ESTRO 2021 Abstract Book

S19

ESTRO 2021

Figure 2. Boxplots comparing the planning techniques. 1) Relative target coverage for IMN and breast. 2) Average doses to OAR.

Conclusion Two radiotherapy techniques for treatments of breast and loco-regional lymph nodes are compared. Both techniques can deliver a proficient target coverage in a robust manner. The VMAT technique results in 9%- points better coverage of the IMN. However, this is at the cost of a lower dose levels spreading through the patient and longer planning times. PH-0045 Comparing robustness of margin and robustly optimised plans to anatomical deformations in H&N J. Robbins 1 , E. Vasquez Osorio 1 , A. Green 1 , M. van Herk 1 1 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom Purpose or Objective With IGRT, the main uncertainty in the anatomical positioning of the patient is due to organ deformations. Previously, a method for modelling and simulating time-dependent anatomical deformations throughout treatment was developed and validated; this study focuses on whether plans can be created with a commercial treatment planning system (TPS) that are robust to such deformations without adaptation. We compared margin-based plans with robustly optimised plans with different settings. Materials and Methods Data from 19 H&N patients, treated over 30 fractions with a prescription of 60-66 Gy to the primary CTV and a second dose level of 54 Gy covering the nodal CTV were used. Margin plans were created using a class solution. Three margin plans were created for each patient, using isotropic PTV margins of 1, 3 and 5mm. Robust plans were created using isotropic robustness settings of 1, 3 and 5mm for both CTVs, keeping the rest of the objectives the same as the margin plan. These plans were then run through an anatomical deformation simulator, simulating 100 full treatment courses for each plan. The minimum dose to the CTV and the mean dose to the parotids were computed. A simulated treatment was considered to have achieved target coverage if the minimum dose to CTV1 was greater than 95% of the prescription dose, and the parotid was considered to have been spared if the mean dose to it was less than 26 Gy. Results Across all patients, increasing settings from 1 to 5mm increased the percentage of treatments achieving target coverage and decreased the percentage sparing the ipsilateral parotid in both the margin and robust plans (Figure 1). There is a large variation for individual patients (Figure 2), likely due to differences in the position and extent of the CTVs, i.e, whether they are in locations more or less likely to deform. Considering just target coverage, the margin plan has a higher number of successful treatments than the robust plan in most patients for the 3 and 5mm plans. Differences in the number of successful treatments