ESTRO 36 Abstract Book

S183 ESTRO 36 _______________________________________________________________________________________________

Torino, Italy 6 Azienda Ospedaliera Città della Salute e della Scienza, Medical Physics, Torino, Italy 7 Humanitas Centro Catanese di Oncologia, Medical Physics, Catania, Italy 8 Ospedale Ordine Mauriziano di Torino- Umberto I, Medical Physics, Torino, Italy 9 Istituto Regina Elena - Istituti Fisioterapici Ospedalieri, Medical Physics, Roma, Italy 10 AUSL di Piacenza, Medical Physics, Piacenza, Italy 11 Istituto Clinico Humanitas, Medical Physics, Rozzano, Italy Purpose or Objective SBRT planning for spinal metastases is particularly challenging due to the high dose required for covering the PTV complex shape, and to the steep dose gradient mandatory for sparing the spinal cord. Many combinations of delivery systems and TPSs are clinically available in different institutions. Aim of this study was to investigate the dosimetric variability in planning spine SBRT among a large number of centers. Material and Methods Two spinal cases were planned by 38 centers (48 TPS) with different technologies (table 1): a single dorsal metastasis, and double cervical metastases. The required dose prescription (DP) was 30 Gy in 3 fractions. Ideal PTV coverage request was: V DP >90% (minimum request: V DP >80%). Constraints on the organs at risk (OAR) were: PRV spinal cord: V 18Gy <0.35cm 3 , V 21.9Gy <0.03 cm 3 ; oesophagus: V 17.7Gy <5cm 3 , V 25.2Gy <0.03 cm 3 . As a last option, planners were allowed to downgrade DP to 27 Gy to fulfil OAR constraints. 3D dose matrixes were analyzed. DVH were generated and analyzed with MIM 6.5 (MIM Software Inc. Cleveland US). Homogeneity index (HI) was computed for each PTV as HI= (D 2% -D 98% )/DP. Planners did not meet the protocol constraints or PTV dose coverage were asked to re-plan the wrong case. Multivariate statistical analysis was performed to assess correlations between dosimetric results and planning parameters.

differences in scatter conditions around the tumour. 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%

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,

Table1: Linac , TPS, delivery technique and kind of inverse optimization used in the intercomparison. Results 14/96 plans did not meet the protocol requests. After the re-planning, still 6/96 plans with different technologies did not respect at least one constraint with differences >0.5 Gy. For the dorsal case, 3 minimum (<0.5Gy) deviations (1 VMAT, 1 IMRT, 1 Tomo), and 2 reduced DP (1 VMAT and 1 Tomo) occurred. For the cervical case, 3 minimum deviation (1VMAT 1IMRT 1Tomo), and 2 reduced