ESTRO 2020 Abstract Book

S1019 ESTRO 2020

the short axis in the direction of the Bragg curve. Dosimeter positioning was verified by lasers at the surface and the digital arm was used to set the depth of dosimeters. After irradiation, dosimeters were separated, fluorescence signal read, and probability of DSB calculated. Results The probability of DSB was measured as 0.079±0.08, 0.081±0.06, 0.082±0.20, and 0.026±0.03 at depths of 15, 17, 17.5, and 18 cm, respectively. Conclusion We were able to benchmark the DNA dosimeter response in a proton beam as a function of depth. Studies to compare protons against photons are in progress. PO-1827 Clinical application of radiation-enhancing nanoparticles N. Scher 1 , P. Poortmans 1 , V. Calugaru 1 1 Institut Curie Ensemble Hospitalier, Paris, Paris cedex 05, France Purpose or Objective Inorganic nanoparticles (NP) activated by radiation therapy (RT) increase, in preclinical studies, radiation dose deposit within the cancer cells compared to RT alone. More recently, clinical evidence of the radiation- enhancing effects of NP begun to emerge, opening innovative fields of clinical applications in oncology. Material and Methods We performed a literature search in English and French from any year until 10 October 2019.The search term was «nanoparticle» and «radiotherapy», yielding 1270 results. Of these, we selected eleven papers related to clinical studies about the combination of RT with NP. Results Two NP are predominantly used in clinical trials: hafnium oxide and gadolinium based NP. The results of three phase 1/2 and one phase 2/3 trials all used hafnium oxide NP, involving 236 patients treated for locally advanced soft tissue sarcoma of the extremity and trunk wall, head and neck squamous cell carcinoma or liver tumours . Based on the data currently available, the safety profile seems manageable for the acute and transient immune reactions. No enhancement of the RT-related adverse events was identified. No serious adverse events were related to the intra-tumoural injection of the NP. Dispersion and migration evaluation by CT scan confirmed hafnium oxide NP to stay within the tumor without negatively impacting adjacent normal tissue or the reliability of the image- guided radiation therapy. As for treatment efficacy, the results of the only phase 2/3 trial available show that preoperative RT combined with hafnium oxide NP confirmed an increased proportion of pathological complete response compared with patients who received RT alone, possibly also related to an enhanced anti-tumour immune response. Currently, six phase 1/2 clinical trials are recruiting to evaluate the combination of gadolinium- based NP and RT for the treatment of brain metastases (NCT03818386) and cervical cancer (NCT03308604); and the combination of hafnium oxide NP and RT for the treatment of head and neck cancer (NCT01946867 and NCT02901483), liver cancer (NCT02721056), rectal cancer (NCT02465593), prostate cancer (NCT02805894) and lung cancer (NCT03589339). Conclusion Overall, available results demonstrate a safe and well tolerated toxicity profile for hafnium oxide NP, with very promising early clinical results. For gadolinium based NP, Poster: Radiobiology track: Biological therapies (e.g. viruses, vaccines)

results remain pending. These data open a large field of applications and justify investing in further research.

Poster: RTT track: Patient preparation, positioning and immobilisation

PO-1828 Alignment of prostate cancer patients for radiotherapy on the MR-linac H. Barnes 1 , J. Mohajer 1,2 , A. Dunlop 1,2 , G. Adair Smith 1 , T. Herbert 1 , R. Lawes 1 , A. Tree 1,2 , H. McNair 1 1 Royal Marsden Hospital, Radiotherapy, Sutton, United Kingdom ; 2 Institute of Cancer Research, Radiotherapy and Imaging, Sutton, United Kingdom Purpose or Objective The MR-Linac (Elekta AB, Stockholm, Sweden) provides a novel treatment modality that can adapt a patient’s treatment plan daily to account for positional errors and organ motion, but also creates new challenges for patient positioning as it is installed without calibrated lasers for alignment. An audit was undertaken to determine accuracy and duration of prostate patient set-up on the MR-Linac (MRL) using two methods for patient alignment, with the aim of establishing a current standard of practice on the MRL. Material and Methods Patients under the PRISM (Prostate Radiotherapy Integrated with Simultaneous MRI) trial (NCT03658525) were aligned for treatment using two different methods. The first (Set-up A) is vendor recommended and requires patients to be aligned right-left (RL) to the MRL sagittal indicator light (SIL) and superior-inferior (SI) in line with the couch index by eye. The second (set-up B), adapted from current protocol, requires the same RL positioning in addition to using MR safe rulers to align lateral tattoos anterior-posterior (AP) and SI to the couch index. Ten patients were included with set-up A and B used for 50% of fractions each. Population mean (M) and systematic (Σ) and random (σ) errors were calculated and compared using a paired t-test. Timing data were also analysed using a Wilcoxon signed rank to determine if there was a variance between set-up times. Table1.1 – Mean (M), standard deviation (SD), random (σ) and systematic (Σ) displacements. There was no significant difference between random and systematic error for the RL and AP directions. There was a significant difference in the SI direction M ( p <0.01) and Σ error ( p = 0.04). The population average set-up times of 4:02min (A) and 4:20min (B) were found not to differ significantly ( p =0.139). Conclusion The significant difference in the SI direction was possibly because of the large SD. As offsets are accounted for in online plan adaption on MRL, this SI displacement does not have a detrimental impact on delivered plans. The vendor recommended method (A) involves less manipulation of the patient and since there was no significant difference in the AP and RL directions, set-up A will be adopted as the standard for prostate and other pelvic patients on the MRL in future. Further investigations will be required to establish standards of practice for other body sites. Results