Topical dosage form does not have a set intake as oral medicines hence application of ICH guidelines for unknown impurities can sometimes be difficult.
Dear Mr. Rao, I know your question was very old but I realised that no one answers. So this reply may be helpful for whom seeks answer to questions like yours. When I searched for setting impurity limits in semi-solids I found that article. You can evaluate ICH Q3b guideline again using the information in this article.
Article Erratum to: Generic Development of Topical Dermatologic Prod...