The present Phase II study aims to assess the rates of late rectal toxicities of grade ≥ 2 after hypofractionated radiotherapy of prostate cancer of 62 Gy in 20 fractions of 3.1 Gy with an injection of hyaluronic acid (HA) in the space between the rectum and the prostate. Thirty-six patients with a low- to intermediate-risk prostate cancer according to the D'Amico classification are included in the present protocol. The main characteristics of the study are that the patients benefit of a reduction of the treatment duration from 40 to 20 fractions, due to the hypofractionated irradiation, and of an injection of 3 to 10 cc of hyaluronic acid in the perirectal fat between the rectum and the prostate.

Inclusion Criteria:

• age superior or equal to 18 years and inferior to 80 years.
• patient with a low- to intermediate-risk prostate cancer, according to D'Amico classification, for an exclusive irradiation.
• prostate cancer histologically proven.
• life expectancy superior to 10 years.
• Karnofsky performance status ≥ 60% (performance status ECOG 0-2).
• the patient has to be the beneficiary of a social security system or other insurance (order n° 2006-477 from April 26th 2006).
• the signed consent form.

Exclusion Criteria:

• age inferior to 18 years and ≥ 80 years;
• history of rectal surgery;
• patient who can't cooperate during the treatment;
• history of pelvic irradiation;
• history of inflammatory bowel disorder such as ulcerative colitis or the Crohn's disease;
• other current neoplasia or history of neoplasia dating from less than 5 months, excepting basal-cell carcinomas;
• patients treated with anti-neoplastic or anti-angiogenic or with other treatments used in rheumatology and which may include methotrexate (in order not to have a radiosensitizing effect);
• patients receiving anticoagulant treatment or PLAVIX;
• other undergoing study that may interfere with the present study;
• patient under legal protection measure.