ESTRO 38 Abstract book

S1164 ESTRO 38

Material and Methods in H&N cases with lymph node involvement, a different BA is strongly advised between the cranial portion of the PTV and the PTV localized in the neck area. Different field directions are used to treat the different PTV portions, with overlapping junctions between the fields (8F, Figure 1). The 6F approach keeps also a different BA between head and neck regions, but the reduction in field numbers is reached by a different splitting of the PTV. In this way, 2 field directions are kept identical for both cranial and neck portions of PTV (6F, Figure 2). The 6F planning technique was studied for two H&N cases clinically treated at PSI using 8F BA and a Simultaneous Integrated Boost (SIB) fractionation of 3 dose levels (54, 60 and 69 Gy-RBE- at 1.8, 2 and 2.3 Gy-RBE- per fraction). 6F plans were designed to be clinically equivalent to the delivered 8F ones, and PR was computed for both 6F and 8F approaches (setup uncertainties for PSI patients positioned with mask: s85%=2.25 mm) for PTVs, CTVs, spinal cord, brainstem, Parotids, chiasma and optic nerves. DT for 8F approach was over 35 minutes while for 6F between 25 and 30 minutes.

Material and Methods An institutional audit of head and patients undergoing radical arc radiotherapy was carried out for the period of April 2017 to March 2018. Specific time points during a patient’s treatment were recorded including the date where dosimetric assessment query was raised to physics team and how long the evaluation took. The total number of dose re-calculations, and rescans were summarised and spinal cord doses were audited.The dosimetric impact of separation change was evaluated by recalculating the RT plans for a sample of 5 patients with irradiation of bilateral nodes, after reducing the external body contour on the TPS by 1cm in all directions. Using the typical fraction at which weight loss is seen a composite weight loss plan was created and spinal cord doses were evaluated. Results

The audit results are as shown in Table 1. In addition, it was found that patients requiring a re-scan were because of poor fitting shell, and thus variability of set-up, rather than the dosimetric impact. Conclusion Where the planned cord PRV doses are within 0.5Gy of tolerance and with more than 50% treatment remaining, a rescan/re-plan should be actioned. This audit has shown that while the VMAT plan is robust to patient separation changes, if variability in set-up is seen a rescan pathway should be implemented immediately. IGRT is necessary to ensure accurate positioning throughout a course of treatment. The results of this audit will help streamline local processes to implement an efficient adaptive pathway for head and neck patients. Further work is being carried out to implement deformable registration to facilitate this accurately and efficiently. EP-2105 Robustness comparison between 6- and 8- fields SIB proton plans on head and neck patients N. Bizzocchi 1 , C. De Angelis 1 , J. Hrbacek 1 , A.J. Lomax 1 , D.C. Weber 1 , A. Bolsi 1 1 Paul Scherrer Institut, Center for Proton Therapy, Villigen, Switzerland Purpose or Objective Pencil Beam Scanning (PBS) proton therapy, compared to conventional radiotherapy (CRT), can improve organs at risk (OAR) sparing in Head and Neck (H&N) patients, especially for median OARs like the spinal cord and brainstem. Nevertheless, plan robustness (PR) needs to be optimal and delivery time (DT) reasonable. For these reasons, bilateral H&N cases with involved lymph nodes chains could represent an issue due to the extension/complexity of the targets and the high number of OAR involved. The PSI standard approach for such cases uses an 8-field beam arrangement (BA) that was proven to satisfy PR specifications. However, with this approach the long delivery time (at least 30 minutes) still represents an issue for intra fractional setup. Therefore a new planning technique was studied to reduce fields number (and treatment time), without compromising the dose distribution. In this study, we report on a comparison of PR between the standard 8 field approach (8F) and a 6 field alternative (6F).

Results Table1 shows the comparison between the 8F and 6F approaches. The values are for one of the 2 analyzed cases (also representative of the other). The fraction of PTV, CTV and OAR volumes (%) with dose errors over 5, 10 and 15% are reported. For OARs results are better for 6F for chiasma and brainstem while for the other VOIs there is no clear trend.