Iginla's power-play tally 4:43 into the final frame broke a 1-1 tie and Letang added another man-advantage goal less than 4 minutes later, providing the eventual game-winner for the Eastern Conference-leading Pens.
Tomas Vokoun turned aside 38-of-40 shots, improving to 12-4-0 on the season.
Brad Marchand and Tyler Seguin scored for the Bruins, seeded fourth in the East.
Tuukka Rask made 21 stops for Boston.
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