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Megavoltage photon minibeam production through the embedded multi-leaf collimator of a clinical linac: A dosimetric
Stefania Linsalata1, Jake Harold Pensavalle2, Fabiola Paiar3
1UO Fisica Sanitaria, Azienda Ospedaliero-Universitaria Pisana, Pisa, Italy.
Background:
Although spatially-fractionated radiation therapy (SFRT) has been receiving increasing clinical interest, its complete and optimized clinical transfer requires several technological and radiobiological issues to be addressed.
Purpose:
The aim of this work is to setup and dosimetrically characterize an experimental platform generating planar megavoltage photon minibeams to exploit quantitative radiobiological SFRT experiments.
Methods:
Minibeam patterns were generated on a Varian TrueBeam™ linear accelerator equipped with a high definition (HD) MLC, which consists of 60 tungsten leaf pairs, whom the 32 inner ones project a thickness of 2.5 mm at the isocenter plane (i.e., 100 cm distance from the radiation source) and the 28 outers of 5 mm. Each pattern was obtained alternating opening one leaf after closing one, two, or three of the smallest MLC leaves. Measurements were performed with a PTW microdiamond detector in edge-on orientation in a water phantom over depths from 1 to 30 cm, varying the beam energy and the source to surface distance (SSD) in different sessions. A Python-based in-house automated fitting procedure was used to obtain from the measured profiles all the minibeam parameters: peak widths (PWs), distance between consecutive peaks (CTCs), peak to valley dose ratios (PVDRs) and depth doses.
Results:
The best performances were obtained at SSD = 70 cm and 6 MV, where PWs and PVDRs respectively range from 1.2 to 2.4 mm and from 2 to 7. For all patterns the average dose varies in depth analogously to the reference field, PVDRs show an almost steady decrease except at shallower depths, while PWs and CTCs widen linearly with the depths. Moreover, their variation over different sessions was within the estimated uncertainty of the single measurement.
Conclusions:
We are claiming that a Varian TrueBeam™ equipped with an HD-MLC represents a practical, stable, reproducible and accessible platform that might be suitable for radiobiological studies on SFRT and for the investigation of the relative underlying mechanisms.
