Boston Medical Center is bracing for dramatic financial losses, which some fear will force it to slash programs and jeopardize care for thousands of poverty-stricken families. The hospital projects that it will lose $175 million in the fiscal year starting Oct. 1, an 18% operating loss that is unusually large even in Massachusetts' up-and-down hospital industry. The hospital estimates that it will close this year $38 million in the red, its first loss in five years.