ESTRO 2021 Abstract Book

S1154

ESTRO 2021

The purpose of the study was to investigate whether the clinically used margins in bladder cancer treatment could be reduced, based on daily images with a plan of the day technique. Materials and Methods Twenty four patients treated for bladder cancer between 2018-12-03 and 2020-04-03 were included in this retrospective study. Treatment was given with curative intention in fractions of 2 Gy; one fraction per day, five days a week up to 64 or 68 Gy. The bladder was considered the CTV. The PTV was generated as an isotropic expansion of 15 mm of the CTV but was modified for 11 patients. Image-guidance during treatment was based on daily CBCT with automatic bone matching in combination with a visual review ensuring that the PTV enclosed the bladder. No bowel regulation was prescribed as standard. Post treatment analysis of reduced PTV margins was done based on three standard margins; small 7 mm isotropic, medium 10 mm cranially and anteriorly with 7 mm in the other directions and large 15 mm cranially and anteriorly with 10 mm in the other directions. A total of 801 CBCT:s were analysed and the smallest PTV that enclosed the bladder was selected. A possible deviation of the bladder position, due to variation in rectum filling, was determined by visually comparing the rectum filling in the planning CT and in the CBCT. Results In the patient data, three outliers were found and rejected from the study. The outliers consisted of two patients that were not treated according to clinical practice and one patient with hip prothesis which made the bladder hard to distinguish. The distribution of the selected margins based on the CBCT:s was 47 % for the small margin, 18 % for the medium margin and 35 % for the large margin. The distribution of selected margins for each patient are shown in figure 1.

The bladder was outside the clinical PTV in 27 fractions (3.4 %). In 11 fractions the bladder was outside the large PTV but inside the clinical PTV. In 9 of these 11 fractions, the rectum filling seemed to affect the bladder shape and position. The colon filling was observed to impact the bladder shape and position for 19 of the 21 patients in at least one fraction. For five patients, the bladder shape and position were affected by the colon filling in more than 50 % of the fractions. Conclusion We showed that the clinical margins could be reduced, paving the way for a plan of the day technique. Using a plan of the day technique would further decrease the irradiated volume of normal tissue. We suggest that the margin reduction can be safetly adapted while a systematic regulation of colon filling is implemented. PO-1405 Decision aid for bladder cancer patients choosing between cystectomy and bladder sparing treatment J. van Tol-Geerdink 1 , A.(. van der Heijden 2 , N.(. Mehra 3 , H.(. Meijer 1 1 Radboud UMC, Radiation Oncology, Nijmegen, The Netherlands; 2 Radboud UMC, Urology, Nijmegen, The Netherlands; 3 Radboud UMC, Medical Oncology, NIjmegen, The Netherlands Purpose or Objective In addition to cystectomy, chemoradiation is becoming an accepted alternative treatment for selected muscle invasive bladder cancer (MIBC) patients. Literature reviews show comparable disease free survival rates. However, in the era of shared decision-making, patients need to be able to balance the pros and cons of both treatment options. A web-based decision aid will be developed to inform patients and to support them in deciding which risks or benefits are most important for them. In addition to the literature, former patients provide an important source of information on treatment aspects that are relevant to patients. Aim of the current qualitative study is to gather information about the perspectives of (former) patients in order to incorporate these in the development of the decision aid. Materials and Methods Semi-structured interviews were held with patients who have undergone either cystectomy or chemoradiation. The patients were recruited by urologists and radiation oncologists and by a call of the Dutch patient association (LMBNK). They were asked about their overall experience, the harms and benefits of their treatment and possible information gaps prior to their treatment. The interviews were performed by telephone, due to Covid restrictions. Results In total, 17 patients who had been treated 1 to 5 years ago in different hospitals in the Netherlands were interviewed. Eleven were treated with cystectomy and six with chemoradiation. The mean age was 66.4 yrs