you can separate the digitiser from the LCD ( HTC Sensation Touchscreen/Digitzer Replacement ) and then carefully clean the dust off both faces of the LCD and digitiser and reassemble. alternatively, you could purchase a new LCD and digitiser module and install that.
the dust may have gotten in there because of a poor fit between the LCD and digitiser, so if you are pulling the phone apart to clean it, then make sure it is a nice seal between the two parts.
having one of these phones my self, i find they are prone to collecting dust and lint from the pocket. i haven't had any issues with dirt going behind the glass, but just keeping the phone clean and taking off the back and giving it a quick blow every now and then should help
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