ESTRO 2021 Abstract Book

S1674

ESTRO 2021

plan doses to organs at risk are close to the planning constraints.

PO-1967 Identifying the collision zone in breast radiotherapy verification and isocentre placement J. Motlib 1 , S.A.A. Moinuddin 2 , S. Wickers 2 1 University College London Hospital, Radiotherapy, London, United Kingdom; 2 University College Hospital London, Radiotherapy, London, United Kingdom Purpose or Objective The ability to acquire verification images with 360° rotation maximises full use of the Varian Truebeam™ imaging capabilities. However, for breast radiotherapy the risk of gantry collision with the patient, accessory equipment or couch is high due to patient positioning (inclined and arms raised) and the anterio-lateral isocentre placement. This collision risk is limiting the implementation of 3D soft tissue verification (CBCT) at Varian sites. Our aim was to identify maximum threshold couch parameters to guide isocentre placement for breast radiotherapy (+/- nodal targets), and enable collision-free 360° gantry rotation for optimal image verification . Materials and Methods Fifteen staff members were positioned on a 5° inclined wingboard with both arms raised. For each volunteer, the longitudinal isocentre was positioned at the level of the suprasternal notch to represent that of a monoisocentric breast technique. At couch vertical values; 25cm, 22.5cm, 20cm, 17.5cm, 15cm, 12.5cm and 10cm, the maximum threshold values for the couch lateral were recorded, whereby a 360° rotation was possible without collision, maintaining a 1cm clearance. The maximum couch threshold values for each volunteer were plotted on a graph and standard deviation calculated for the cohort. Results 15 staff members were recruited, 11 females and 4 males with a median height of 164cm (152cm-182cm). At 25cm couch vertical, the gantry was only able to complete a collision-free 360° rotation with a maximum couch lateral of 1.3cm ( ±1.2) , due to collision between the gantry and the couch. At couch verticals 22.5cm, 20cm and 17.5cm, the maximum couch lateral values achievable were 2.8cm (±0.9), 4.3cm (±0.6) and 5.6cm (±0.6) . At couch vertical 15cm, 12.5cm and 10cm, the maximum couch lateral values achievable were 6.1cm (±1.2), 5.7cm(±1.7cm) and 4.4cm(±1.8cm) due to collision between the gantry and the patient (most commonly, the elbows). Conclusion This study has provided data of the collision-free zone for 360° gantry rotation at specific couch vertical and lateral positions for breast cancer radiotherapy on a Varian Truebeam™ linear accelerator. This informs the threshold values for the lateral and vertical isocentre placement to facilitate collision-free 360° verification. Purpose or Objective Bladder cancer radiotherapy (BCRT) is challenging regarding treatment margins. The plan of the day concept (POD) has shown to be a possible solution in Webster et al 2013. We have investigated potential reduction in normal tissue complications probability (NTCP) for rectum and bowel bag using POD, based on daily image analysis. Materials and Methods A retrospective analysis of the use of POD for twenty BCRT patients was performed. Three different PTV margins were used: small – isotropic 7 mm, medium – 10 mm anterior and cranially and 7 mm in the remaining directions, large – 15 mm anterior and cranially and 10 mm in the remaining directions. VMAT plans with 6 MV were generated with the Eclipse treatment planning system (Varian V15.6) for all cases. The prescribed dose defined as median dose to PTV was either 64 Gy in 32 fractions or 68 Gy in 34 fractions. A previous study determined the percentage use of each POD plan. These were used to generate a final summed POD treatment plan. NTCP calculation was performed for the summed POD treatment plan as well as the clinical plan (PTV margin 15mm isotropic). The NTCP for rectum (NTCPR_gr2) was estimated by a generalized equivalent uniform dose-based NTCP model in Michalski et al 2010 for grade ≥2 late rectal toxicity using n=0.09, m = 0.13 and TD50=76.9 Gy. Two different NTCP measures for bowel bag were used 1) 10% risk of early grade ≥ 3 toxicity for bowel bag if V45GyBB > 195 cc, based on a study from Kavanagh et al 2010 and 2) a NTCP model for V45GyBB late grade ≥ 1 toxicity (NTCPBB_gr1) using a logistic model with upper limit at 1, estimated from figure 3 in McDonald et al 2015. Median bowel bag V45Gy and median mean rectum dose were also compared. A paired Wilcoxon signed rank test was used for all statistical analysis. A difference was considered statistically significant if p < 0.05. Results Out of the twenty patients analyzed, the number of patients with V45GyBB > 195 cc were nine and four for the clinical plans and summed POD plans, respectively. A statistically significant but not clinically relevant NTCP difference was found for rectum (Table 1) and a statistically significant reduction was noted for the mean rectum dose (Table 1). A statistically significant reduction was found for both NTCPBB_gr1 and V45GyBB for all patients (Table 1). Digital Poster: Motion management and adaptive strategies PO-1968 Potential reduction of NTCP using plan of the day for bladder cancer S. Talozi 1 , I. Bengtsson 1 , N. Svanberg 1 , A. Gunnlaugsson 1 , E. Wieslander 1 , J. Scherman 1 1 -, Department of Hematology, Oncology and Radiation Physics, Lund, Sweden