[Dosimetric impact of hydrogel spacer use for stereotactic body radiotherapy of localised prostate cancer]. / Impact dosimétrique de la pose d'un espaceur rectal dans le traitement de cancer de la prostate localisé par irradiation en conditions stéréotaxiques.

PURPOSE: Stereotactic body radiotherapy (SBRT) of prostate cancer is associated with rectal toxicities, which can be reduced by using a hydrogel spacer. The object of this retrospective study was to show the feasibility of spacer placement under local anesthesia and utility of hydrogel spacer to reduce the dose to the rectal wall. MATERIAL AND METHODS: We collected data from all patients with localised prostate cancer treated with SBRT (40Gy in 5 fractions) between 2018 and 2020. A hydrogel spacer (SpaceOAR®) was placed depending on the availability of the product. We collected dosimetric data for target volumes and organs at risk. We calculated mean values, which were compared using non-parametric tests. RESULTS: Among 35 patients, mean age was 75 years. Seventeen had a spacer placed, with a mean space created of 10mm. No complication was reported during the intervention. High doses to the rectal wall were significantly lower in spacer group (V38: 0.39 cm3 vs. 0.72 cm3; P=0.02). PTV were better covered in spacer group (P=0.07). Doses to the bladder wall were similar in both groups. CONCLUSION: Spacer procedure under local anesthesia was well tolerated. Hydrogel spacer allowed to reduce doses to the rectum while improving PTV coverage.

Mechanisms of phosphene generation in ocular proton therapy as related to space radiation exposure.

Particle therapy provides an opportunity to study the human response to space radiation in ground-based facilities. On this basis, a study of light flashes analogous to astronauts' phosphenes reported by patients undergoing ocular proton therapy has been undertaken. The influence of treatment parameters on phosphene generation was investigated for 430 patients treated for a choroidal melanoma at the proton therapy centre of the Institut Curie (ICPO) in Orsay, France, between 2008 and 2011. 60% of them report light flashes, which are predominantly (74%) blue. An analysis of variables describing the patient's physiology, properties of the tumour and dose distribution shows that two groups of tumour and beam variables are correlated with phosphene occurrence. Physiology is found to have no influence on flash triggering. Detailed correlation study eventually suggests a possible twofold mechanism of phosphene generation based on (i) indirect Cerenkov light in the bulk of the eye due to nuclear interactions and radioactive decay and (ii) direct excitation of the nerve fibres in the back of the eye and/or radical excess near the retina.