The present invention relates generally to radiotherapy systems, and more particularly to schedule optimization for delivery of external beam radiotherapy treatment. 
Advances in technology have led to great progress in the treatment of many diseases.  However, full utilization of the capabilities of the most powerful machines have not been fully realized because often the use of complex machinery must be carefully balanced against safety requirements.  Adherence to safety requirements, which, of course, may not be compromised, often results in non-optimal use of machinery, unnecessarily long treatment sessions, and unnecessary expense both from non-optimized use of expensive equipment and from poor utilization of human resources. 
The foregoing may be more true for radiation oncology than for any other medical field in that radiation oncology has been driven more by technological innovation involving complex, expensive and potentially hazardous equipment than most, if not all, other medical fields.  External beam radiotherapy is a sophisticated treatment method for treatment of tumors and other medical conditions by aiming one or more collimated beams of energized ionizing particles to a desired location, a treatment volume, in a patient. 
External beam radiotherapy is a form of treatment of, for example, cancers.  A radiation source is used to produce a focused radiation beam, e.g., an x-ray beam that is aimed at a volume of interest in a patient.  Typically, the volume of interest is a tumor.  In many forms of radiation therapy, the treatment involves aiming a sequence of radiation beams from various angles towards the volume of interest.  One reason for doing so is that by using beams from various angles may avoid delivering damaging radiation energy to healthy tissue in the path from the radiation beam delivery mechanism to the volume of interest and beyond.  By having the multiple beams converge on the volume of interest a necrotic dose is delivered to the volume of interest while minimizing the damage to any healthy tissue that surrounds the volume of interest.  This procedure involves many steps, which include positioning the radiation beams, re-configuring the cross-sectional profile of the beam and its intensity, and repositioning the patient and verifying correct positioning of the patient through re-imaging the patient.  To allow for patient positioning, a patient is placed on a table that may be moved in six degrees of freedom, namely, x, y, and z-axis positions, and pitch, roll, and yaw rotations.  Such a table may be robotically controlled. 
Proton therapy is one particularly advanced form of external beam radiotherapy.  Proton beams are produced for proton therapy by accelerating protons (hydrogen atoms without their electrons) in particle accelerators such as cyclotrons or synchrotrons.  From the accelerator the protons are transmitted to a treatment room through a beam transport system.  In the treatment room, the proton beam is focused through a nozzle placed very close to the patient. 
An important advantage of proton therapy over, for example, photon radiation therapy, is that most of the energy of a proton beam is released at a specific location that may be accurately controlled.  As protons enter the patient only a small amount of energy is released.  However, the protons slow down as they pass through tissue and release a large amount of energy immediately before coming to rest.  As a result, the energy associated with the protons may be delivered at a precise location with significantly less impact on surrounding tissue. 
Manipulation of the lateral shape, through collimators, and energy of the beam using compensators, the proton beam may be given a three dimensional shape.  Thus, the proton beam may be made to conform to the three-dimensional shape of a volume of interest.  It must be appreciated that precise configuration of the proton therapy equipment and precise location of the patient, thereby precisely locating the volume of interest, is of critical importance.  Consequently, a large portion of a treatment session is concerned with placement of the patient in the treatment room, modeling of the patient to allow for precise placement, and configuration of the equipment.  Actual delivery of the treatment is usually a very small portion of the total treatment session. 
Proton treatments are traditionally delivered using gantry systems, such as the system described in U.S. Pat.  No.  7,348,579 to Eros Pedroni.  In a gantry proton beam delivery system, a patient is positioned on a table and the proton delivery system is located on a rotating structure that may rotate around the patient to allow for delivery of the proton beam from many angles in the plane of rotation.  In an alternative delivery mechanism the proton delivery system is fixed, for example, as fixed horizontal beam.  In yet another alternative, the proton beam delivery system may incline between various angles to the vertical.  In each of these systems, a patient positioning system is used to ensure that the patient, and, consequently, the volume of interest are positioned accurately so that the proton beam will deliver its energy to precisely the correct location. 
In all forms of radiation therapy, accurate patient positioning is critical.  In one form of proton therapy, known as Image-Guided Proton Therapy (IGPT), the patient positioning system is guided by a digital radiographic panel that may be moved in numerous ways around the patient positioning system to acquire x-ray images of the patient to verify accurate positioning of the patient and the volume of interest.  The patient positioning systems may be further enhanced from additional imaging equipment such as optical tracking, ultrasound and electromagnetic signal emitting positioning systems. 
Thus, treatment delivery may include many independent but linked steps involving patient simulation, patient positioning, equipment configuration and equipment movement.  These steps may include several pieces of equipment that may move about the treatment room including the patient positioning system, the proton beam delivery system, and various forms of imaging equipment. 
The delivery of radiation therapy is usually broken down into three stages: patient simulation, treatment planning, and treatment delivery.  In patient simulation a patient is digitally modeled so that treatment planning and treatment delivery systems may have an accurate understanding of the shape and internal anatomy of the patient.  Treatment planning includes designing a sequence of beams delivered from various directions relative to the patient and each beam having a particular shape and delivering a particular dose to the target.  The main objective is to ensure that the beams in the treatment plan collectively deliver a destroying dose of radiation to the volume of interest while leaving surrounding tissue unharmed.  U.S. Pat.  No.  6,546,073 to Eva K. Lee, the entire disclosure of which is incorporated herein by reference, describes several treatment planning approaches including standard planning (or forward planning) in which a physician solves the problem of determining the appropriate dose distribution given a known set of beam characteristics and delivery parameters, and inverse treatment planning in which a computer optimization is performed from a set of parameters and dose distributions specified by physicians based on a set of pre-selected variables.  Treatment delivery is the stage in which the treatment plan is executed to deliver the desired radiation treatment to a patient. 
U.S. Pat.  No.  6,546,073 further describes techniques for optimizing the treatment plan in terms of the selection of beams.  However, while such optimization may produce more efficient and effective delivery of radiation to the tumor, the limitations of the equipment, the room setup, and delivery process are not considered.  Furthermore, the order of beams in a plan may not be optimal with respect to treatment time or equipment motions.  Hence there is still a need to optimize the actions of the actors involved in the delivery of a radiation treatment.  Actors involved in the delivery of a radiation treatment include the radiation therapists (RTTs), the patient positioning system, multiple patient imaging devices, and the radiation delivery system.  Because of the many independent actors involved, it is possible that a given workflow plan for delivery of a treatment plan involves steps that present danger to the patient or the equipment due to collisions between pieces of equipment or between equipment and patient.  Because such situations are unacceptable, RTTs typically ensure safety by completely retracting equipment that may be retracted when other equipment is being positioned or when a patient is re-positioned.  Such conservative measures are not efficient in terms of equipment use and time.  A typical treatment session may take as long as one hour while the actual delivery of treatment is only 2-3 minutes of that time.  Equipment placement, patient positioning, and patient imaging consume the remaining time of a treatment session. 
Treatment planning fails to address duration of treatment sessions in terms of minimizing the time expended on inefficient workflow steps. 
Interactive simulation of treatment rooms and radiotherapy equipment combined with accurate 3D patient specific data has been proposed, for example, in Hamza-Lup F. G, Sopin I, Lipsa D, and Zeidan O, (2007) X3D in Radiation Therapy Procedure Planning, International Conference on Web Information Systems and Technologies (WEBIST 2007), March 3-6, Barcelona, Spain (available at http://galati*armstrong*edu/research/webist07*pdf1), for improving treatment planning by using 3D equipment modeling and simulation to detect collisions among components used in delivering external beam radiation therapy treatments thereby saving time and resources in generating a treatment plan.  This system, however, only provides for qualitative evaluation and visual inspection of different treatment configurations in a treatment plan.  It is desirable to provide a quantitative evaluation of the quality of an entire treatment plan, rather than just individual configurations.  Specifically, estimates of the treatment time, treatment safety, and treatment schedule are desired.  1 Due to the prohibition of functioning hyperlinks in patents, all URLs herein are listed as www*xyz*com wherein each “*” is to be interpreted as a “.”.  Furthermore, this and other similar systems only model traditional couches and radiotherapy equipment used in photon therapy.  Unlike existing patient positioning couches, robotic couches may execute arbitrarily complex trajectories in 3D space. 
From the foregoing it will be apparent that there is still a need for an improved radiation therapy system and method of radiation therapy with improvement in duration of treatment sessions.  Primary to such an improvement would be estimates for expected time required for execution of a workflow to deliver a treatment plan and evaluation of safety to patient and equipment expected during the delivery of a treatment according to a given workflow for delivery of a treatment plan.