RBE-weighted proton radiation dosimetry: Algorithm development

The main advantage of Proton beam therapy (PBT) over conventional radiotherapy (RT) is the more precise geometrical shape of the energy deposition inside the patient. The Bragg peak at the end of the proton range allows delivery of accurate dose in a deep seated cancer, that also reduces dose to surrounding normal tissues [4],[15],[16]. The proton beam causes higher density of ionization events along its track, which can result in irreparable damage. The irreparable damage is more apparent at the end of the beam path and is the origin of the enhanced biological efficiency in the Bragg peak region. This biological efficiency is called Relative Biological Effectiveness (RBE) and depends upon many biological and physical parameters [23].;The RBE can often be measured by cell survival experiments in-vitro or by biophysical models [2]. Proton radiation has been shown to be more biologically effective for cell killing compared with X-rays for human tissue because of the higher density of ionization tracks [1],[4]. Clinically to date, RBE of 1.1 (WRBE=1.1) is applied to all treatments independent of dose/fraction, position in the Spread Out Bragg Peak (SOBP), initial beam energy and the tissue type [4],[2],[27]-[29]. However several studies reported that the RBE depends on the Dose-averaged Linear Energy Transfer (LETd), cell or tissue type which is a function of its (&agr;/beta) x and the dose per fraction. The variations of (LET d) values have been observed within the exposure volume in proton treatment. The RBE values are directly proportional to (LET d) and inversely proportional to (&agr;/beta) x [30]-[35]. These dependences make the RBE values vary from point to point along the proton track especially where an SOBP is employed to treat the planning target volume (PTV) region.;In treatment planning, any potential variation of RBE over the SOBP could result biological hot spots with wide variations in biological dose that make dosimetry difficult [25]. This thesis presents simplistic proton RBE-weighted treatment plans in two-dimensions and compares them with standard proton plans using WRBE=1.1. The isodose distribution profiles were accomplished using matrixes that represent coplanar intersecting beams. These matrixes were combined and contoured to clarify the distribution of dose using standard RBE or other various values of RBE (WRBE=ref[25]).;There are some differences in dose distribution between the (WRBE=1.1 ) and the modeled values of RBE (WRBE=ref[25]). The hot spots of WRBE=ref[25] remain inside the PTV with higher RBE values. However increased dose also appeared outside of the PTV that may cause damage to healthy tissue in the body.