position text at the top of cell in table

Question

I want to position the text at the top of the box as it is declared, but when the column on the right contains a long text, it automatically changes to the bottom of the box :

There is my code :

\documentclass[12pt]{report}

%% Je suis francophone !
\usepackage[francais]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fancybox}
%% Je veux utiliser néanmoins des fontes qui « paraissent bien » en PDF
\usepackage[cyr]{aeguill}
\usepackage[table,svgnames]{xcolor}
\newcolumntype{C}[1]{>{\centering\arraybackslash }b{#1}}
\newcolumntype{L}[1]{>{\raggedright\arraybackslash }b{#1}}
\newcolumntype{R}[1]{>{\raggedleft\arraybackslash }b{#1}}
\begin{document}
\begin{center}
\begin{table}[h]
\begin{tabular}{||C{0.5cm}||C{5cm}||C{4cm}||C{6.5cm}||} 

\hline 
\cellcolor{Gray} \textbf{RG} & \textbf{Définition} & \textbf{Mapping} & \textbf{Règles de gestion spécifiques}\\  
\hline
\hline  
\multicolumn{1}{||L{0.5cm}|}{\parbox{0.5cm}{1}} & \multicolumn{1}{|L{5cm}|}{\footnotesize {Portail \-Date Traitement Courant}} & \multicolumn{1}{|L{4cm}|}{\scriptsize{RBP \_ vTBADMRBP \_ SUIVI \_ APPLI.D \_ TRAIT}} & \multicolumn{1}{|L{6.5cm}||}{\footnotesize {CASE
WHEN [RBP \- Presentation View].[Dimension \- Référentiel Datamart].[L\_DATMR] = 'PORTAIL' THEN [RBP \- Presentation View].[Fait \- Suivi Application].[D\_TRAIT]
END}} \\
\hline 
\end{tabular}
\caption{Équipe de travail}
\end{table}
\end{center} 
\end{document}

There is a result that i have :