ESTRO 36 Abstract Book

S241 ESTRO 36 _______________________________________________________________________________________________

TV coverage. To avoid the common mistakes encountered, a new approach to training including verbal instructions is being developed for testing. Our aim is to demonstrate benefits in clinical workflow and expand validation to other RT centres. OC-0453 Stereotactic radiosurgery for multiple brain metastases: Results of multi-centre benchmark studies D.J. Eaton 1 , J. Lee 1 1 Radiotherapy Trials QA group RTTQA, Mount Vernon Hospital, Northwood, United Kingdom Purpose or Objective Stereotactic radiosurgery (SRS) is strongly indicated for treatment of multiple brain metastases. Various treatment platforms are available, and comparisons have been made between modalities, but mostly in single centre studies. A pre-requisite for all providers selected as SRS/SRT centres in England was to participate in a quality assurance process, informed through collaboration between the national trials QA group and a multidisciplinary expert advisory group. All clinical centres undertook planning benchmark cases, providing a unique dataset of current practice across a large number of providers and a wide range of equipment. This was used to facilitate sharing of best practice and support centres Two brain metastases cases were provided, wit h images and structures pre-drawn, involving three and seven lesions respectively. Centres produced plans a ccording to their local practice, and these were reviewed centrally using metrics for target coverage, selectivity, gradient fall-off and normal tissue sparing. Results 38 plans were submitted, using 21 differe nt treatment platforms, including Gamma Knife, Cyberknife, Varian (Novalis / Truebeam STx / 2100), Elekta (Synergy / Versa HD using Beam Modulator / Agility MLC) and Tomotherapy. 6 plans were subsequently revised following feedback, and review of 4 plans led to a restriction of service in 3 centres. Prescription doses were typically 18-25Gy in 1 fraction (or 27/3fr), except for a lesion within the brainstem, which was prescribed 12-20Gy in 1 fraction (or 18-30Gy/5fr). All centres prioritised coverage, with the prescription isodose covering ≥95% of 208/209 lesions. Selectivity was much more variable, especially for smaller lesions, and in some cases this was combined with high gradient index, resulting in large volumes of normal tissue being irradiated. Both Tomotherapy submissions were outliers in terms of either selectivity or gradient index, but all other platforms were able to give plans with relatively low gradient indices for larger lesion volumes, although there was more variation among Varian and Elekta plans, than for Gamma Knife and Cyberknife (first figure). There were also larger differences for the smaller volumes, with increasing variation both inter- and intra- treatment-platform. Doses to normal brain and brainstem were highest when PTV margins were applied, but substantial improvements were possible by re-planning, even without changing margin size (second figure). with less experience. Material and Methods

Purpose or Objective Published data demonstrate an overall survival benefit associated with including the internal mammary chain (IMC) in the radiotherapy target volume (TV) for women with node positive breast cancer. Implementation of IMC- RT will be facilitated by development of resource efficient techniques. However, even relatively simple techniques rely on time consuming clinician outlining of lymph nodes to achieve adequate dose to the TV (not well covered by standard fields based on bony landmarks). In order to reduce the resource burden of nodal contouring, an anatomical point based algorithm for guiding field placement was developed and tested for its ability to

ensure TV coverage. Material and Methods

We identified six points, representative of regional lymph node level borders according to ESTRO consensus guidelines, and tested these for their ability to inform field placement adequately covering the TV (Table 1). Written instructions were developed and a cohort of 20 cases identified as a validation group. ‘Gold standard’ nodal volumes (Levels 1-4 and IMC) were delineated according to ESTRO consensus guidelines by four clinical oncologists with experience in locoregional breast radiotherapy. Six independent testers (three clinicians and three radiographers blinded to the nodal volumes) were invited to place points and consequently fields on the cases. In four cases a clinician placed both the points and fields, in eight cases a clinician placed the points and radiographer applied the fields and in eight cases points and fields were placed by a radiographer. Cases were planned using forward planned techniques. The dose objective to the nodal PTV was V 32Gy ≥90%. Results

Fourteen of 20 cases met the dose objectives when testers followed the written algorithm alone (Figure 1). Of the remaining six cases, four failed due to the subclavian vein being mistaken for the subclavian artery. Two failed due to point 3 being placed at the inferior part of the pectoralis minor muscle resulting in insufficient coverage of level 3. When the points were re-placed correctly nodal TVs were well covered. Conclusion The results suggest that, in the majority of cases, by following the algorithm clinicians and radiographers can appropriately place fields which result in acceptable nodal