ESTRO 36 Abstract Book

S181 ESTRO 36 2017 _______________________________________________________________________________________________

lungs, intestine and stomach; smaller than 5Gy for scalp and kidneys (Figure 2). Moreover, protons provide the smallest non-PTV integral doses (V1Gy: 53% 3D-CRT, 69% photons MT, 15% PBS; V5Gy: 23% 3D-CRT, 43% photons MT, 12% PBS). A considerable variation in PTV and OAR dosimetry was observed within a certain technique.

Initial analysis showed a high proportion of plans where PTV coverage was compromised. Plan quality metrics were therefore developed which were independent of PTV coverage. These metrics are defined in eqn1 and eqn2:

where V 100% are the volumes covered by 100% and 50% of the prescription dose (the dose intended to cover the target) respectively. The mean, median and standard deviation are reported for both metrics, split into PTV V 100% volume ranges of 0-20cc, 20-40cc and >40cc. Results 38 lung and 77 non-lung (lymph node, liver, adrenal and bone) plans were reviewed, produced for treatment using Cyberknife (29), Tomotherapy (7), VMAT (71), fixed gantry angle IMRT (5) or 3D conformal (3) modalities. 11% of lung patients and 29% of non-lung patients had significantly compromised PTV coverage (PTV V 100% < 90%). The spillage results for lung and non-lung sites were similar. Modified Gradient Index (MGI) values were higher for lung than non- lung sites and decreased with increased treated volume (see table 1). No clinically significant differences were seen between treatment platform or modality. and V 50%

Conclusion Modern radiotherapy techniques demonstrate superior conformity and homogeneity, and reduced mean dose the OARs compared to 3D-CRT. PBS produced the case with the lowest mean dose for each OAR and integral doses. However, the variability among centres using the same technique means it is not possible to clearly identify the best technique from this data. Efforts should be made to improve inter-centre consistency for each technique. OC-0346 Multicentre audit of SBRT oligometastases plan quality J. Lee 1 , R. Patel 1 , C. Dean 2 , G. Webster 3 , D.J. Eaton 1 1 Mount Vernon Cancer Centre, National Radiotherapy Trials QA RTTQA Group, Northwood, United Kingdom 2 Barts Health NHS Trust, Radiotherapy Physics, London, United Kingdom 3 Worcestershire Oncology Centre, Radiotherapy Physics, Worcester, United Kingdom Purpose or Objective SBRT for oligometastases is currently being used to treat patients at 17 centres in England, as part of the NHS England “Commissioning through Evaluation” programme. The national trials QA group conducted QA for the programme, which included establishing appropriate clinical plan quality metrics for auditing submitted SBRT plans. The purpose of the audit was to inform future guidance on plan quality metric tolerances and help centres determine whether a given plan is optimal. Material and Methods Plans included were either benchmark plans using pre- delineated CT images planned by all cen tres prior to patient recruitment; or plans of initial patients reviewed prior to treatment. VODCA software (Medical Software Solutions) was used for independent plan review. Lung plans were analysed separately due to the inherent differences in scatter conditions around the tumour.

Table 1. The mean, median and standard deviation of the “Spillage” and “Modified Gradient Index” plan quality metrics for lung and non-lung oligometastatic SBRT plans. Conclusion The high proportion of non-lung patient plans with compromised target coverage suggests that future guidance documents should use plan quality metrics which are independent of coverage, such as those proposed here. The similar spillage results for lung and non-lung sites suggest that for this metric, site specific tolerances are not required. The MGI is higher for lung plans, as expected with the increased scatter in low density surroundings. MGI lung and non-lung results are similar in absolute terms and so equivalent planning tolerances could be applied to both groups. These data provide evidence of what plan quality is achievable across multiple treatment platforms, modalities and clinical sites. These are particularly useful for non-lung oligometastatic SBRT plans where there is currently a lack of data in the literature. OC-0347 Key factors for SBRT planning of spinal metastasis: Indications from a large scale multicentre study M. Esposito 1 , L. Masi 2 , M. Zani 3 , R. Doro 2 , D. Fedele 3 , S. Clemente 4 , C. Fiandra 5 , F.R. Giglioli 6 , C. Marino 7 , S. Russo 1 , M. Stasi 8 , L. Strigari 9 , E. Villaggi 10 , P. Mancosu 11 1 Azienda Sanitaria USL centro, S.C. Fisica Sanitaria, Firenze, Italy 2 Centro CyberKnife IFCA, Medical Physics, Firenze, Italy 3 Casa di cura San Rossore, Radioterapia, Pisa, Italy 4 Azienda Ospedaliera Universitaria Federico II, Medical Physics, Napoli, Italy 5 Università degli Studi di Torino, Medical Physìcs, Torino, Italy