ESTRO 2020 Abstract book

S951 ESTRO 2020

Purpose or Objective The objective of this study is to evaluate the dosimetric impact of rectum and bladder filling during hypofractionated prostate radiation therapy based on daily CBCT calculation of delivered dose. Material and Methods The prospective study consisted of 15 prostate carcinoma patients that were treated with a hypofractionated radiotherapy protocol: 67.5 Gy in 25 fractions, 2.7 Gy per fraction, with a simultaneous boost irradiation of 69 Gy. Planning CT images were acquired in supine position using a knee wedge and foot block for immobilization. Three fiducial gold markers were implanted into the gland under ultrasound guidance at least two weeks before the CT planning acquisition. The patients were reminded to follow an empty rectum and fill bladder preparation protocol before the CT acquisition and each fractional treatment. At each fraction a pre-treatment CBCT was acquired and a rigid 3D fiducial markers-registration to the planning CT was performed to an accurate positioning of the patient. Organ at risk were re-contoured in each CBCT, the results of rectum and bladder daily filling were described in [1]. The plan was recalculated on daily CBCT once the positioning errors had been corrected. Rectum and bladder delivered doses were analyzed. [1] Impact of rectum and bladder anatomy in intrafractional prostate motion during hypofractionated radiation therapy. M. Roch, A. Zapatero, P. Castro. Clin Transl Oncol DOI 10.1007/s12094-018-1960-y Results The average delivered doses in rectum are similar to the plan doses except the maximum dose that is slightly higher. However, the average doses imparted to the bladder are greater than plan doses over the entire dose range. The variability for bladder dosimetric data is greater than for the rectum (Figure 1).

No correlation is found between bladder volumes and delivered maximum dose. However, in patients who suffer a considerable weight loss during the treatment, the maximum delivered dose in the bladder is higher (CI = - 0.32; p <0.01) as well as the maximum delivered dose in the body (CI = -0.55; p <0.01). Conclusion Daily anatomy of rectum and bladder has a considerable impact on delivered dose. The patient preparation protocol makes the plan reproducible with regard to the rectum, but not to the bladder in which delivered doses are greater than plan doses over the entire dose range. PO-1641 Role of surface imaging for verification of mono-isocentric multi-focal stereotactic radiosurgery D. Saenz 1 , V. Bry 1 , K. Zourari 2 , E. Zoros 2 , E. Pappas 3 , K. Rasmussen 1 , N. Papanikolaou 1 1 UT Health San Antonio, Radiation Oncology, San Antonio, USA ; 2 National and Kapodistrian University of Athens, Medical Physics Laboratory, Athens, Greece ; 3 University of West Attica, Department of Biomedical Sciences Radiology & Radiotherapy Sector, Athens, Greece Purpose or Objective Stereotactic radiosurgery (SRS) requires high accuracy and precision when targeting intracranial lesions. Cone-beam CT has been shown to be effective for initial setup and alignment but cannot be conducted at table positions other than 0°. We aimed to demonstrate that surface imaging can be used to provide the necessary verification of patient positioning at other table angles when other x- ray-based imaging technologies are not available. Material and Methods An SRS treatment plan was devised on an RTsafe Prime phantom using Elekta high definition dynamic radiosurgery (HDRS) in Monaco. Five targets of varying diameter (7-20 mm) were targeted with a single isocenter (targets 12-38 mm from isocenter) to 8 Gy in a 3D printed polymer gel phantom matching patient anatomy. The phantom was setup on the Elekta VersaHD linear accelerator at table angle 0° in a frameless SRS mask and aligned to the isocenter based on CBCT and a tabletop capable of six- degree of freedom corrections. A reference image was then created using the CRAD Catalyst HD three-camera surface imaging system. Subsequent arcs treated at different angles were verified making corrections as suggested by surface imaging (shifts > 0.7 mm). The irradiated phantom was subsequently scanned on a 1.5 T MRI unit using a 2D multi-slice, multi-echo PD to T2- weighted sequence using a head coil with 2 mm slice thickness. In the resulting scans, the absorbed dose is directly proportional to the relaxation rate. Results were compared with the planned dose distribution and assessed using 3D gamma analysis, target mean dose and D95, and geometric offset. Results 3D GI (gamma index) comparison between the calculated and measured dose distributions resulted in passing rates

The correlation analysis between daily organ filling and delivered doses shows that for larger rectum volumes the maximum dose increases (Correlation Index (CI) = 0.34; P <0.01) and the average dose decreases (CI = -0.32; p <0.01). In our study, the rectum filling is greater than the plan volume in 55% of the sessions. For the bladder, smaller daily volumes compared to plan volumes results in higher delivered doses (CI = -0.83; p <0.01). The daily bladder is emptier than plan bladder in 66% of the treatment sessions, this explains the higher delivered dose values compared to those planned. The relation between average delivered dose and daily volume fits a potential model (R 2 = 0.84) (Figure 2).