ESTRO 2022 - Abstract Book

S860

Abstract book

ESTRO 2022

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. 1 University Hospital Zurich, Radiation Oncology, Zurich, Switzerland; 2 University Hospital Zurich , Radiation Oncology, Zurich, Switzerland; 3 University Hospital Zurich , Radiation Oncology, Zürich, Switzerland; 4 North Carolina State University, Mathematics, Raleigh, USA; 5 University Hospital Zurich, Radiation Oncology, Zürich, Switzerland Abstract Text Most radiotherapy treatments are fractionated because normal tissues can tolerate higher total doses if the dose is split into multiple fractions. However, more fractions also require higher doses in the tumor to achieve the same level of tumor control. In that regard, the ideal treatment would simultaneously achieve hypofractionation in the tumor along with more uniform fractionation in normal tissues. While this may appear impossible at first glance, it can be partially achieved by delivering distinct dose distributions in different fractions. These dose distributions are designed such that different fractions deliver high single-fraction doses to complementary parts of the tumor while achieving a similar dose bath in the surrounding normal tissue, thereby increasing the biological effect in the tumor while exploiting the fractionation effect in the normal tissue [1,2]. We refer to this concept as spatiotemporal fractionation. The first part of the presentation will introduce spatiotemporal fractionation and discuss potential clinical applications in the context of in-silico treatment planning studies. We will introduce treatment plan optimization methodology to simultaneously optimize multiple dose distributions for different fractions based on objectives and constraint functions evaluated for the cumulative biologically effective dose (BED) of all fractions. Spatiotemporal fractionation is then demonstrated for patients with multiple brain metastases, illustrating a potential clinical application. The figure illustrates spatiotemporal fractionation for a patient with 30 brain metastases, out of which 9 are shown on the axial slice. A 3-fraction SRS treatment is shown with a prescription corresponding to the BED10-equivalent of 3x9 Gy or 1x18 Gy. Spatiotemporal fractionation delivers high single fraction doses to small metastases. For example, metastasis M5 receives most of the dose in fraction 1 whereas M9 receives most of the dose in fraction 3. In addition, larger metastases such as M4 are compartmentalized into different regions that receive high doses in alternate fractions. Thereby, the prescribed BED10 can be achieved with a lower physical dose. Since some degree of fractionation is achieved in parts of the normal brain between the metastases, the lower physical dose in the tumor results in a net reduction of the mean BED2 in the normal brain. Compared to a treatment delivering the same dose distribution in all 3 fractions, spatiotemporal fractionation achieves mean brain BED2 reductions of approximately 15%. SP-1014 Spatiotemporal fractionation J. Unkelbach 1 , N. Torelli 2 , I. Telarovic 3 , I. Vetrugno 3 , D. Papp 4 , M. Pruschy 5