ESTRO 2020 Abstract Book

S809 ESTRO 2020

Medical Systems Imaging Laboratory GmbH, CH-5405 Dättwil- Switzerland, Dättwil, Switzerland

The aim of this study was to compare the dosimetric benefit of robust optimised Intensity Modulated Proton Therapy (IMPT) with photon Volumetric Arc Radiation Therapy (VMAT) for HNC patients. Material and Methods 11 patients with locally advanced oropharyngeal cancer were replanned using eclipse TPS 15.5. 5 plans were made for each patient: (MCO) c)IMPT- (no robust optimisation parameters included) d)IMPT+3mm (3mm perturbations included in the optimisation process) e)IMPT+5mm (5mm perturbations included in the optimisation process) Prescribed dose and fractionation for each patient was 65Gy in 30 fractions for the high-risk PTV (defined as gross disease with a margin and entirety of involved nodal level) and 54Gy in 30 fractions for the low-risk PTV (defined as areas considered at risk of containing microscopic disease). The PTV margins for all patients were 3mm added isotropically to CTV. A PRV of 3mm was applied to critical organs at risk (OARs). ‘Larynx’ was used to describe the midline mucosa from hyoid to cricoid. Plans were normalised such that the mean target dose (PTVs for photons, CTVs for protons) was 100% of the prescribed dose. Standard planning constraints derived from the PARSPORT trial were followed. Target volume dosimetry and dose to standard OARs were evaluated. Results Target volume constraints were met for both CTVHR and CTVLR with all planning modalities, see table 1. The PRV brainstem constraint of D1% <48 Gy was met comfortably across all planning techniques. This dose did increase with increasing IMPT perturbations, but the increase was not clinically significant. PRV Spinal Cord constraint of D1% <44 Gy was also met across all techniques. Mean dose to larynx was around 40Gy with RP, MCO and IMPT+5mm but IMPT- and IMPT+3mm achieved a mean dose of 31.0±12.7Gy and 33.7±15.4Gy respectively. Lowest contralateral mean parotid dose was achieved with IMPT- (16.0±7.8Gy) and the highest with IMPT+5mm (24.0±10.0Gy). A similar trend was observed for the ipsilateral parotid however no modality met the constraint of mean dose < 24Gy. a)Rapidplan TM (RP) b)RP+Multi-Criteria Optimisation

Purpose or Objective Arteriovenous malformation (AVM) is an abnormal connection between arteries and veins, bypassing the capillary system. In order to correctly locate an AVM, typically, a digital subtraction angiography (DSA) is carried out. To use the DSA for target definition in radiosurgery, an accurate image registration between CT and DSA is required. Carrying out a non-invasive, frameless procedure, i.e. having no invasive head frame in place that serves as a fixed coordinate system, registration of the 2D- DSA images with the CT is critical. A new software prototype called Brain Clinic is enabling this frameless procedure. The aim of this work was to evaluate Brain Clinic in terms of targeting accuracy and reliability based on phantom measurements as well as with the aid of patient data. In addition, the user's ability to recognize mismatches between CT and DSA was assessed. Material and Methods Targeting accuracy was determined with a simple cubic, as well as with a more realistic anthropomorphic head phantom. Clearly defined academic targets within the two phantoms were contoured on the CT. The center of mass (COM) coordinates of these reference structures were compared with the COM coordinates of the structures generated within Brain Clinic, based on the 2D-DSA images. A similar approach was used with patient data, where the clinically contoured target served as the reference structure. In order to check if the user is able to recognize registration errors on blended digitally reconstructed radiographs (DRR) and 2D-DSA projection images, a set of different registration errors (translations and rotations) was introduced to the correctly registered CT and 2D-DSA image data sets of three different patients. Each of six different users rated the whole set of registrations within Brain Clinic. Results The target accuracy of the Brain Clinic was found to be below 0.04 cm for the simple cubic phantom and below 0.05 cm for the anthropomorphic head phantom. The mean target accuracy for fifteen patient cases was found to be below 0.3 cm. Artificially introduced registration errors above 1° or 1 mm were detected by the six users and correctly rated as not The method of this study shows, that the prototype is a useful tool that has the potential to fill the gap towards a frameless procedure when treating AVMs with the aid of 2D-DSA images in radiosurgery. The target accuracy of the prototype is similar to other systems, which are already well established in clinical routine. PO-1502 A comparison of photon and proton radiotherapy treatment planning techniques in head and neck cancer D. Church 1 , C. Paterson 2 , S. Currie 1 , P. Houston 1 , R. Valentine 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics Department, Glasgow, United Kingdom ; 2 Beatson West of Scotland Cancer Centre, Radiation Oncology Department, Glasgow, United Kingdom Purpose or Objective Although proton therapy is becoming more widely available it remains a limited and high-cost resource, being used where only the most tangible benefits are anticipated. Although dosimetric advantages of proton therapy have been previously reported for head and neck cancer (HNC), it is unclear if those benefits persist when compared with advanced photon planning technology or when robustness is applied to proton planning. acceptable. Conclusion