ETD system

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Tesi etd-09082016-201751

Thesis type
Tesi di specializzazione (3 anni)
Corso di studi
relatore Dott.ssa Ravaglia, Valentina
Parole chiave
  • radiochromic films
  • radiotherapy
  • skin dose
  • in vivo dosimetry
  • multichannel dosimetry
Data inizio appello
Riassunto analitico
The development and the spreading of complex treatment modalities in radiotherapy, such as Intensity Modulated Radiation Therapy (IMRT), Stereotactic Radiosurgery (SRS), and Volumetric Modulated Arc Therapy (VMAT), have provided great achieving in reducing dose to healthy tissues and delivering radiation targeted for very specific lesions. On the other hand, these techniques are characterized by high dose gradients and considerable time and space variations of dose rate and beam fluence, leading to an increase of the complexity of treatment planning and delivery. For these reasons, more specific dosimetric solutions are necessary to the radiotherapy treatment verification and quality assurance (QA).
Among the various types of dosimeters, at present GafChromic EBT3 films represent one of the most powerful tools to perform QA with high accuracy and measure absolute dose in radiotherapy. They have very high spatial resolution, weak energy dependence in a wide photon energy range, and no angular dependence. Furthermore, radiochromic films present the advantages of easy handling, being insensitive to room light, self-developing, and they can be digitalized using a common flatbed charge couple device (CCD) scanner. The characteristics of near-tissue equivalence, effective point of measurement very close to the clinically relevant and recommended depth for skin of 70 μm (ICRP 1991; ICRU 1985) and the possiblility of being cut and positioned at various locations on the patient, allow to use EBT3 film as in vivo dosimeter and evaluate skin dose during radiotherapy treatments.
This thesis aims to describe the use of GafChromic EBT3 films for pre-treatment QA applying both single channel and multichannel methods, and to report and analyse skin dose measurements using radiochromic film as in vivo dosimeter.
In first place, an overview of the main principles, characteristics and applications of radiochromic films and of flatbed scanner dosimetry is provided. Secondly, the setting out of the scanning protocol and the measurements and analyses performed to characterize the scanner behaviour with differently exposed films along many positions on the scanbed are described.
Then, an account of the calibration measurements, performed both at Linac and in Plesio-Röntgen therapy, is given, focusing on fit analyses performed for single channel dosimetry with red color channel. Moreover, Linac calibration has been also performed with a method that combines green and blue channels data together (Fiandra et al. 2013; McCaw et al. 2011) and using multichannel dosimetry (Pérez Azorín et al. 2014).
With the purpose of validating GafChromic film dosimetry method and investigating which of the EBT3 calibration approaches better responded to the range of doses commonly used in radiotherapy, some films irradiated at Linac with different standard fields have been compared with data measured through PTW 2D-Array 729. Afterwards, pre-treatment QA measurements have been performed with EBT3, and absolute dose maps have been compared with TPS calculations using γ-analysis (Low et al. 1998).
Finally, in vivo measurements performed during radiotherapy treatments are described. Skin dose was evaluated on patients undergoing helical tomotherapy treatments for different pathologies, including head and neck and sarcoma. Moreover, measurements on patients with breast cancers treated at Linac both with 3D conformal radiotherapy (3D-CRT) and VMAT, and skin lesions treated with Plesio-Röntgen therapy have been considered. Then, in vivo measurements, analysed with GafChromic dosimetry, have been compared with TPS planned doses, investigating on factors possibly related to discrepances between expected and measured doses.
If compared with the other calibration approaches, multichannel method resulted the most accurate in determining the absolute dose maps regarding both validation fields and pre-treatment QA measurements, leading to a mean value of 92% for the γ(3%, 3mm) local dose pass rate.
In vivo measurements showed a good reproducibility, especially regarding measuremnts performed in tomotherapy with the application of thermoplastic masks, that lead to a more reliable repositioning of the film if compared with skin measurements performed on breast treatment at Linac. Furthermore, absolute percentage differences between EBT3 reading and TPS expected dose often were higher than 10%, confirming a higher uncertainty of TPS estimated dose in the first mm of skin.