TomoTherapy treatments of multiple brain lesions: an in-phantom accuracy evaluation

Abstract

Aim of the present study was to evaluate the accuracy which can be obtained with helical TomoTherapy ® (HT, Accuray) systems in the case of multiple intracranial targets treatments. Setup accuracy was measured, for different registration options and MegaVoltage CT (MVCT) slice thickness, by applying known misalignments to an ad-hoc developed phantom. End-to-end (E2E) tests were performed to assess the delivery accuracy in phantoms containing multiple targets by using radiochromic films: measured dose distribution centroids were compared with physical and calculated target positions on axial and coronal planes. A Gamma index analysis was carried out on planned and measured planar dose maps. The bone and tissue algorithm with the fine MVCT reconstruction grid gave the best results among the automatic options. The most accurate registration modality resulted to be the manual one with a sub-voxel accuracy shifts and a capability in the detection of rotations within 0.3°. For the E2E along the coronal plane (six targets), a mean deviation between measured dose distribution centroids and physical barycenters of 0.6 mm (range 0.1 mm-1.3 mm) was observed. Along the axial plane (five targets), a mean deviation of 1.2 mm (range 0.7 mm-2.1 mm) was found for the centroids shifts. Gamma index (5%, 1 mm, local) passing rates higher than 87.5% between planned and delivered dose distributions were measured. These results demonstrate that multiple brain lesion HT treatments are feasible with an accuracy at least comparable to frameless linac-based delivery, when a setup capable to assure angular corrections and a reliable patient immobilization is employed.

Technological quality requirements for stereotactic radiotherapy

“…Helical radiotherapy requires special consideration, as the MLC leaf width might be larger compared to the gantrybased MLC systems used for SRS and FSRT (i.e., 6.25 mm) but the delivery technique is different (helical) and it is in fact claimed to be used clinically for SRS and FSRT (e.g., [149][150][151][152][153][154]). However, in connection with helical treatment, especially for SRS, the inherent coplanarity of this modality (and for others, too) might be an additional disadvantage [155][156][157].…”

“…Concerning international guidelines, the IAEA technical report series 483 advised a geometric accuracy of 2 mm and a dosimetric accuracy of 3-10% in system-specific end-toend tests [154]; however, this advice is for radiotherapy in general and not specifically tailored to stereotactic radiotherapy. The ESTRO/ACROP guideline for lung SBRT on the other hand advised a geometric and dosimetric accuracy of 0.5-4 mm (median 1.25 mm) and 2-5% (median 3%), respectively [20].…”

Systematic end-to-end testing of multiple target treatments using the modular RUBY phantom

“…For E2E tests or other systematic tests of the special case of single-isocenter multiple target treatments, radiochromic films [5,[13][14][15][16] or gel dosimetry [7,17] have been used, where the gamma passing rates were determined as a measure of plan quality. Although these dosimetry methods offer 2D or 3D measurements, they are also time-consuming [14].…”