Stereotactic radiotherapy for choroidal melanomas by means of HybridArc™ : Physics and technique of linac-based photon beam therapy.

Abstract

Stereotactic radiotherapy (SRT) is suitable to treat ocular tumours. The optimal beam geometry for SRT, however, has not been defined. Here we evaluate acombination technique with dynamic conformal arcs (DCAs) and intensity-modulated static fields (IMRT), known as HybridArc™ (HA). For the first consecutive 25cases with choroidal melanomas with volumes of 0.02 to 1.18 cm3 treated with 50 Gy in five fractions, the results with respect to dose conformity, homogeneity, and dose distributions were summarised. To describe the dose distribution at the planning target volume (PTV) boundary, we defined aspatially averaged dose gradient (SADG) and compared it with Paddick's gradient index (GI). We made dosimetric comparisons between HA and other irradiation techniques. The PTVs ranged from 0.42 to 3.37 cm3. The conformity index (CI) was 1.25 ± 0.15, and the homogeneity index (HI) 0.08 ± 0.02. The SADG was (-3.5 ± 0.5) Gy/mm or (-7.0 ± 1.0) %/mm between the isodose levels 95 and 20%; local minima reached -11.5 Gy/mm or -22.9%/mm. The coefficient of determination for anonlinear regression of GI on SADG was 0.072. After amedian follow-up time of 19.6months, local tumour control was 100% without any case of post-therapeutic enucleation. Two patients (8%) developed liver metastases. SRT of ocular tumours by HA is highly appropriate, and HA is superior to intensity-modulated arc therapy (IMAT) concerning dose reduction in organs at risk (OARs). The novel gradient measure SADG is more informative than Paddick's GI.