Jan 23 - Fitch Ratings has assigned a 'BB/RR1' rating to Tenet Healthcare Corp.'s (Tenet) 4.5% senior secured notes due 2021. The ratings apply to approximately $4.9 billion of debt at Sept. 30, 2012. The Rating Outlook is Stable. A full list of ratings follows at the end of this release. Tenet will use the proceeds of the $850 million senior secured notes issuance to retire $714 million of its 10% senior secured notes due 2018 in a tender offer. The balance will be used for general corporate