Study of in vivo Dosimetry for Proton Therapy

The Chang Gung Memorial Hospital (CGMH) is hosting a cutting edge new proton therapy facility which will begin to treat patients in 2013. A few other hospitals in Taiwan are also planning to import and construct proton facilities for clinical applications. Comparing to photon therapy, proton therapy beams have excellent physical properties of delivering higher dose to the tumor and lower doses to surrounding normal tissues. Radiation doses to the critical organs right behind the Bragg peak are essentially zero so that healthy tissues can be totally protected. As a result, the tumor control probability (TCP) is increased, the normal tissue complication probability (NTCP) is reduced, and the occurrence of secondary cancers is minimized. Quality assurance (QA) is very important in proton therapy. The QA includes pre-treatment phantom dose measurements, treatment-planning CT dose simulations, and on-treatment in vivo dose verifications. In the Bragg peak region where tumor is located, the dosimetry QA is rather difficult since proton energy is rapidly degraded, dose gradient is sufficiently large, and relative biological effectiveness (RBE) is increased at the distal edge. Because of these, ultra-thin dosimeters, Monte Carlo simulations, and microdisimetry methods are needed for the dose measurements, dose calculations, and RBE determinations. It is recommended in International Commission on Radiation Units and Measurements (ICRU) Report 78 that RBE-weighted absorbed doses should be recorded and reported in proton therapy, especially around the spread-out Bragg peak (SOBP) region. In this project, we propose to study the in vivo dosimetry for proton therapy by applying the following techniques: (1) measurements of absorbed doses using calibrated ionization chambers to establish dose standards for comparisons, (2) studies of characteristic properties of metal oxide semiconductor field effect transistors (MOSFETs) for in vivo dose verifications, (3) measurements of microdosimetric lineal energies using self-designed mini tissue-equivalent proportional counters (mini TEPCs) for RBE determinations, (4) simulations of MOSFETs and the mini TEPCs by the Monte Carlo FLUKA code for absorbed dose and RBE calculations. In CGMH, two different techniques are applied to generate SOBP proton beams, i.e. the wobbling technique and the pencil-beam scanning technique. The advanced pencil-beam technique produces fine proton beams of unique properties different from broad beams of the Wobbling technique. Due to the characteristic feature of pencil beam scanning technique, it requires thorough investigations of in vivo dosimetry. The dosimeters and methodologies to be studied in this project involve some advancement of newly developed technologies. The present study is important not only for academic research but also for clinical application. Results of this study will be provided to hospitals for clinical use in QA and in vivo dosimetry. Medical physicist training is an integral part of the objectives of this study.

Project ID:PC10207-0438
External Project ID:NSC102-2314-B182-048