ESTRO 2024 - Abstract Book

S3522

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

Four patients with PTVs in the very small volume range (0.01 to 0.5cc) or with complex shapes, that had previously been treated with SRS using the CyberKnife VSI system at our institution, were chosen for investigation. Two were patients with multiple brain metastases: the first with 26 lesions (minimum PTV volume 0.01cc), the second with 5 lesions (minimum PTV volume 0.11cc), the third was a patient with pituitary adenoma (smallest PTV width 5mm close to the optic pathway) and the fourth a patient with acoustic neuroma (smallest PTV width 3mm in the auditory canal, close to the cochlea). All four patients were re-planned with Varian HyperArc using 4 VMAT arcs with 6FFF beam quality, attempting to emulate the plan quality metrics for prescription dose spillage and modified gradient index that were achieved on the clinical CyberKnife plans. HyperArc plans were produced in Varian Eclipse v17.1, calculated using the AAA algorithm (v 17.1.0.1) using a 1mm dose calculation grid size. Each HyperArc plan was then recalculated on a CT scan of an in-house water-equivalent modular phantom that allows for measurement of dose in clinically meaningful planes using high-resolution gafchromic EBT3 film (Ashland inc.). Each plan was delivered to the phantom containing the film under image guidance. The film was analysed in absolute dose mode using an established calibration methodology in Sun Nuclear SNC Patient (v 8.2). The measured dose plane was co-registered with the known phantom film plane position to allow for end-to-end comparison of planned vs delivered dose. Analysis of absolute dose magnitude, dose gradient and dose positional accuracy were performed in multiple planes (sagittal and coronal) using multiple 1D dose profiles and using a 2D 5%/1mm gamma test for each target.

Results:

Tabulated results for planned and delivered results appear in figure 1. In all cases, the HyperAc plans were very similar to the clinical plans in terms of plan quality metrics. The SRS planned maximum dose gradients of 20 23%/mm (up to 5.8 Gy/mm) were achieved for all plans. Comparing planned and delivered dose, the maximum geometric correction required was no more than 1 mm in any cardinal direction for any plane of analysis, with most planes not requiring any geometric offset at all. For the 5%/1mm gamma test, pass rates are very high (average >95%) in the region of the targets examined. Figure 2 is an example case from the sagittal plane of the pituitary adenoma plan showing excellent agreement throughout the measured dose range.