S1652
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
with an interchangeable detector insert (ion chamber and Gafchromic film). Four co-planar spherical air pockets (4mm diameter) were built into the 3D print to facilitate image-based phantom setup.Measurements were performed on a Varian TrueBeam Edge linac, equipped with the High Definition 120 leaf MLC (HDMLC). The plans utilized in this study were calculated in the Eclipse treatment planning system (TPS) v18.1 (Varian Medical Systems, Palo Alto, CA). Validation of the phantom involved static-gantry, dynamic-conformal-arc (DCAT) and 9 SIMT SRS VMAT plans using a PTW cc01 ion chamber and film. Doses were compared to measurements in PW and the TPS. For the SIMT VMAT validation plans, each plan consisted of two equally sized targets of varying off- axis distances located on the central plane. Results: Static-gantry measurements in the 3D-printed phantom agreed with PW to within ±1%, while DCAT measurements agreed with TPS doses to within ±2% for all energies and field sizes. For the 9 SIMT SRS VMAT plans film-measured dose planes compared to TPS dose planes yielded a mean gamma pass rate (5%/1.5mm) of 98.5% (range: 92.4% to 100%). Including both off-axis targets in each plan, the average error between the film measured target dose and the TPS dose across the 9 SIMT SRS plans was calculated to be 1.9% ± 2.8%, with a maximum and minimum error of 10.0% and -2.2%, respectively. Conclusion: The 3D printed phantom designed in this study is a cost-effective tissue-equivalent solution for SIMT SRS dose verification. References: [1] D. Hartgerink et al., LINAC based stereotactic radiosurgery for multiple brain metastases: guidance for clinical implementation. Acta Oncologica, 58(9), 2019.[2] G. H. Raza et al., Single-isocentre multiple- target stereotactic radiosurgery for multiple brain metastases: dosimetric evaluation of two automated treatment planning systems. Radiation Oncology, 17(16), 2022.[3] S. Ahmed et al., Comprehensive evaluation of the high-resolution diode array for SRS dosimetry, J. Appl. Clin. Med. Phys. 20(10), 2019.[4] C. L. Teng et al., Dosimetric commissioning of a high- resolution CMOS 2D detector array for patient-specific QA of single-isocentre multi-target VMAT stereotactic radiosurgery. Journal of Radiation Research, 65(6), 2024. Keywords: 3D-printing, SRS, Treatment verification
Digital Poster 2593 Diminished benefits from lead shielding for pregnant patients during cranial radiotherapy in a flattening-filter-free world Tanya Kairn, Jemma Blyth, Scott B Crowe Cancer Care Services, Royal Brisbane and Women's Hospital, Brisbane, Australia Purpose/Objective: The increased availability of flattening-filter-free (FFF) radiotherapy [1], in combination with an increased appreciation for both the low energy of scattered linac head leakage [2,3] and the physical and toxicological hazards associated with lead use [1,4,5], has raised the question of whether the previously recommended use of thick lead shielding during radiotherapy for pregnant patients [6] remains justifiable. Since the brain and head-and-neck regions are commonly the sites of urgent and unavoidable radiotherapy treatments for pregnant patients [7], this study focused on the effects of FFF beams and lead shielding for cranial radiotherapy treatments in particular. Material/Methods: Head, neck and thorax sections of an Alderson RANDO phantom (1974 model) were set up with slabs of water-equivalent plastic positioned to allow calibrated ionisation chamber measurements 50 cm inferior of isocentre. A Varian Edge linac was used to deliver 6x6 cm2 square 6 MV radiation beams, in flattened and FFF modes, from a range of gantry angles, with and without 5 cm thick lead blocks or 2 cm thick bolus sheets placed over the abdominopelvic region of the phantom. Results:
Figure 1: Shielding comparison results at 10 cm depth
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