物理师是肿瘤放射治疗中非常非常重要的成员，可以毫不夸张的说，没有物理师，放射治疗工作就开展不了。特别是随着近年来肿瘤放射治疗设备和技术的飞速发展，物理师在保证辐射安全，提高治疗技术水平，为患者提供高质量服务等方面所起的作用也越来越重要。在欧美国家医院里的肿瘤放疗科，物理师作为一个职业已有很长的历史，从事物理师职业的人数也由于设备和精确放疗技术的发展不断增加，同时所担负的责任也越来越重。
在肿瘤放射治疗中，放射肿瘤学医师无疑将对整个放射治疗过程负责，基于这样一个角色，他或她的责任就是确定一个合适的能胜任工作的物理队伍，在这个队伍中不同人员（包括物理师，剂量师或其他人员）的职责是明确指定的。没有足够的物理支持，就无法为患者提供高标准的治疗和服务。而物理师则必须领导物理组的工作，对应用于患者的所有物理数据和过程负责，不管这些过程是否由物理师本人直接实施。
每一个放射治疗部门都需要不断提高自己的治疗水平，这就意味着需要不断引入新的治疗技术和手段，同时有选择地保留原有的治疗项目。在这个过程中，物理师都扮演了重要的角色。例如在近30年里，加速器技术的发展、CT成象、三维治疗计划、适形和动态治疗、远程后装近距离照射、调强放射治疗以及立体定向治疗等新技术的相继出现和发展，都不断地改变着物理师的工作内容和职责范围。由于每家临床医院的肿瘤放射科所拥有的治疗设备各不相同，治疗水平和开展的项目也不一样，所以工作在不同医院里的物理师的具体工作和职责也就不尽相同。在具备大多数先进的放射治疗设备的肿瘤放疗科里，物理师这个职业的具体任务大致包括以下几个方面。
1． 针对放射治疗设备方面的工作
现代放疗设备包括远距离照射设备、近距离照射设备及模拟机等等。考虑到放疗设备的迅速发展、针对的病症种类和相对昂贵的价格，物理师有责任对本单位需要购买的放射治疗设备进行性能价格比方面的选择，就如何开展该治疗项目提出自己的建议，并提出厂家的设备需要满足的指标和条件。这不仅要求物理师不断了解最新的放射治疗技术，同时也要清楚各种技术和手段的适用范围和局限性，并对这些技术实施过程的复杂程度有所了解。
放射治疗设备的安装一般都是由厂家完成的，但随后该设备的验收检测和机器数据测量都是医学物理师的工作。对每种放疗设备来说都可列出正式的验收检验条目，其指导原则是用于患者的任何设备都必须经过检测以确保满足使用要求和安全标准。例如对直线加速器，就需要做以下几方面的检测：辐射防护测量，独立准直器的对称性的检查、各部分中心轴是否一致、机架和机头的转动对等中心点位置的影响、X射线的能量、射野平坦度及射野对称性的检测、电子线的能量、射野平坦度及射野对称性的检测、监测电离室的稳定性和线性度的检测等等。每一项检测都有不同的内容、步骤和指标, 可以列成表格的形式逐一完成。
通过验收检测的一部分放疗设备可直接开始临床使用，但还有一部分不能直接使用，需要获取更多的数据，如直线加速器在进行临床使用之前，必须通过刻度，测量得到治疗计划系统所需要的所有射束参数和机器参数并将它们输入治疗计划系统，然后检验该治疗计划系统所计算的剂量分布是否与实际测量结果相符合，这些都是物理师的工作。经过物理师授权的机器才能被用于治疗患者。
放疗设备的质量保证（QA），是一个临床机构进行高质量放射治疗服务的必要条件。每台放疗设备都需要有每天应该做的质量保证内容，每月应该做的质量保证内容以及每年应该做的质量保证内容，并将其列在文档中，按时间安排人员逐一实施。一些常规的质量保证任务既可以由物理师来完成，也可以由剂量师来完成，但物理师必须建立质量保证的内容条目和步骤，指导整个过程并检查最后的结果。
2． 辐射治疗计划方面的工作
首先，辐射治疗计划系统硬件和软件的验收检验、数据测量、日常的系统和数据维护都需要物理师来完成。对硬件系统的检验内容包括检查数字化输入和输出设备的精度和线性度；对软件系统的检验就是选择一系列治疗条件，检查在这些条件下计算数据和测量数据相比的精确度，如在三维水箱中可进行的各种计算和测量数据的比较。另外一个重要的方面是对治疗计划系统中的各种算法进行检验，如它们的精确度、限制条件和特点等等。这里医学物理师的职责是保证治疗计划系统能够得到正确的使用。
其次，辐射治疗计划过程一定需要物理师的参与。虽然患者的治疗方案由放射肿瘤学医师全面负责，但具体的治疗计划则由放射肿瘤学医师和物理师共同来完成，因为治疗计划过程中许多方案的设计和优化包含复杂的物理概念。一般的模式是：①放射肿瘤学医师根据患者的病情决定是否做CT检查或MR检查，或两者都做，并确定CT模拟的定位方式和定位点；②物理师将CT图象数据及MR图象数据输入治疗计划系统；③如果有MR图象数据，物理师先进行CT图象和MR图象的融合，然后在CT图象上进行外轮廓、重要器官的轮廓勾画；④放射肿瘤学医师勾画靶区，与物理师讨论如何设置射野，在DRR图象上勾画射野中的挡块形状，此时物理师在领会医生的治疗方案后，考虑实际的物理条件和设备条件，提出自己的建议；⑤物理师进行参数设定和剂量计算，不断对计划进行改进和优化，以尽量实现医生的治疗方案；⑥最后由医生决定治疗计划是否可接受，并在病历上签字认可。在整个过程中，放射肿瘤学医师和物理师都应该是密切配合的。在很多治疗中心，一般的治疗计划是由剂量师完成的，同样需要遵循以上的步骤，物理师主要起监督和指导的作用，当涉及到复杂的治疗计划时，则由物理师来完成。
另外，物理师还有一个重要的任务，那就是对治疗计划的质量保证。所有的治疗计划经过医生的认可后，一方面需要输出到控制治疗设备的计算机中以控制实际的治疗过程，另一方面需要输出到患者的病历中，这两方面的输出都要求非常准确，物理师需要对每一项内容进行检查，保证计划输出、控制输出和患者的病历三者的数据是一致的；另外因为放射治疗一般要进行分次治疗，为了检查每次治疗是否是按计划要求进行，治疗师需要按照表格填入每天的治疗情况，如日期，每一射野治疗时输出的实际剂量等等，而物理师则每隔一周左右检查这些记录，发现问题及时纠正。为了尽量不出错，上述的检查一般需要由两名物理师进行双检。
如果患者的治疗计划是一个调强放射治疗计划（IMRT），那么需要对它进行专门的质量保证过程。每个放射治疗部门可根据本部门的设备条件制定IMRT的质量保证内容。如对一个IMRT 治疗计划，可以把该治疗计划应用于一个固体水的体模中，计算得到在这个体模中每个射野的等剂量分布；同时用Mapcheck实际测量每个射野的等剂量分布，其中每个射野由几十个甚至上百个子野组成。将计算值和测量值进行比较，如果80%的点的剂量误差在5%以下，那么这个计划就得以通过，可进行下一步的治疗。或者用小的空腔电离室测量某一点的绝对剂量，用EDR2胶片测量某一平面的等剂量分布，然后与计算结果相比。如果一个放射治疗部门拥有两种不同厂家的IMRT治疗计划系统，可以用被称为混合计划验证的方法进行质量保证。具体做法是将一个系统产生的IMRT计划应用于一个固体水的体模中，计算得到在这个体模中每个射野的等剂量分布；同时在另一个治疗计划系统中用同样的射束条件进行固体水体模中的剂量分布计算，比较两个系统的计算结果，等中心剂量的计算结果的差异应小于5%。该方法与用独立的剂量计算系统进行QA验证的方法类似。
3． 培训和研究方面的工作
由于放射治疗技术本身的复杂性和飞速发展，每一个放射治疗部门不仅要求有一支能满足临床任务的物理师队伍，而且对其人员的不断培训也非常重要。这些培训不仅包括常规的临床训练，同时也包括对新的技术和治疗方式的逐步掌握。首先，对新进入医学物理领域从事物理师工作的人员，必须要有一段合理的临床训练时间，对临床工作中许多实际的操作必须有一个熟悉的过程；其次，将一种新的治疗手段引入到一个放射治疗部门，如全身照射、电子线照射、三维适形放射治疗、调强放射治疗、立体定向放射手术、低能量源植入式内照射、高剂量率内照射等等，对物理师来说一方面是要掌握治疗技术本身，另一方面是要了解开展该治疗技术的治疗设备，并针对这一治疗设备制定相应的操作规程和质量保证计划，全面开发该设备的各种功能。所以，医学物理师的职业培训应该是一个长期的继续教育和自我培训的过程。这样才能保证治疗设备处于良好的工作状态，为患者的诊断和治疗提供最佳的技术支持。另外，物理师还负有培训本单位的剂量师和治疗师在物理方面的知识的责任。
现代社会中飞速发展的各种高、精、尖技术也集中地体现在现代放射治疗设备的开发和应用上，如电子技术、精密仪器、计算机网络、图形图象处理、自动控制技术等等。在提高放射治疗技术、发展新的治疗设备的过程中，特别是在它们的设计和临床应用方面，医学物理师都扮演了重要的角色。而涉及医学物理领域各个方面的研究工作是促进放射治疗技术不断发展的源泉。对放射治疗技术本身的精益求精也是医学物理师的职责之一。肿瘤放射治疗过程中的物理支持工作并不是每一项都要物理师亲自完成，其中的一些具体技术工作可以由剂量师来做，由物理师进行检查。这样物理师可以有一定的时间开展一些研究工作，提高治疗技术水平，发展新的治疗手段。
每一个医学物理师在肿瘤放射治疗中的角色和职责非常强烈地依赖于他或她所在的放射治疗部门内所拥有的设备种类和开展的治疗项目，同时也与所在的部门内物理师人数多少有关，另外一些物理师还要负担一些教学和管理任务，因此很难详尽地进行概括。但他们共同的目标都是协助肿瘤放射学医师，将处方剂量正确而有效地打到病灶靶区，提高和发展临床治疗技术，为患者提供高标准的治疗服务。
THE ROLE OF A MEDICAL PHYSICIST IN RADIATION ONCOLOGY
The first responsibility of the radiation oncology physicist is to the patient--to assure the best possible treatment given the state of technology and the skills of the other members of the radiation oncology department.
A radiation oncology physicist brings a unique perspective to the clinical team in a radiation oncology program: that of a scientist trained in physics, including radiological physics, and also in clinical,basic medical, and radiobiological sciences. The physicist performs an important role working along with the radiation oncologist, the radiotherapy technologist and others, to assure the accurate delivery of all aspects of a treatment prescription.
Responsibilities of Radiation Oncology Physics
In radiation oncology, physicists have the primary responsibility for the following, except where the responsibility is noted as shared: Planning for resource allocation with radiation oncologists, administrators,and technologists, including:
· Equipment usage, selection and replacement,
· Staff requirements, assignments, and recruitment,
· Budget preparation,
· Program operation, and
· Continuing review of the program’s policies and procedures.
Physical aspects of all radiation sources (radioactive materials and radiation producing machines) used in a radiation oncology program,including:
· Performance specification, acceptance testing and commissioning of new equipment,
· Calibration of the sources and maintenance of all information necessary for their appropriate use,
· Development and maintenance of a quality assurance program for all treatment modalities, localization procedures, and computational equipment and programs to assure that patients receive prescribed doses and dose distributions, within acceptable degrees of accuracy,
· Maintenance of all instrumentation required for calibration of sources, measurement of radiation, and calculation of doses, and
· First-order maintenance of treatment units (in conjunction with any inhouse electronic technician).
The radiation safety program (possibly shared with an institution’s radiation safety officer, including:
· Development and administration of the radiation safety program, including compliance with all regulating and certifyingagencies(e.g., the Nuclear Regulatory Commission, the Joint Commission on Accreditation of Health Care Organizations, the Occupational Safety and Health Administration, and appropriate state and local agencies),
· Administration of a personnel radiation monitoring program,
· Supervision of source preparation and handling during brachytherapy,and the continual maintenance of the brachytherapy source inventory,
· Participation on the institutional Radiation Safety Committee,and other committees (e.g., General Safety) as needed, and
· Calculation of shielding required for new or renovated treatment rooms, radioactive-source storage and handling facilities, and brachytherapy patient rooms.The physical aspects of patients’ treatments, including:
· Consultations with radiation oncologists on the physical and radiobiological aspects of patients’ treatments, and the development of treatment plans.
· Acquisition and storage of data for treatment plans,
· Calculation of dose distributions and machine settings for patient treatments,
· Design and fabrication of treatment aids and treatment-beam modifiers.
· Assurance of the accuracy of treatment unit parameters and settings used for a patient’s treatment, includingcorrect transfer of parameters between the simulator, treatment plan and the treatment unit, and periodic review of each patient’s chart.
· In-vivo measurement to verify the dose delivered to a patient.
· Assisting the radiation oncologists in statistical analysis for evaluation of treatment efficacy, and participation in clinical trials,
· Development of techniques (hardware, software, or procedural)to improve the delivery of radiation treatments,
· Participation at patient-discussion conferences, and
· Continuing education of the radiation oncology staff.
Interaction with the medical physics community, including:
· Participation at radiation oncology physics or related medical meetings to receive and disseminate state-of-the-art information,and
· Participation in peer review.
Because of the training received in analytical processes and scientific principles, the physicist plays a principal role in development of systems and policies, the review of consistency between plans and their execution, and problem-solving. After the development and testing of a procedure, a physicist may delegate to other appropriately trained personnel the routine performance, while maintaining responsibility and supervising as required. The work of dosimetrists involved in the computation of dose distributions and machine settings is a common example of such delegation. Because of the scientific complexity and regulatory specifications, the performance of calibrations cannot be delegated.
Physics Staffing
While the list in the previous section outlines most of the major,identifiable activities performed by a radiation oncology physicist, it is by no means exhaustive. The nature and relative importance of the different activities depend on the particular working relationships between the physicists and radiation oncologists in a given program.However, the list does present a typical set of responsibilities. Someof the activities, such as maintaining a calibrated ionization chamber, provide basic support for a program regardless of the number of patients treated each year or the number of treatment machines operated. Other activities, such as routine quality assurance, depend on the number of treatment units to be checked, while still others (e.g., patient consultations) depend on the number of patients treated.Research and teaching (other than inservice and continuing education sessions necessary for safety, compliance, or patient care) explicitly fall outside of the considerations of this report. Radiation Oncology in Integrated Cancer Management,the report of the Inter- society Council for Radiation Oncology (on which the American Association of Physicists in Medicine participates) gives staffing levels considered as the minimum “necessary for patient care.” Physics staffing should never fall below the levels suggested in the current edition of that report.Because of the difficulty in obtaining qualified physicists (see the section below on Qualifications of a Radiation Oncology Physicist),some administrators of radiation oncology programs hire undertrained medical physicists or surrogates for radiation oncology physicists, such as physicists trained in other fields, dosimetrists, or technologists. In some cases, radiation oncologists have endeavored to perform the physics services themselves. To assure proper and safe treatments, all radiation oncology facilities must have at least one qualified physicist responsible for the physics program, with a sufficient time commitment to allow familiarity with the daily operations of treatments and authority to make changes in procedures as necessary. A qualified physicist can supervise, delegate, and coordinate the activities of any ancillary physics staff. The Physics Section of a radiation oncology facility may include some, or all, of the following trained specialists:
Physics assistants-- Physics assistants, who usually begin their training with either a bachelor’s degree in science, or prior training in radiotherapy technology, may perform many routine physics functions under the supervision of a physicist, such as:
· Treatment-unit quality-assurance measurements,
· Radiation-level readings around brachytherapy patients,
· Brachytherapy source inventory, ordering, and shipping,
· Personnel radiation monitors handling.
Dosimetrists-- Dosimetrists are personnel specially trained in performing specified, and usually patient-oriented physics tasks. Examples of tasks commonly performed by dosimetrists include:
· Assembling patient data required for dose calculations,
· Calculating dose distributions,
· Manufacturing compensators, immobilization molds and related devices, and custom-made blocks,
· Computing treatment times or control monitor units,
· Performing in-vivo dosimetry for patient treatments,
· Performing periodic checks on treatment records.
Many of these functions apply to both external beam and brachytherapy treatments. Dosimetrists perform these tasks under the supervision of a physicist, who holds the actual responsibility for their proper execution. Dosimetrists, or other individuals specially trained to perform certain tasks under the purview of a physicist,form an integral part of the radiation oncology team.The difference between a dosimetrist and a physics assistant may be quite arbitrary in some institutions, but usually a dosimetrist only performs duties directly related to treatment planning and dose calculation.
Mold-room technologists and block cutters-- The demand for increased precision in treatment delivery has led to an increased use of individualized immobilization devices, such as body molds, biteblocks,and casts. The fabrication of low-melting-temperature alloy shielding blocks for patients’ treatments improves reproducibility,and has become commonplace. Persons fabricating such blocks or immobilization devices on a continuing basis develop increased skill and efficiency. Since financial support for a full-time block cutter or mold-room technologist may require a substantial patient load(depending on charge structures), a dosimetrist or physics assistant may fulfill this function in departments with smaller workloads.
Electronic technicians-- Electronic technicians make repairs on the program’s equipment. Adequate staffing reduces down-time waiting for outside service personnel. Many physicists have not had formal training in electronic equipment repair, but, through experience, can often complement an electronic technician during repairs.In addition to these specialized support persons, the physics section requires access to machine-shop facilities for the construction or modification of special devices needed quickly or not commercially available. Routine clinical physics functions should be transferred to such specially-trained, support persons. The basic functions performed by a physicist should center around problem-solving. This delegation of routine activities to technical staff saves funds compared to having a physicist perform these functions, and frees the physicist for other necessary activities not appropriate to the training of other individuals.
Qualifications of a Radiation Oncology Physicist
Evaluating the competency of a physicist for a position in a radiation oncology program based on interviews and a curriculum vitae can prove difficult. The statement by the AAPM defining a Qualified Expert can serve as a guide. The excerpt cited below lists only those subspecialty certifications appropriate to radiation oncology.Although individuals may exist who, by virtue of their training and experience, may also be qualified, certification in an appropriate area by one of the organizations listed below is the only way easily to determine adequacy of preparation to function independently as a clinical medical physicist. The AAPM encourages its members to obtain certification in the fields of desired specialization and recommends that expertise be sought among properly certified individuals.It is of critical importance that the agency or employer seeking expertise insure that the type and/or subspecialty of certification match the expertise being sought.
