http://www.wordreference.com/definition/thorough

Arabic term (edited): متأنّية

"Deliberate" is defined as "unhurried in action, movement, or manner, as if trying to avoid error" (American Heritage Dictionary). If you feel that is what the writer meant by متأنّية, then "deliberate" would fit very well.

However, if you feel that the writer meant something more along the lines of "attentive to all aspects or details" or "extremely careful," then I would recommend "painstaking" or "meticulous."

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Note added at 15 hrs (2006-06-27 22:06:12 GMT)
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On second thought, these choices may be too much for this context. "Deliberate" is accurate by itself, but in this context, I feel that the writer simply meant "should be handled carefully" or "with care." You can even qualify it by saying "with deliberate care" if you feel that helps. Without additional context, I think this is the route I would go. I will therefore withdraw my answer and vote for Mayssa Allaf's answer.