The spatially fractionated radiation therapy (SFRT) technique uses high spatial dose modulation to create dose profiles with valleys and peaks that increase the tolerance of healthy tissue, in addition to activating biological processes by stimulating the immune response. Currently, the most established uses of SFRT are ablative irradiation for tumor debulking or as boost treatment. Bystander and abscopal effects are achieved in certain tumors, but the immunological response caused by SFRT is still under active investigation.
This presentation focuses on the commissioning of SFRT with conventional LINACS, which allows the treatment using SFRT with the techniques of GRID (both with physical block and with the use of MLC modulation) and lattice therapy, with spheres of very high dose distributed inside the GTV or an inward margin structure LTV (Lattice Tumor Volume), and where PTV periphery receives the dose in a conventional fractionation without increasing the toxicity to adjacent OARs.
The knowledge of these techniques is based on the experience of a few centers, which have reported parameters on the dose, fractionation, peak-to-valley dose contrast and spatial frequency, and physics quality assurance (QA). In this talk, we will review published articles that cover these topics.
A recent publication of a white paper by the physics working group of the Radiosurgery Society on photon GRID RT is presented. It provides consensus recommendations on dose prescription, treatment planning, response modeling and dose reporting. It recommends commissioning with water phantom measurements with microionization chamber and transverse and radial dose profiles, as well as film dosimetry and Monte Carlo simulation before clinical use of the GRID collimator. For MLC-generated GRID dose distributions, it is recommended film dosimetry in solid-water slabs. Furthermore, criteria are established for the dose prescription and the treatment report.
The experience published by single institutions on commissioning for both GRID with physical block and MLC-based is also reviewed, where several studies report measurements of profiles and output factors in water phantoms and with film dosimetry. Likewise, it is shown the experience of a center commissioning the physical GRID block in the treatment planning system with a PinPoint ion chamber in a water phantom and with radiochromic film within solid water slabs.
A LINAC commissioned for radiosurgery and SBRT is appropriate for delivering a plan using the lattice RT technique. Published articles that provide information on the lattice layouts (number, diameter and separation of high-dose spheres), specifications of dose (peak, valley and tumor peripheral dose), treatment planning, patient-specific QA and reporting are reviewed.
Following our experience, a step-by-step approach to generate and verify lattice RT plans is presented. It consists of the administration of a near-maximum dose (V2%) from 15Gy to 18Gy in a single fraction over at least 5 spheres distributed inside of GTV. Treatment planning and reporting need to establish the specific volumes of interest LTV, VTV (Vertex Tumor Volume, set of 1cm diameter spheres placed inside LTV and separated by 2.5cm to 3.0 cm) and VV (Valley Volume, subtraction of LTV-VTV).
To homogenize the prescription criteria for LRT treatments, we have used the following dosimetric constraints. Vertex index (VI) defined as the ratio D50%/D2% in VTV: VI>0.75 ; V8Gy <30% of Valley Volume (VV) ; V5Gy<50% of PTV; V18Gy/VGTV<3% in VTV.
The VMAT technique with two non-coplanar double arcs with a single isocenter has been used to generate the treatment plans. The patient-specific QA process includes point measurement with a small volume ionization chamber at points chosen outside the steep dose gradient areas. The verification of the agreement between calculated and measured fluence maps is estimated with two methods: radiochromic film and an SRS 2D array.
In a cohort of 17 patients, the QA showed that the mean difference in the measurement of the absolute dose is 0.6%±1.3%, while gamma function mean values are γ(2%,2mm) = 93.8%±3.4% for radiochromic film and γ(2%,2mm) = 98.8%±1.0% for 2D array.