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Optimisation Strategy and Conformity Guidance for Mono-Isocentre SRS VMAT of Targets

K Blythe*, D Eaton, Guys And St Thomas Nhs Foundation Trust


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

The challenges of treating targets <0.5cm in mono-isocentric non-coplanar SRS VMAT diameter are mitigated by introducing a novel planning technique using an optimisation target volume of 0.8cm diameter. The actual PTV (GTV +1mm margin) is used to determine plan assessment metrics including conformity, selectivity, paddick and gradient indices. This work reports the achieved planning metrics using this strategy and presents a new set of planning guidelines for the Conformity Index (CI = Body V100%/PTV V100%) to reflect the expected results of this practice.

For 109 individual targets of <0.1cc, all in multiple metastatic treatments, an optimisation PTV was created with a diameter of 0.7-0.8cm. This limit was chosen to prevent the HDMLC’s attempting to create apertures smaller than techniques such as Gamma Knife (0.4cm) and Cyberknife (0.5cm).
Treatment was planned and optimised to this larger optimising PTV, but the plan normalised to the actual PTV (GTV+1mm). Consequently the planning indices used to determine an acceptable plan do not fall into line with targets which do not require this larger planning volume. CI thresholds have now been set based on this data.

The median CI reduces as the GTV size increases, because the optimizing PTV and true PTV become closer in size as the GTV volume increases. This planning guidance recommends that as GTV volume increases from 0.01cc, the maximum recommended CI will decrease by 0.5 for each 0.01cc of GTV volume. This guidance has been implemented.

A novel strategy has been presented to allow treatment of targets =0.5cm diameter, along with guidance for limits of CI for a range of target sizes. This guidance provides meaning to currently un-utilised (in small GTV cases) plan assessment metrics and produces more consistent and conformal plans. These guidelines will be reviewed and reduced further in due course.


Stereotactic Radiosurgery, Treatment Planning, Optimization


TH- External Beam- Photons: intracranial stereotactic/SBRT

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