ESTRO 2020 Abstract Book

S1020 ESTRO 2020

PO-1829 Can empty bladder filling protocol be implemented for localised prostate external beam radiotherapy? G. Chetiyawardana 1 , C. Holborn 2 , P. Hoskin 1 , Y. Tsang 1 1 Mount Vernon Cancer Centre, Radiotherapy, Northwood, United Kingdom ; 2 Sheffield Hallam University, Allied Health Department, Sheffield, United Kingdom Purpose or Objective With the recent developments in radiotherapy (RT) delivery, volumetric modulated radiotherapy (VMAT) can be used to reduce dose to organs at risk (OARs) when bladders are not filled. This study aims to examine the impact of bladder filling protocols on patients receiving radical RT for localised prostate cancer on post RT related toxicity and biochemical progression free survival (bPFS) at our institution. Material and Methods A retrospective review of treatment records of prostate patients who followed two different bladder-filling protocols was carried out. This included each patient’s bladder size on planning computed tomography (CT), daily online image guided radiotherapy (IGRT) setup data, treatment time and post treatment follow up (f/u) data. The f/u data included each patient’s bPFS and gastrointestinal (GI) and genitourinary (GU) toxicity scoring at 1, 6, 12, 24 and 36 months post RT using the CTCAE v4.0 protocol. bPFS and cumulative GI/GU toxicity rates were calculated using the Kaplan-Meier method and the resulting curves of patients using two different bladder filling protocols were compared using the Mantel-Cox log- rank test. The daily online IGRT setup data were defined as the vertical, longitudinal and lateral couch shifts required for each treatment fraction. The treatment time for each fraction was calculated as the time difference between the starts of first imaging field and the last treatment field. Mann Whitney U tests were used to investigate any significant differences in IGRT setup and treatment times between the two bladder filling protocols. All patients included in the study were planned and treated under the same departmental clinical protocol with VMAT and daily online IGRT corrections. Results 66 patients with localised prostate cancer were treated between March 2014 to August 2015 at our institution with a median follow up of 36 months. At 3 years bPFS in the empty bladder group was 100% and 97% in the full bladder group (p=0.32). As indicated in the figure, no greater than grade 3 GU and GI toxicities were found for both bladder filling groups over the 3 years post RT f/u period. There were no statistically significant differences in cumulative grade 1+ GU (p=0.37) and GI (p=0.85) toxicity rates between the two bladder filling protocols. As shown in the table, there were no statistically significant differences in couch shifts for the daily IGRT between the two groups. Despite the fact that the treatment times of the two groups are not significantly different, it was noted that all patients under the full bladder filling protocol were required to arrive at the department 30 minutes earlier than their scheduled appointment time for every fraction.

Conclusion It’s suggested that bladder filling protocols have no significant impact on the bPFS, GI and GU toxicities at 3 years in patients with localised prostate cancer using advanced RT techniques. This empty bladder approach can provide better patient comfort during the whole treatment course. PO-1830 Clinical verification of the pitch functionality and setup accuracy of the Solstice SRS System C.L. Ong 1 , E. Franken 1 1 Hagaziekenhuis, Radiotherapy, Den Haag, The Netherlands Purpose or Objective Reliable patient immobilization is one of the most crucial part in treatment of frameless stereotactic radiosurgery (SRS) for brain metastases on a linear accelerator. Various commercial products offer promising rigid fixation, but most of these system are solely used for immobilization as any setup errors can only be corrected by the treatment couch. And if any pitch-rotational setup errors were detected, it can only be corrected using a 6D-couch, which is not commonly available. The purpose of this study was to investigate the stability of the new CIVCO Solstice SRS immobilization system and the accuracy of its manually adjustable pitch-rotational functionality. Material and Methods The Solstice mask comprised of a carbon fiber head support, customizable cushion and thermoplastic mask (Fig 1). Pitch setup errors can be corrected by rotating the screw at the back of the head support with a rotation of 0.5° per turn. Initially, 12 palliative cranial patients receiving 1–10 RT fraction were treated using this system,