Sairos Safai

Talk:   Physical Modelling for Proton Therapy Planning and Beam Delivery
Sairos Safai, PhD. Paul Scherrer Institut, Switzerland

Friday, March 22 nd, 2013      ………………….    at IFIC ‘s seminars room, 16:00 h
(Av. Agustín Escardino; Parque Científico de la Univ. Valencia, 300m from Canal 9 TVV. )

Abstract: Over the past decade it has been demonstrated that intensity modulated radiation therapy can be safely delivered to patient with both photons and/or ions. With protons (or other ions) this technique is known as intensity modulated proton (or particle) therapy (IMPT). IMPT offers much better spare of organs at risk with the potential to further escalate target-dose compared to classical proton therapy. The underlying paradigm of this presentation is the following:
“The best technique to fulfil the mission assigned to radiation therapy is IMPT, that is, the ability to modulate each particle pencil beam to obtain an inhomogeneous dose distribution within the target per field direction”.
This paradigm holds in particular when we think of the great potential that biological dose targeting may play in the future. We would then bring intensity modulated therapy to its extreme, in which not only the dose per field is inhomogeneous but also the overall dose distribution over all fields. What improvements are therefore necessary to be able to use IMPT to its full potential? In this regard we will consider the status and improvements of current physical modelling for proton therapy planning and beam delivery, in particular we will focus on:

• Modelling of nuclear interactions: nuclear interactions affect both the integral depth dose curve as well as the lateral profile. Up to 9% dose differences could be observed between measured and calculated dose when nuclear interactions are not taken into account.
• Pencil beam angular-spatial distribution and multiple Coulomb scattering in different materials: the challenge of low density tissues, e.g. lung.
• Beam delivery:
o   the need of small pencil beams
o   the challenge of delivering the dose to shallow tumours
• Biology aspects:
o   RBE modelling for protons: even though a constant global RBE value of 1.1 is widely accepted for protons, studies have shown that also for protons RBE may change as a function of LET, i.e., at the distal fall-off RBE could be as high as 20. Therefore, RBE may cause an increased biological dose (10-20%) in some localised positions and extend the biologically effective range by 1-2 mm. However, since RBE is also dependent on the end-point, dose, dose-rate and cell-type it may still be premature to move away from the global value of 1.1; the biological uncertainties are just too large.
• Range uncertainties: even though in water we can achieve very good agreements between calculations and measurements, the patient may, to some extent, reduce this confidence. Range uncertainties are indeed the biggest challenge in proton therapy. The redistribution of pencil beams within the target by the introduction of robust optimization methods can improve the robustness of the IMPT plans against range errors.

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