db.collection.aggregate([{$group: {_id : "$PATIENT_ID", MASS:{$max: "$max_field"}}}]);

{ "_id" : "TCGA-A2-A0YD", "mass" : 0.994683301742671 }
{ "_id" : "TCGA-A2-A3XX", "mass" : 0.993455527786917 }

db.collection.aggregate([{$group: {_id : "TCGA-A2-A3XX", MASS:{$max: "$max_field"}}}]);

{ "_id" : "TCGA-A2-A3XX", "mass" : 0.994683301742671 }