The optimization of intensity modulated radiotherapy in cases where the planning target volume extends into the build-up region

Abstract

A common clinical problem in IMRT, especially when treating head and neck cases, is that the clinical target volume (CTV) stops short of the skin surface, whilst the margin for geometric uncertainties may take the planning target volume (PTV) to the skin surface or beyond. In these cases, optimization leads to over-dosing of the skin, unless the planner resorts to procedural tricks to avoid this, such as the use of pretend bolus or reduction of the PTV followed by adding 'flash' after optimization. This paper describes a method of avoiding the need for these tricks by using a multiple-isocentre CTV-based objective function. This enables plans to be produced that will give good coverage of the CTV for all the geometrical uncertainties that would have been covered by the PTV without causing the problem of over-dosing the skin. Eight isocentre shifts, equally distributed on the surface of a sphere with a radius equal to the CTV-PTV margin, are shown to be adequate for the optimization process. The resulting fluence maps are much simpler than those resulting from PTV optimization and will therefore be simpler to deliver. The method also permits better sparing of organs at risk such as the spinal cord.

Robustness of VMAT and 3DCRT plans toward setup errors in radiation therapy of locally advanced left-sided breast cancer with DIBH

“…Some dosimetry studies have found that VMAT improves target coverage and lowers dose to organs at risk in LSBC compared to 3DCRT [12,13,[17][18][19]. However, since the target is close to the external contour, it is important to be cautious with complex modulations in order to ensure target dose due to lack of buildup and lateral scatter [20,21]. Some studies have used a virtual bolus outside the breast contour to ensure coverage, and then removed it before the final dose calculation [22][23][24].…”

The optimization of superficial planning target volumes (PTVs) with helical tomotherapy

“…Nguyen et al (3) suggested a “Multiple‐Isocentre CTV‐Based objective function (miCTV)”, performing optimization on a set of potential patient positions, averaging over a sphere with radius equal to the CTV–PTV margin. The use of this method implicitly considers areas of increased dose resulting from fluence boosting effects, without recourse to PB or PTV clipping.…”

“…In situations where the PTV extends up to or near the surface of the patient, the treatment planning system compensates by boosting beamlets tangential to the superficial region. When the patient is displaced from their planned position, notable regions of high dose are presented upon a small movement in the patient's position, even when the original plan shows good homogeneity 1 , 2 , 3 . Such an effect is distinct from other potential sources of increased superficial dose, such as the bolus effect of immobilization masks (4) .…”

Evaluation of robustness of optimization methods in breast intensity-modulated radiation therapy using TomoTherapy

“…In super cial targets, the tangential uence increases because of a lack of build-up material. A plan that provides excessive uence to the skin may have a lower objective function [13,14]. In breast IMRT, the beam weights in the tangential direction are high to avoid irradiation to the heart and ipsilateral lung.…”

“…When inverse planning is performed on a super cial target, uence of the tangential beam may increase near the skin surface to compensate for a lack of build-up material [13,14]. To avoid this, target clipping or the virtual bolus technique is used during IMRT optimization for super cial targets [15].…”