Margin evaluation measurements.

<p>The CTV (red) is propagated to the fraction control CT image (fCTV, orange) to calculate CTV volume missed by the treatment <b><i>V</i>_{fCTV \ ITV}</b> and the volume of healthy tissue being hit .</p

Mean distance between ITV and transformed spinal cord d_SC(z) of different margin approaches for all patients.

<p>Mean distance between ITV and transformed spinal cord d_SC(z) of different margin approaches for all patients.</p

Margin sizes for all three different margin generation methods.

<p>Margin sizes for all three different margin generation methods.</p

Distance between ITV and OARs per CT slice.

<p>The slice wise distance between ITV and spinal cord <b><i>d</i></b>_<b>SC</b><b>(</b><i><b>z</b></i><b>)</b> is used to estimate the risk to the spinal cord.</p

ITV margins (green) of an exemplary patient (#19).

<p>Left: Approach 1 with a 3 mm constant margin; center: Approach 2 with a variable margin based on distances; right: Approach 3 with a variable margin approach using biomechanical modeling.</p

The finite element model of the patient is deformed.

<p>Left: the finite element mesh model of the patient’s skin in the planning configuration (grey) and the selected anatomical landmarks (green) as well as their position in the fraction control CT image (red). The patient model is deformed by a force based boundary condition between the landmarks. Right: Landmarks were pulled towards their position in the fraction CT, the mesh is deformed resulting in rising of the right shoulder.</p

Clinical Target Volume (CTV) of a head and neck cancer patient.

<p>Left: Volume rendering of the planning CT image for an exemplary head and neck cancer patient receiving an IMRT treatment under daily image guidance. The clinical target volume (CTV) is displayed in red, brainstem in green. The stereotactic external frame is used as positioning device. Center: A transversal CT slice of the same patient of the mandibular region is displayed. The clinical target volume (CTV, red) and the spinal cord (green) as organ at risk are shown as planning contours. Right: CTV contours of different treatment days are plotted on the same planning CT slice to visualize daily variations.</p

Schematic representation of the competing margin generation concepts to cope with residual deformations after a simulated IGRT correction.

<p>Approach 1: The constant margin is applied by increasing the CTV (red) contours by 3 mm resulting in the internal target volume (green). Approach 2: The variable margin is applied locally by increasing the CTV (red) by a value calculated using measured landmark displacements of adjacent landmarks. Approach 3: The variable finite-element-model-based margin is created by statistical sampling of the propagated target volumes based on DVFs generated from landmark displacements by the patient-specific biomechanical model.</p

Sensitivity of post treatment positron emission tomography/computed tomography to detect inter-fractional range variations in scanned ion beam therapy

<p><b>Background:</b> Ion therapy, especially with modern scanning beam delivery, offers very sharp dose gradients for highly conformal cancer treatment. However, it is very sensitive to uncertainties of tissue stopping properties as well as to anatomical changes and setup errors, making range verification highly desirable. To this end, positron emission tomography (PET) can be used to measure decay products of β⁺-emitters created in interactions inside the patient. This work investigates the sensitivity of post treatment PET/CT (computed tomography) to detect inter-fractional range variations.</p> <p><b>Material and methods:</b> Fourteen patients of different indication underwent PET/CT monitoring after selected treatment fractions with scanned proton or carbon ion beams. In addition to PET/CT measurements, PET and dose distributions were simulated on different co-registered CT data. Pairs of PET data were then analyzed in terms of longitudinal shifts along the beam path, as surrogate of inter-fractional range deviations. These findings were compared to changes of dose-volume-histogram indexes and corresponding dose as well as CT shifts to disentangle the origin of possible PET shifts.</p> <p><b>Results:</b> Biological washout modeling (PET simulations) and low (<55 Bq/ml) activity concentrations (offline PET measurements, especially for ¹²C ions) were the main limitations for clinical treatment verification. For two selected cases, the benefit of improved washout modeling based on organ segmentation could be demonstrated. Overall, inter-fractional range shifts up to ±3 mm could be deduced from both PET measurements and simulations, and found well correlated (typically within 1.8 mm) to anatomical changes derived from CT scans, in agreement with dose data.</p> <p><b>Conclusions:</b> Despite known limitations of post treatment PET/CT imaging, this work indicates its potential for assessing inter-fractional changes and points to future developments for improved PET-based treatment verification.</p