ESTRO 36 Abstract Book

S238 ESTRO 36 2017 _______________________________________________________________________________________________

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

Figure 1: Measurement of D LAD Results For proximal, middle and distal LAD, 78%, 72% and 61% of subjects had both the longest D LAD in end-inspiratory (90- 10%) phase and shortest D LAD in end-expiratory (40-60%) phase. The average D LAD in end-inspiratory phase and end- expiratory phase were 13.1±2.0mm, 12.7±1.8mm, 11.6±1.5mm and 10.9±1.5mm, 10.5±1.4mm, 9.5±1.3mm for proximal, middle and distal LAD respectively. While the D LAD decreased from proximal to distal portion of LAD in both phases, the extension of D LAD from end-inspiratory phase to end-expiratory phase were similar in all LAD portions (proximal:2.1±0.9mm; middle: 2.2±0.8mm; distal: 2.1±0.5mm). The average Maxdisp LAD due to cardiac motion were also similar in proximal, middle and distal portion, which were 2.6±0.6mm, 2.4±0.5mm and 2.6±0.7mm respectively. When accounting both cardiac and respiratory motions, D LAD could be shorter than expected. To account the effect of both cardiac and respiratory motions, the shortest distance between LAD and chest wall (D LAD - 0.5 x Maxdisp LAD ) was estimated for end-inspiratory and end-expiratory phase. The averages were 11.8±2.1mm, 11.5±1.8mm, 10.3±1.6mm and 9.6±1.7mm, 9.3±1.4mm, 8.2±1.4mm for proximal, middle and distal LAD respectively. Conclusion Most patients could be benefited from gated radiotherapy using end-inspiratory phase (90-10%). However, the distance between LAD and chest wall could be shorter than expected due to random cardiac motion during actual treatment delivery. Special attention should be put on distal portion of LAD as it had the closest proximity to chest wall. A minimum clearance of 2mm (~0.5 x Maxdisp LAD ) from the LAD to the high dose zone during treatment planning is recommended to compensate for LAD displacement due to cardiac motion for patient receiving gated left breast radiotherapy. OC-0452 Evaluation of a novel field placement algorithm for tangential internal mammary chain radiotherapy A. Ranger 1 , A. Dunlop 1 , M. Maclennan 2 , E. Donovan 3 , E. Harris 3 , B. Brigden 4 , C. Knowles 4 , K. Carr 4 , E. Henegan 4 , J. Francis 4 , F. Bartlett 5 , N. Somiah 1 , I. Locke 1 , C. Coles 6 , A. Kirby 1 1 Royal Marsden Hospital Trust & Institute of Cancer Research, Clinical Oncology, London, United Kingdom 2 Edinburgh Cancer Centre, Clinical Oncology, Edinburgh, United Kingdom 3 Royal Marsden Hospital Trust & Institute of Cancer Research, Physics, London, United Kingdom 4 Royal Marsden Hospital Trust, Radiotherapy, London, United Kingdom 5 Portsmouth Hospital NHS Trust, Clinical Oncology, Portsmouth, United Kingdom 6 Cambridge University Hospitals NHS Trust, Clinical Oncology, Cambridge, United Kingdom

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