Surgical system comprising: (i) a handheld device comprising: - a base designed to be held in a user's hand, - an end-effector for mounting a surgical tool, respectively a guide for guiding a surgical tool, said surgical tool being designed to treat a planned volume of a part of a patient's body, - an actuation unit connected to said base and said end-effector for moving said surgical tool, respectively tool guide, with respect to the base in order to treat said planned volume, - a support unit connected to the base, respectively to the end-effector, comprising at least one element (50, 51, 52, 53, 54b, 54c, 55, 56, 57, 58) designed to make contact with the part to be treated or a region of the patient's body adjacent to the part to be treated so as to provide a partial mechanical link between the base, respectively the end-effector, and the part to be treated, (ii) a tracking unit configured to determine in real time the pose of at least one of the tool, the end-effector and the base with respect to the part to be treated, (iii) a control unit configured to: (a) compute in real time an optimized path of the tool or of the end-effector with respect to the base depending on said measured pose, (b) detect whether said computed path of the tool, respectively of the end-effector, can be achieved without changing the pose of the base, and, if this is not the case, determine a possible repositioning of the base with respect to the part to be treated, (c) configure the actuation unit so as to move the end-effector according to said computed path, and (d) iterate steps (a) to (c) until the planned volume has been treated, (iv) a user interface configured to indicate feedback information to the user. (i) a handheld device comprising: - a base designed to be held in a user's hand, - an end-effector for mounting a surgical tool, respectively a guide for guiding a surgical tool, said surgical tool being designed to treat a planned volume of a part of a patient's body, - an actuation unit connected to said base and said end-effector for moving said surgical tool, respectively tool guide, with respect to the base in order to treat said planned volume, - a support unit connected to the base, respectively to the end-effector, comprising at least one element (50, 51, 52, 53, 54b, 54c, 55, 56, 57, 58) designed to make contact with the part to be treated or a region of the patient's body adjacent to the part to be treated so as to provide a partial mechanical link between the base, respectively the end-effector, and the part to be treated, - a base designed to be held in a user's hand, - an end-effector for mounting a surgical tool, respectively a guide for guiding a surgical tool, said surgical tool being designed to treat a planned volume of a part of a patient's body, - an actuation unit connected to said base and said end-effector for moving said surgical tool, respectively tool guide, with respect to the base in order to treat said planned volume, - a support unit connected to the base, respectively to the end-effector, comprising at least one element (50, 51, 52, 53, 54b, 54c, 55, 56, 57, 58) designed to make contact with the part to be treated or a region of the patient's body adjacent to the part to be treated so as to provide a partial mechanical link between the base, respectively the end-effector, and the part to be treated, (ii) a tracking unit configured to determine in real time the pose of at least one of the tool, the end-effector and the base with respect to the part to be treated, (iii) a control unit configured to: (a) compute in real time an optimized path of the tool or of the end-effector with respect to the base depending on said measured pose, (b) detect whether said computed path of the tool, respectively of the end-effector, can be achieved without changing the pose of the base, and, if this is not the case, determine a possible repositioning of the base with respect to the part to be treated, (c) configure the actuation unit so as to move the end-effector according to said computed path, and (d) iterate steps (a) to (c) until the planned volume has been treated, (a) compute in real time an optimized path of the tool or of the end-effector with respect to the base depending on said measured pose, (b) detect whether said computed path of the tool, respectively of the end-effector, can be achieved without changing the pose of the base, and, if this is not the case, determine a possible repositioning of the base with respect to the part to be treated, (c) configure the actuation unit so as to move the end-effector according to said computed path, and (d) iterate steps (a) to (c) until the planned volume has been treated, (iv) a user interface configured to indicate feedback information to the user.