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life enjoyment ( P ≤ 0.05). In addition, at the 6M time point patients treated with HN-SBRT had less drowsiness, less interference with general activity, less limitations to food eaten, less discomfort, more satisfaction with mouth/throat dryness, and less effect on all aspects of life than patients treated with IMRT or PBT ( P ≤ 0.05). At the 12M time point patients treated with HN-SBRT reported less difficulty swallowing at the end of the day, less frustration with a swallowing problem, less coughing when drinking, greater feeling to swallow a huge amount of food, less fatigue, less shortness of breath, less drowsiness, less mucus production, less choking, less hoarseness, and less concerned about the appearance of their teeth/gums than patients treated with IMRT ( P ≤ 0.05). Conclusion In this prospective analysis we demonstrate that HN-SBRT has a favorable patient reported short term toxicity profile in the setting of treatment of recurrent head and neck cancers. Abstract text The biological effects of radiotherapy depend on dose distribution, treated volume, dose-rate, fractionation and treatment duration. Low dose rate has been very popular for a long time. It has some radiobiological advantages compared to external beam radiotherapy: subletal damages repair during irradiation leading to a relative protection of healthy tissues, no tumour cell repopulation, cell cycle redistribution and a low oxygen enhancement ratio. The effects of dose rate in low-dose rate irradiation have been controversial in several decades. Paterson proposed that the total dose should be corrected for overall treatment time. In 1952, he published an isoeffect curve. The standard treatment was 60 Gy in 6 days. The dose had to be decreased to 46 Gy if delivered in 3 days, and increased to 62 Gy if given in 9 days. In the seventies and eighties, the validity of the Paterson’s curve was questioned. Several clinical analyses failed to demonstrate an effect of dose-rate on clinical outcome and concluded that Paterson’s correction was overestimated. These conclusions were not supported by radiobiological data, and a reappraisal of the issue was carried out in the nineties, with a more appropriate use of statistics in the analysis of clinical series. An effect of dose-rate on clinical outcome was clearly demonstrated in several large retrospective analyses and a randomised trial, in the range of 0.3-1 Gy/hr. These effects seemed greater for late reacting tissues than for local control. Indeed, the effect of dose rate in low-dose rate brachytherapy can be compared to the effect of dose per fraction in fractionated external beam radiation therapy. Decreasing the dose rate increases the therapeutic ratio, as does lowering the dose per fraction in external radiotherapy. Isoeffect curves can be drawn for both tumoral and normal tissues. The isoeffect curve is very close to that previously published by Paterson. In contrast, the isoeffect curve of carcinoma is much less steep (i.e. carcinoma with a large a/b). This means that Paterson’s correction is appropriate for late effects, but overestimated for early effects and cancer control. It is therefore not advisable to adapt the dose to the dose-rate in the range of 0.3-1 Gy/hr. It seems more appropriate to deliver a high dose to maximise local control, while using a low-dose rate (0.3-0.6 Gy/hr) to minimise late effects. High dose rate and pulsed dose Award Lecture: Iridium Award Lecture SP-0434 The dose rate effect in brachytherapy J.J. Mazeron 1 1 Hôpital Pitié-Salpétrière, Radiation Oncology, Paris, France

Conclusion Our data confirmed the prognostic value of clinical and patient related factors. We were able to find a prognosticator role of VRec and total dose that could potentially allow for a more refined patients’ selection to re-RT. PV-0433 Prospective analysis of patient reported outcomes after head and neck reirradiation C. Pollard- III 1 , T.P. Nguyen 1 , A.S. Garden 1 , S.J. Frank 1 , C.D. Fuller 1 , D.I. Rosenthal 1 , J. Phan 1 1 UT MD Anderson Cancer Center, Radiation Oncology, Houston- TX, USA Purpose or Objective Head and neck reirradiation is associated with increased risk of patient symptom burden. Highly conformal radiotherapy can potentially mitigate this risk. In this study we prospectively analyzed patient reported short- term toxicity outcome differences in patients treated for head and neck recurrences with intensity modulated radiation therapy (IMRT), head and neck stereotactic body radiation therapy (HN-SBRT), and proton beam therapy (PBT). Material and Methods 109 patients treated for recurrent head and neck malignancies with IMRT, HN-SBRT, or PBT from 2014-2017 were prospectively enrolled onto our patient reported outcome (PRO) protocol and assessed. Pre- and post- treatment symptom and interference scores were assessed using multiple patient reported outcome surveys. Enrolled patients answered each survey at a pre-treatment baseline (BL), within 2 weeks post- treatment (2W), 3 months post-treatment (3M), 6 months post-treatment (6M), and 12 months post-treatment (12M). The mean scores were calculated and compared for each treatment type at each time point using one way ANOVA. Significance = P ≤ 0.05. The Kaplan-Meier method was used to estimate local control (LC) and overall survival (OS) outcomes. Results 52 (48%) patients were treated with IMRT, 32 (29%) patients were treated with HN-SBRT, and 25 (23%) patients were treated with PBT. Median follow up for all patients was 10.3 months. Overall survival (OS) in the entire patient population at 1 year was 89% and 71% at 2 years. Local control (LC) for the entire cohort was 79%. ~70% of all patients received chemotherapy. At the 3M time point patients treated with HN-SBRT reported easier swallowing at end of day and more comfortable speaking in front of others when compared to IMRT ( P ≤ 0.05). At the 6M time point patients treated with HN-SBRT reported less effort swallowing, less of an impact on income, greater feeling to swallow a huge amount of food, less feelings of exclusion, better appetite, better swallowing/chewing function, and less interference with