In the presence of geometric uncertainties in fractionated stereotactic radiotherapy (SRT), use of planning target volume (PTV) margins is a common mitigating strategy to ensure that the gross tumor volume (GTV) received the prescribed radiation dose.

     The purpose of this study was to calculate PTV margins for frame-based linac and Perfexion (PFX) SRT using the relocatable head frame (RHF) eXtend™.

     Patients with large brain metastases are enrolled on the clinical trial.

     Patients with large brain metastases are currently undergoing hypofractionated (3 fractions) SRT on PFX enrolled on a phase 1 dose-escalation clinical trial. In prior investigation, the performance of the RHF was quantified in stereotactic coordinates using cone-beam CT (CBCT) in fourteen patients undergoing linac-based SRT (median: 30 treatment fractions). Considering only where the check-tool indicated a positional displacement <1mm (action threshold), standard performance metrics – group mean (m), systematic (S) and random (s) uncertainties – were determined in stereotactic coordinates for both positioning and intra-fraction motion using CBCT. These metrics were input to a published margin-determination formula (2.5*S+0.7*s) that assumes many fractions and linac dose distributions. An additional factor of (s/v3) was added to the systematic component of the margin formula when initially designing the PTV for 3 fractions in PFX-SRT. To more accurately account for PFX dose distributions and only 3 treatment fractions, a population-based stochastic modelling approach is being developed to refine the PTV margin for hypofractionated PFX-SRT.

     The RHF positioning performance for linac-SRT was mposition={0.3;-0.5;-0.7}mm, Sposition={0.8;0.9;0.6}mm, and sposition={0.3;0.5;0.4}mm in {Right;Superior;Anterior}. One subject outlier was indentified and subsequently removed from analysis, yielding mposition={0.1,-0.2,-0.6}mm, Sposition={0.2;0.8;0.6}mm, and sposition={0.3;0.6;0.4}mm. For intra-fraction motion, mmotion={-0.1;-0.1;0.0}mm, Smotion={0.2;0.2;0.1}mm, and smotion={0.2;0.4;0.2}mm. The standard margin formula indicated an expansion of {1.0;2.6;1.8}mm and {1.6;3.1;2.3}mm for frame-guided linac (30 fractions) and PFX-SRT (3 fractions), respectively. The mposition of 0.2 and 0.6 mm along inferior and posterior directions indicate an anisotropic margin expansion. For two patients treated to date on PFX, mposition=(0.2mm;-0.9mm;-0.8mm), indicating similar trends to the linac cohort.

     This is a retrospective study.

     To ensure that the GTV receives the prescription dose, PTV margins have been calculated to account for the geometric uncertainties present in PFX-SRT.

     The margins will be reviewed as more data are collected, RHF refinements are made, and stochastic-based modelling is used.