ESTRO 36 Abstract Book

S994 ESTRO 36 2017 _______________________________________________________________________________________________

Results In total 137 scan sets were acquired during this time period. In the majority of cases (89.78%) treatment was delivered as planned after analysis of initial CBCT. In 14 cases (10.22%) after acquiring the CBCT the patient was taken off the treatment couch. Three were due to the patient experiencing urinary urgency and needing to void. The remaining 11 cases after analysis of the CBCT variation in rectal distension and/or bladder filling was observed and the radiographers did not continue to treatment delivery (Figure 1).

The goal of the study was to compare two different respiratory gating techniques to treat left-sided breast cancer with Deep Inspiration Breath Hold (DIBH): infrared tracking camera with a reflective marker (RPM, Varian) and the optical surface monitoring system (OSMS, Varian) and to improve our daily radiation therapy workflow. Material and Methods 9 breast cancer patients, undergoing DIBH were treated in our clinic using 3D tangential fields. They were positioned supine, on the C-QualTM Breastboard (CIVCO Medical Solutions). Before the first treatment the patient reference breathing curve was imported to a linac treatment workstation together with calculated thresholds. Additionally, the reference surface from the CT scan in free-breathing (FB) as well as in DIBH were imported to the OSMS and the region of interest was selected. Patients were leveled according to the CT reference marks and then positioned with OSMS using FB surface from CT (5 patients) or acquired FB surface on the first day of treatment (4 patients). After aligning the patient, MV imaging in DIBH based on RPM was done and bone match on the chest wall was used to correct for positioning error. OSMS deltas using the DIBH surface, acquired before performing the couch shifts, were assessed against MV imaging results in the breath hold for every patient to compare the two methods. Results Positioning based on OSMS was in good agreement with the positioning based on RPM and MV imaging. The mean 3D deviation between the two techniques was within 5mm accuracy. The FB reference surface from CT was found less reliable than the one obtained on the first day of the treatment. For 2 patients, the CT reference DIBH surface shown more than 5mm discrepancy compared to MV imaging with RPM and a new one was taken on the first day of the treatment and used for consecutive treatments. OSMS detected patient pitch of up to 10 degrees. Conclusion According to our preliminary data, DIBH patient positioning based on OSMS is feasible and reproducible. More data will be collected to confirm these findings and shifts of patients based on the DIBH reference surface, before performing MV imaging, will be implemented into the workflow. EP-1843 An audit evaluating the frequency of patient re-preparation after CBCT analysis in prostate IMRT K. Crowther 1 1 Cancer Centre- Belfast City Hospital, Radiotherapy, Belfast, United Kingdom Purpose or Objective As imaging techniques have advanced and kV-CBCT is now routinely used to verify prostate radiotherapy (RT), changes in bladder and rectal volume affecting the position of the prostate can be seen. The consequences of this can potentially lead to a reduction in PTV coverage, an increase in treatment toxicity, and even biochemical failure. Patients should be treated with a ‘comfortably full’ bladder and an empty rectum with the aims of reducing rectal distension and minimising prostate movement . Material and Methods All kV-CBCT images for patients receiving prostate IMRT on a treatment unit were examined over a one month period. All patients followed the departmental Bladder/Bowel preparation pathway- patients' self- administer daily micro-enemas and follow a simple bladder filling protocol (500 mls of water, 45 mins prior to treatment). The departmental prostate CBCT protocol was used (minimum day 1-3 and weekly). A record of patients requiring re-prep after analysis of CBCT was kept with details of the action taken. The data recorded was reviewed to identify any trends and to quantify the impact on daily workflow.

A summary of the results is shown in Table 1.The mean delay caused on the treatment unit was 15 minutes (mins) (range 0-20), this equates to on average 210 mins per month or 21 treatment slots. The time taken from image acquisition to decision to take the patient off the treatment couch mean was 3.21 mins (range 1-6). Scheduled appointment time to time of CBCT acquisition mean was 7.6 mins (range 1-18). A variety of instructions were given to patients with inconsistencies observed with regards to bladder filling.

Conclusion This is a small sample but it has highlighted important issues seen within this patient population. Patient compliance with preparation and the instructions given to patients who require re-prep are important. Guidance and training should be available to ensure consistency in patient instructions. With the introduction of more advanced prostate RT, the demand for IGRT will continue to increase; this will have an impact on the daily workflow with the potential to increase patient waiting times if issues such as patient compliance with preparation instructions are not addressed.

Electronic Poster: RTT track: Motion management and adaptive strategies

EP-1844 Clinical introduction of simple adaptive radiotherapy for transitional cell bladder carcinoma N.J.W. Willems 1 , P.S. Kroon 1 , J.C.J. De Boer 1 , G.J. Meijer 1 , J.R.N. Van der Voort van Zyp 1 , J.L. Noteboom 1 1 UMC Utrecht, radiotherapy, Utrecht, The Netherlands

Purpose or Objective