ESTRO 36 Abstract Book

S826 ESTRO 36 2017 _______________________________________________________________________________________________

Bologna, Italy 5 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Radiation Oncology Department, Roma, Italy Purpose or Objective The aim of this study was to assess the feasibility in the delivery of highly heterogeneous doses to symptomatic large tumor using VMAT technique and simultaneous integrated boost during a short course palliative accelerated radiotherapy. Material and Methods For this dosimetric analysis we selected a patient with a large symptomatic sarcoma. A Planning Target Volume (PTV) and a Boost Target Volume (BTV) were defined as the GTV plus and minus 1cm, respectively. Two different doses were simultaneously delivered to the PTV and BTV according to a dose-escalation protocol in 4 fractions. Five dose levels were planned: Level 1 (basal plan: PTV: 20Gy/5Gy), Level 2 (PTV: 20Gy/5Gy; BTV: 25Gy/6.25Gy), Level 3 (PTV: 20Gy/5Gy; BTV: 30Gy/7.5Gy), Level 4 (PTV: 20Gy/5Gy; BTV: 35Gy/8.75Gy) and Level 5 (PTV: 20Gy/5Gy; BTV: 40Gy/10Gy). The aim was to irradiate the central part of the tumor up to 10Gy/fraction while maintaining the border area of the tumor and the surrounding healthy tissues with <5Gy/fraction. SIB-VMAT plans were generated using Oncentra Masterplan TPS, in the dual-arc modality. The mean dose, D98%, D95% and D2% doses were scored for each target. A conformity index, PTV_CI, defined as the volume encompassed by the PTV 95% isodose divided by the PTV volume, was calculated. A dose contrast index (DCI) was defined as the mean dose to the BTV divided by the mean dose to the PTV (excluding BTV). For healthy tissue, an integral dose, Dint, was defined as the product of mean dose and volume of normal tissue, excluding the PTV. This was reported together with the irradiated volumes at the dose levels of 5, 10, 15 and 20Gy (V5, V10, V15 and V20). Results Overall results are reported in Table 1. When BTV dose escalated up to 200% of PTV prescription, the PTV_CI increase was <8% (from 1.11 to 1.20), proving that SIB strategy was able to reduce the dose to the BTV surrounding volume despite the major dose escalation. Similarly the percentage increase of ID to normal tissues was 11%. The increase in healthy tissues receiving more than 5, 10 ,15 and 20 Gy was about 2%. Deviation from the ideal contrast dose slightly increased with increased BTV dose.

Conclusion We quantified the capability of SIB-VMAT to deliver highly heterogeneous doses in the treatment of large tumors. Despite the major dose escalation in the BTV, the dose conformity to PTV and the integral dose to the normal tissue minimally increased, with a slow increase of dose spillage from PTV to normal tissue. The safe delivery of ablative dose in the central part of the tumor has the potential to greatly improve the palliative effect. EP-1555 Improving inter-planner variability in head and neck (H&N) VMAT H. James 1 , C. Scrase 2 , K. Yip 2 1 Suffolk Oncology Centre The Ipswich Hospital, Radiotherapy Physics, Ipswich Suffolk, United Kingdom 2 Suffolk Oncology Centre The Ipswich Hospital, Clinical Oncology, Ipswich Suffolk, United Kingdom Purpose or Objective Inverse plan optimisation for H&N VMAT is resource intensive. Variation exists between planners when determining optimal solutions. Re- optimisation of sub-optimal plans impacts upon the patient pathway. In our institution class solutions and dose assessment criteria improve consistency in prostate VMAT planning. In this study inter-planner variability was assessed for H&N VMAT. Analysis of plans and shared learning informed the development of optimisation templates to improve consistency and meet clinician expectations. Material and Methods VMAT plans for a radical tonsil treatment were created by 10 individuals with varying levels of experience. Plans were expected to meet or exceed pre-defined PTV and PORV dose requirements. Planners used their own judgement to determine optimisation objectives and priorities, define dummy structures and assess whether an optimal plan had been produced. Quantitative dosimetric analysis of the plans covered 3 areas – PTV coverage (conformity index (CI), homogeneity index (HI)), PORV doses and dose spill. Parameters were scored against a gold standard and ranked. Plans were independently reviewed by 2 clinicians specialising in H&N RT and their assessments were compared with quantitative analysis. Optimisation objectives and priorities and the use of dummy organs were analysed in conjunction with the dose distributions. Results Each plan met the pre-defined PTV and PORV doses. There was no significant variation in HI (1.05-1.09) regardless of the priorities applied in the optimisation. A larger variation in CI (1.07-1.18) was attributed to use of the Normal Tissue Objective function. There were variations in dose to normal tissues as planners applied varying dose constraints to keep doses as low as reasonably practicable without compromising PTV coverage. Clinician reviews picked up more subtle but potentially clinically relevant variations between plans – high dose spill, dose spread across mid-line and over- zealous sparing of normal tissues. The plans scored most highly by the clinicians were created by the most