ESTRO 2020 Abstract Book

S63 ESTRO 2020

instead of 5 mm planning target volume (PTV) margins [1– 3]. However, the loco-regional (LR) control rate was not preserved in one study [3]. The current study investigates the occurrence, location and trends where clinical target volume (CTV) coverage and critical organ at risk (OAR) sparing may be at risk in the context of reduced PTV and planning risk volume (PRV) margins. It specifically focusses on locations where the high-dose target coverage is commonly tight during treatment planning. Material and Methods VMAT plans for 12 patients were optimised using either 3 or 5 mm PTV and PRV margins (prescribed dose, Dpresc 66 Gy in 30 fractions). The planning CT (pCT) was first registered to each daily CBCT using demons deformable image registration (DIR). Subsequently, the inverse registration was used to reconstruct and accumulate the delivered dose (accD) over all fractions. The 95% level of confidence of accD was determined using the structure- and voxel-specific uncertainty as described previously. The CTV and critical OAR doses were assessed using the dose-volume histogram (DVH) metrics D99% and D0.1cc of accD, respectively. In addition, the planned and accumulated minimum dose (planDmin and accDmin) were determined for local target volumes defined at the point of minimum distance to the brainstem (BS), spinal cord (SC) and skin. Results For each critical OAR and for both margin expansions, accD0.1cc was below the specific OAR tolerance dose. For the CTVs, accD99% was less than 95% Dpresc at the 95% level of confidence in two and one cases for 3 and 5 mm PTV/PRV margin plans, respectively, with the under dosage occurring near the skin. Fig. 1 shows that target volumes closer to the skin generally exhibited a lower planDmin and a larger drop in local coverage during treatment compared to less superficial CTVs, independent of the applied PTV/PRV margin. Analysis of the local target coverages near the BS showed that for one patient, an increased BS PRV dose could be avoided using 3 mm margins. For cases with more proximal CTVs, a higher planDmin to the local PTV was achievable with 3 mm plans. Nevertheless, the drop in coverage during treatment for the local CTV was always less than 2%, with accDmin > 95% Dpresc (Fig 2). Similar results were obtained for CTV coverage near the SC.

Conclusion The workflow requires that the therapist set up the patient similarly to conventional 4DCT. The overall scan time is 2- 3 minutes, and the dose is nearly the same as conventional 4DCT. The resulting images are sorting artifact free and the model 95 th percentile residual is typically less than 4 mm (regardless of whether the breathing is regular), although we have seen two cases of >4 mm, both of which had highly irregular breathing. PH-0123 DIR based dose accumulation to validate reduced PTV and PRV margins in head-and-neck radiotherapy N. Lowther 1 , S. Marsh 2 , R. Louwe 1 1 Wellington Blood & Cancer Centre, Department of Radiation Oncology, Wellington, New Zealand ; 2 University of Canterbury, School of Physical and Chemical Sciences, Christchurch, New Zealand Purpose or Objective Emerging literature has reported reduced treatment toxicity in head-and-neck radiotherapy (HNRT) with 3