ESTRO 37 Abstract book

S1224

ESTRO 37

Results The reference output dose mean deviation was 0.25% (sd:+/-0.86) across institutions. After correcting for output deviations, the deviations between chamber doses and point-planned doses were 1.29% (range:-3.37; 6.71) and 0.91% (-2.76; 5.02) for the 15mm and the 25mm spheres in the static situation, respectively. Agreement with the planned dose for the motion encompassing planning techniques (centre of the PTV taken as reference point) for the 15mm sphere in motion was 3.13%(-3.17; 15.68). The film gamma mean pass rates were 81.2% (32% ; 99%)for 15mm static, 85.4%(45% ; 99%) for 25mm static and 70.5%(24% ; 95%) for 15mm dynamic. The performed wilcoxon test showed that the dynamic condition resulted, in comparison to the static, in significant overdosages with a p-value of 0.038 and 0.012 for the point dose and the film measurements, respectively. Conclusion The beam output results showed a consistently high accuracy among the institutions, while in contrast the phantom measurements highlighted important variations. Finding a significant difference between static and dynamic irradions is expected as motion encompassing planning techniques account for respiratory motion but do not assume to deliver the dose equally as planned. Instead the found difference identifies the need for novel QA methods that would allow the verification of adequate dose delivery to moving targets. These results form a starting point for discusssion. EP-2210 A review of QA submitted for the EORTC 1308 ROAM trial R. Brass 1 1 The Clatterbridge Cancer Centre, Physics, Bebington, United Kingdom Purpose or Objective To provide an overview of contouring and planning QA submissions for ROAM trial (radiotherapy vs observation following surgical resection of atypical meningioma: a randomised controlled trial). Material and Methods Two benchmark cases are available. One case is used to assess the PI's ability to contour targets and OARs according to trial guidelines. This case contains pre and post-op MR scans and a planning CT. The case is difficult as GTV abuts the orbital bone and, inferiorly, is divided by optic canal. ROAM guidelines dictate that the GTV-CTV margin is 5mm, extended to 10mm at dural edges. The other case is to assess the ability of the centre to plan according to the RT guidelines. The case is challenging as CTV approaches the patient surface, requiring the creation of a PTV_Report structure. PTV_Report consists of PTV cropped back from the surface by 5mm. The CTV overlaps with Optic apparatus, requiring a compromise of PTV coverage; the ipsilateral cochlea also sits within CTV.

Data from plan assessment forms is used to compare distributions for different treatment modalities. Results Of 35 contouring submissions, 32 were approved first time while 3 centres were asked to re-submit. Two of these subsequently passed while one is still to be re- submitted. Out of 37 total submissions, 10 were approved with no minor deviations. The remaining 24 submissions that passed were all returned with feedback due to one or more minor deviations. The most common area of feedback, as expected, was the delineation of the GTV and CTV. In particular, excluding the medial portion of GTV visible on the pre-op MR, extending the CTV margin at the dural edges and editing off uninvolved bone. Of the 33 planning cases approved, 7 failed the initial review. One further case has been rejected and is awaiting re-submission. Of the 8 rejections, 5 were wholly or partially due to the incorrect creation of PTV_Report. The others were all due to either missing OARs / PRVs or failing to keep the ipsilateral lens or cochlea within tolerance. The variety of treatment techniques permitted allowed for comparison of the distributions for each modality.

Conclusion The contouring standard for ROAM is high, with 2 rejections from 37 submissions; however the same deviations frequently occur. More guidance is required to ensure PIs understand what is required. A European contouring workshop has since been held to try to rectify this. Tomotherapy provides the most uniform coverage, with the highest D98% and lowest D1cc. Fixed field IMRT provides the greatest benefit to OARs, due to the increased degrees of freedom enabling the planner to avoid critical structures, however this at the expense of a uniform covering dose. NC VMAT offers more freedom than coplanar, allowing OARs to be avoided. Dose uniformity in the target is also maintained.

Benchmark QA for ROAM has been completed for 32 centres in 8 countries, with a further 3 centres partially submitted. Contouring benchmark reports are reviewed to investigate common deviations.