ESTRO 2020 Abstract Book

S899 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).

Figure 1 : Distribution of manual CBCT corrections after Catalyst setup. Z was corrected for table sag.

Figure 2: CBCT displacements after Catalyst positioning for Halcyon breast treatments (X: red, dots, Y: green, dashed, Z: blue, dashdot (corrected for table sag)). The black line corresponds with the inter-user MVCT fusion variability taken from Crop et al 2016. Conclusion The adapted 3-camera setup for breast patient positioning by Catalyst on Halcyon is precise and CBCT use could be reduced. The use of the MPC halcyon phantom (with the addition of a white scotch on the handle) is precise for daily Catalyst QA. There remains a small table flex for halcyon, which can be compensated by SIGRT. PO-1640 Dosimetric impact of rectum and bladder anatomy during hypofractionated prostate radiation therapy M. Roch 1 , P. Castro 1 , C. Anson 1 , D. Hernandez 1 , F. Garcia de Vicente 2 , A. Zapatero 3 , A. Viñals 1 , R. Fayos-Sola 1 , L. Perez 1 1 Hospital Universitario La Princesa, Radiophysics / Radiation Oncology, Madrid, Spain ; 2 Hospital Universitario Ramón y Cajal, Radiophysics, Madrid, Spain; 3 Hospital Universitario La Princesa, Radiation Oncology, Madrid, Spain

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).