ESTRO 2020 Abstract Book

S263 ESTRO 2020

the session MRI and post-treatment MRI due to the impact of bladder filling. All patients completed treatment as planned. The treatment was well tolerated with no unexpected acute toxicity seen at one month.

perturbation factors for both phantoms using the EGSnrc egs_chamber user code. Results The depth of maximum dose and the PDD at 10 cm for the 2.5 MV low-Z beam were 4.8 mm and 50.15%, compared to 5.8 mm and 52.85% for the low-X beam. Compared to the low-X beam, the low-Z beam provided improved CNR for all four contrast objects across the investigated range of imaging doses for both thin and thick phantoms. At imaging doses above which the cortical bone, CB2-30%, and breast contrast objects are visible in the thick phantom (CNR~1), the low-Z CNR advantage becomes increasingly significant with increasing dose. For the thin phantom, this CNR trend is consistent for the breast and CB2-30% contrast objects, whereas the low-Z CNR advantage of cortical bone and brain decrease with increasing imaging dose. Conclusion This study marks the first investigation of a sintered diamond target in the target arm of a Truebeam linear accelerator. Compared to the commercial low-X beam, the low-Z beam offers improved CNR for low-, medium-, and high-contrast objects in both thin and thick phantoms across a range of clinically relevant imaging doses. The improvements in CNR of visible low-contrast and medium- contrast objects at higher imaging doses (~1 cGy) could prove beneficial in the case of MV volumetric imaging, which will be investigated in forthcoming work.

Conclusion MR-guided online adaptive radiotherapy is feasible with acceptable tolerability even in patients unfit for radical daily radiotherapy. Current PTV margins maintain acceptable intra-fraction coverage of the CTV. Recruitment to PERMIT is ongoing.

Proffered Papers: Proffered papers 26: New technology

OC-0470 Investigation of a low-Z sintered diamond target for 2.5 MV imaging J. Borsavage 1 , D.A. Cherpak MCCPM 1,2,3 , D.T. Monajemi MCCPM 1,2,3 , D.J. Robar FCCPM 1,2,3 1 Dalhousie University, Department of Physics and Atmospheric Science, Halifax, Canada ; 2 Dalhousie University, Department of Radiation Oncology, Halifax, Canada ; 3 Nova Scotia Health Authority, Department of Medical Physics, Halifax, Canada Purpose or Objective A 2.5 MV 'low-X' imaging mode is available on the Truebeam (Varian Medical Systems, Palo Alto, CA) platform for beam's eye view imaging. Compared to therapeutic photon beams, this mode offers improved contrast-to-noise ratio (CNR) due to the softer energy spectrum. Previous research indicates that the 2.5 MV beam can be further optimized by using a low-Z sintered diamond target, which should reduce the self-absorption of photons in the diagnostic energy range. This work describes the first installation of a sintered diamond target A 5 mm-thick sintered diamond target was machined into the Truebeam target arm replacing the copper low-X imaging target and installed (see Figure 1). The new 2.5 MV sintered diamond target beam (low-Z beam) was characterized dosimetrically in terms of inline, crossline, and percent depth dose (PDD) profiles for various square fields. CNR versus dose was evaluated for four tissue equivalent materials, including cortical bone, CB2-30%, breast and brain, using in-house thin (4 cm) and thick (20 cm) phantoms for the 2.5 MV low-X and low-Z beams and 6 MV (see Figure 2). Imaging dose was measured for all three beams at phantom mid-separation and corrected for detector energy sensitivity using Monte-Carlo derived in the Truebeam unit. Material and Methods

Figure 1 Modified Truebeam target arm containing 2.5 MV low-Z sintered diamond target.

Figure 2 CNR versus imaging dose for breast, brain, cortical bone and cortical bone-30% for a) thin and b) thick phantoms.