A study of lateral fall-off (penumbra) optimisation for pencil beam scanning (PBS) proton therapy

Abstract

The lateral fall-off is crucial for sparing organs at risk in proton therapy. It is therefore of high importance to minimize the penumbra for PBS. Three optimisation approaches are investigated: Edge-collimated uniformly weighted spots (collimation), pencil beam optimisation of uncollimated pencil beams (edge-enhancement) and the optimisation of edge collimated pencil beams (collimated edge-enhancement). To deliver energies below 70MeV, these strategies are evaluated in combination with the following pre-absorber methods: field specific fixed thickness pre-absorption (fixed), range specific, fixed thickness pre-absorption (automatic) and range specific, variable thickness pre-absorption (variable). All techniques are evaluated by Monte Carlo simulating square fields in a water tank. For a typical air gap of 10cm, without pre-absorber collimation reduces the penumbra only for water equivalent ranges (WER) between 4cm to 11cm by up to 2.2mm. The sharpest lateral fall-off is achieved through collimated edge-enhancement, which lowers the penumbra down to 2.8mm. When using a pre-absorber, the sharpest fall-offs are obtained when combining collimated edge-enhancement with a variable pre-absorber. For edge-enhancement and large air gaps, it is crucial to minimize the amount of material in the beam. For small air gaps however, the superior phase space of higher energetic beams can be employed when more material is used. In conclusion, collimated edge-enhancement combined with the variable pre-absorber is the recommended setting to minimize the lateral penumbra for PBS. Without collimator, it would be favourable to use a variable pre-absorber for large air gaps and an automatic pre-absorber for small air gaps.

What will the medical physics of proton therapy look like 10 yr from now? A personal view

“…The whole picture is also complicated by the need to use pre‐absorbing material (a range shifter) in the nozzle to deliver superficial Bragg peaks, that is, those with a residual range in the patients of about 4 cm or less. Such pre‐absorbers scatter the beam, and if the distance to the patient is not minimized can lead to considerable broadening of the pencil beam widths …”

“…Such pre-absorbers scatter the beam, and if the distance to the patient is not minimized can lead to considerable broadening of the pencil beam widths. 20 The lateral penumbra for superficial fields can be improved by a number of approaches. The first, and probably the hardest, is to reduce the minimum energy that can be applied through the treatment machine.…”

Parametric characterization of penumbra reduction for aperture-collimated pencil beam scanning (PBS) proton therapy

“…Multiple studies have been conducted investigating the use of apertures in PBS proton beam delivery (Rana et al, 2013, Charlwood et al, 2016, Gottschalk, 2011, Urie et al, 1986, Safai et al, 2008, Winterhalter et al, 2017, Titt et al, 2010, Baumer et al, 2018. For example, Charlwood et al performed Monte Carlo (MC) simulations of a PBS nozzle and characterized lateral penumbra as a function of depth for varying energies and found adding collimation to a PBS beam can sharpen penumbra by 2 -4 mm depending on depth (Charlwood et al, 2016).…”

Development, commissioning, and evaluation of a new intensity modulated minibeam proton therapy system

“…In comparison to legacy double scattering systems, IMPT systems offer advantages in flexible application, use efficiency, and proximal dose sparing. The conformity of IMPT is determined by optimization type and strategy, as well as the physical properties of the proton beamlets . However, lateral and distal target penumbra of noncollimated IMPT fields are not necessarily smaller than from double scattering delivery .…”

“…The conformity of IMPT is determined by optimization type and strategy, as well as the physical properties of the proton beamlets. 1 However, lateral and distal target penumbra of noncollimated IMPT fields are not necessarily smaller than from double scattering delivery. 2 Aside from optimization dependencies, the conformity and lateral penumbra of an IMPT system are proportional to the proton beamlet spot size.…”