ESTRO 2021 Abstract Book

S460

ESTRO 2021

Conclusion For severe implant failure and dental lesions, the threshold dose is typically 60 Gy [1, 2]. For a titanium implant the dose constraint is reduced to ~52 Gy while for PEEK it is ~59 Gy. While the dose to the pores decreases with depth within the implant and towards the centre of the pores, the osseointegrating cells close to the tissue interface are at a greater risk. Excessive dose can damage the integrating tissue and ultimately jeopardise the success of the implant. We believe that PEEK provides a superior alternative since the perturbations are within a lower fraction and the radiotherapy can be planned without causing significant spikes in radiation. We make a recommendation that the integrating tissue in implants be classified as an organ at risk to allow for tighter constraints on the dose and minimise the incidents of implant failure.

PH-0596 Relationship between treatment logs and pre-treatment QA results from a Halcyon 2.0 linac D. Sánchez-Artuñedo 1 , M. Hermida-López 1 1 Hospital Vall d'Hebron, Servei de Física i Protecció Radiològica, Barcelona, Spain Purpose or Objective The Halcyon linac (Varian Medical Systems) is designed with a dual-layer stacked and staggered MLC. In a previous work (PO-1381, ESTRO2020, Radiother. Oncol. 152, Suppl. 1, 2020), we found different RMS values in treatment logs depending on the anatomical site. It is not clear if these differences in RMS values are correlated with differences in pre-treatment QA results. This work studies the relationship between the results of the pre-treatment QA for five class solutions with MLC performance parameters calculated from the treatment logs. Materials and Methods Retrospectively, we randomly selected and analyzed 245 pre-treatment QA plans representing the most common sites in our institution (50 breast, 60 lung, 50 prostate, 35 pelvic, and 50 gynecologic plans) corresponding to 115 IMRT plans and 130 VMAT plans. All plans were calculated in Eclipse 15.6.04 (Varian Medical Systems) with AAA using 6 MV FFF modulated fields. Measured fluence with EPID and predicted fluence were compared using the Portal Dosimetry module. Standard criteria of 3%/3mm lead to passing rates above 99% for all the plans. Therefore, the more restrictive