA TECDOC1455 comparing TRS398 to air kerma based codes of practice differences of up to 4% were found for low energies but agreement was within 1% for medium energies. There are two ways in which the output of the machine can then be measured: a measurement of air kerma in air together with the application of a backscatter factor or a calibration at a relevant depth in a phantom. In the latter case a perturbation correction is needed. The magnitude of these corrections is calculated using Monte Carlo methods. Changes to codes of practice of around 7% (at 100kV) have been required (IAEA TRS277 and IPEMB revision 2005). The consensus seems to be that measurement at 20 mm depth in water is the preferred approach for medium energy x-rays but codes of practice are divided about whether measurements at very low energy (40kV) should be in air (e.g. TG61) or a phantom (e.g. IPEMB). There is general agreement that plastic phantoms need to be used with care. Newer devices such as the Zeiss Intrabeam device and the Ariane Papillon machine use 50kV x-rays with more challenging geometries. For these devices special jigs are needed to ensure geometric accuracy and GafChromic EBT2 film may also offer an appropriate means of reference dosimetry especially for small fields. For low energies the requirement for controlled geometry suggests the use of plastic phantoms but for medium energies the use of liquid water reduces uncertainty. SP-0018 Monte Carlo for low and medium energy x-ray beam modelling Purpose/Objective: The Monte Carlo method is the most accurate method for radiation measurement simulation and dose calculation. This presentation reviews the recent advances of Monte Carlo for low and medium energy x-ray beam modeling. Material/methods: In the last 20 years significant developments have been made in the areas of radiation transport theory, Monte Carlo simulation techniques and computer technology, which have enabled the Monte Carlo method for widespread applications in radiation measurement and clinical radiotherapy dosimetry. Kilovoltage x-rays are different from megavoltage x-rays due to their excessive scattering properties and short electron ranges,which make it possible to simulate photon transport only in some Monte Carlo applications such as brachytherapy dose calculation. Results: We will first start with the Monte Carlo modeling of the airfilled ionization chamber response in low and medium energy x-ray beams where the fractional contributions of the secondary electrons from the cavity air and the surrounding chamber media were investigated accurately. This will be followed by Monte Carlo studies of ionization chamber corrections factors for the chamber stem and waterproofing sheath that demonstrated the accuracy and efficiency of Monte Carlo simulations of the photon attenuation and scattering effects with the use of correlated sampling techniques. More detailed reviews of the Monte Carlo method for radiotherapy dosimetry and treatment planning will be given with a focus on radiation source modeling, kilovoltage CT dose calculation, and treatment planning dose calculation for external beam therapy and brachytherapy. Conclusion: The Monte Carlo method has been demonstrated as a useful simulation tool for accurate dosimetry measurement and radiotherapy dose calculation. SP-0019 Radiobiology of kilovoltage x-rays A
doi:10.1016/s0167-8140(15)32323-9 fatcat:c2nq3iflynehdgxnonvsepqkou