Can CT imaging improve targeting accuracy in clip-based proton therapy of ocular melanoma?

Abstract

To evaluate the benefit of adding CT imaging to the simulation process of clip-based proton therapy of ocular melanomas. For thirty ocular melanoma cases, the clip position in the eye model was determined based on orthogonal radiographs as well as on a CT image set. The geometrical shift of the clips between the standard simulation process and standard simulation process with addition of CT imaging (CT-guided) was determined. The dosimetric impact was evaluated by developing treatment plans based on both the standard-process model and the CT-guided model. In 40% of the studied cases, the difference in clip position between eye models created with and without CT was less than 0.5 mm. This difference was more than 1 mm in 17% of cases. The dosimetric impact of shifts below 1 mm was low because these shifts did not exceed the planning margins. For the four cases with a shift of more than 1 mm a reduction in target coverage (ΔV99%) of −3% to −6% was observed. Changes in macula and optic-disc mean dose of up to 16% and 35% of the prescribed dose were seen when these structures abutted the target. Adding CT imaging to the simulation process is beneficial in select cases where discrepancies between the eye model and ophthalmology measurements occur or where a critical structure is located close to the target and improved localization accuracy is wanted. For the majority of patients, addition of CT imaging does not result in quantifiable changes in dosimetry. Nevertheless, CT imaging is a valuable tool in the quality control of the modeling and treatment-planning process of clip-based eye treatments.

Potential and pitfalls of 1.5T MRI imaging for target volume definition in ocular proton therapy

“…At the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Berlin, for example, volumetric eye imaging has been integrated in their in-house treatment planning system (TPS; OCTOPUS) [7][8][9][10]. Slopsema et al also argue for using CT imaging to improve the geometrical eye model and Marnitz et al [11] report reductions of target volumes by close to two when comparing fiducial based definition to delineation in MRI scans [3]. In contrast, Daftari et al [9] found more similar volume ratios of between 0.993 and 1.02 for delineations using EyePlan or T2weighted MRI imaging [12].…”

Three‐dimensional MRI‐based treatment planning approach for non‐invasive ocular proton therapy

“…Nevertheless, those geometrical models may in some cases significantly differ from the real patient anatomy. Slopsema et al 18 recently evaluated the differences in clip position between an eye definition based on geometrical modeling as provided by the EYEPLAN TPS and a modified eye model based on CT‐guided simulation. They demonstrated that the position of the clips is more accurately modeled for specific cases in the latter when 3D CT information was incorporated.…”

“…The clinical workflow, including surgery for clips placement, in‐room simulations and positioning, planning and treatment completions, has been described in detail by a wide spectrum of publications over the last four decades 8,14–18 . In short, patients undergo surgery prior to proton therapy, with clips stitched onto the sclera to outline the tumor boundary.…”

Measuring eye deformation between planning and proton beam therapy position using magnetic resonance imaging

“…Slopsema et al [20] showed that the shape change of the eye due to gravity is less than 0.6 ± 0.3 mm, by comparing the tantalum clip positions on supinely acquired CT images, with clip positions obtained from a geometrical eye-model based on orthogonal X-rays acquired in sitting position. As this observed difference is probably largely the result of uncertainties in the geometrical eye-model used for PBT planning, such as the rotational center of the eye, the actual change in eye shape is expected to be less than the observed differences between the CT-based and X-ray based eye-model.…”