Radiation Therapy - A Technology Perspective

Introduction

Thirty three percent of all deaths under 65 years in Ireland can be attributable to cancer. Figures indicate that there are 18,000 new cases annually. A National cancer strategy was developed by Government in November 1996, with a medium term target to reduce the death rate in the under 65 year age group by 15% in a 10 year period. As part of ongoing development of cancer services in Ireland, St.Luke's Hospital in Dublin has undergone considerable expansion with major investment in new equipment and involvement in new techniques and their further development. St. Luke's Hospital is the major centre in the country for radiation therapy and deals with approximately 4000 new patients each year.

Radiotherapy patients are either treated with curative intent or for palliation, that is, relief or prevention of specific symptoms. When considering treatment involving radiotherapy, either alone or in conjunction with surgery / chemotherapy, the standard approach is to divide the total radiation dose delivered to the tumour into a number of daily fractions over a number of weeks. This fractionation exploits the ability of normal and cancer tissue to repair after exposure to radiation. Normal tissue is better organised and repairs better than the tumour after a dose of radiation. It is important that a treatment prescription is completed once it has been initiated. This is because regeneration of tumour cells can result in some tumours speeding up their growth rate by as much as a factor of 10 after initiation of a course of radiotherapy.

A key department within St. Luke's Hospital is the Physics Department, responsible for the provision of a clinical support service. This service includes the development of treatment prescriptions for the patients (treatment planning), quality assurance protocol development and implementation, in addition to full, in-house, technical support for all electromedical equipment. Fractionation has major implications for equipment performance and thus effective equipment management procedures are essential for the provision of a good clinical service. Most importantly, there is a requirement for good linac reliability. There is a need to have maximum equipment "up-time". Unscheduled "down-time" creates many problems and ultimately can compromise treatment, patient schedules and patient faith in the process. Technical support is provided by the Clinical Engineering Group within the Physics Department. This group has developed a good working relationship with the equipment manufacturers and/or suppliers.

This service allows for;

• an immediate response to equipment problems and
• a link between the clinicians and manufacturers to be better developed, often providing feedback and research opportunities leading to improvement in equipment design and performance.

Equipment

The primary systems used are Medical Linear Accelerators which deliver a radiation dose of a specific energy, electrically generated, to a depth in the tissue which corresponds to the site of the tumour (i.e. the target volume). Another method of treatment is with the use of a radioactive source material, usually Cobalt-60. While Cobalt-60 has its applications, linacs have become the treatment unit of choice for many types of treatment because of its superior radiation beam characteristics (penetration, precision, versatility and dose rate). In the industrialised world over 75% of radiation treatment units are Linacs. St. Luke's Hospital has 6 Linacs and 1 Cobalt-60 machine. These 6 linacs are divided between two manufacturers, requiring a greater range of expertise within the Clinical Engineering Group.

In medical linacs the charged particle is an electron, which is accelerated down a waveguide by an rf electric field which oscillates at typically 3000 MHz. The electrons are provided by thermionic emission from a hot cathode, shaped into a narrow pencil beam and injected by a pulsed dc electric field into the waveguide. The rf field in the waveguide (accelerating structure) then forms the electron stream into bunches and accelerates them up to > 99% of the velocity of light. The electrons then strike a tungsten target to produce emission of "hard" (penetrating) x-ray photons in a forward lobe. Microwave power is provided by a magnetron or klystron, which is pulsed by a high voltage modulator. Auxiliary systems provide a high vacuum inside the accelerator structure and cooling / temperature control of its internal conducting surfaces. A system of monitors and automatic feedback systems and interlock circuitry maintains stable safe operation at values selected at the control console.

The control system on modern linacs comprises a wide range of associated electronics, with a PC workstation at the front end as a user interface. This replacement of the front end of the linac with a PC allows for the possibility of networking the treatment machines to each other as well as to treatment planning workstations where the patient's treatment prescription is produced. This has allowed for greater patient safety with the establishment of a record and verification regime. The treatment will not commence until the machine set-up matches that detailed in the patient's prescription. Once the treatment has been completed a record of the treatment is stored on the network.

Design Criteria:

A fundamental set of clinical requirements must be satisfied by any type of radiotherapy accelerator. These requirements are:

A precise delivered dose at depth
The goal is to kill the cells of a primary tumour without excessive damage to surrounding normal tissue. The margin for error therefore should be small and the dose to the tumour should be known to a high degree of accuracy. Thus there is a requirement for high precision of linac performance, with ease of patient set-up for treatment and thorough quality assurance procedures.
Precise position, orientation and size of treatment fields
Practical limitations exist to the precision with which patients can be routinely positioned from day to day. Thus treatment fields need to be larger than the assumed tumour volume to ensure the tumour is always included in the target volume. However it is necessary to avoid injuring neighbouring critical organs and thus there is a need for sharp, precisely located field edges.
A wide variety of radiation modalities
Modern linacs are designed to provide delivery of a radiation dose at different user selectable energies and modes (e.g. electron treatment).

Linacs are designed to provide delivery of a radiation dose accurately defined in terms of its energy, uniformity, distribution and duration. Cancer patients usually need to lie on their backs for radiation treatment. Also the patient's anatomy shifts markedly when lying on their back to lying on their front. Thus in order to irradiate the target volume from different directions successfully without turning the patient over, 360o rotation of the gantry is necessary. The gantry is rotated to spread the dose out over normal tissue, thus minimising the resulting dose to the tissue whilst maximising the dose to the tumour. This rotation occurs about an isocentric point in space to a very high degree of accuracy. The isocentre needs to be at a reasonable working height for patient set-up with adequate space between it and the machine head. Thus there is limited space for the various machine components and the large amount of radiation shielding material required in the head which protects areas of the patient outside the intended treatment beam. This radiation shielding is normally lead and/or tungsten, the weight of which poses difficulties with the mechanical isocentric accuracy of rotation which is essential. This is further compounded with the introduction of Stereotactic Radiosurgery. This involves the ablation of small tissue structures, usually in the head, by a large radiation dose delivered by the linac with high spatial precision, typically with combined errors within 1 mm.

New Direction

One new direction of equipment development in radiation therapy is the development of 3-D Conformal Radiation Therapy. New developments in medical imaging in areas such as image acquisition techniques and computational image processing techniques, allows for high speed provision of 3-D displays of the patient's anatomical structure. This allows for the linac radiation beam dose distributions to be computed (Treatment Planning) and superimposed on the anatomical display for radiation beams from many different directions. Thus optimal treatment plans can be developed for particular patients. There is a need to shape the radiation field, either by use of manual placement of static blocks or computer controlled shaping devices. In conventional radiation therapy at least 2 or 3 treatment fields are used at different gantry angles. The machine operator needs to enter the room each time to change the field shaping devices for each treatment field. With 3-D Conformal Radiation Therapy more treatment fields can be employed, the unit can be programmed to change the shape of the treatment field to match the tumour more precisely. In addition, use of computerised control allows for precision movements of the gantry and treatment couch automatically. Thus there is a movement toward developing new computer algorithms to control the radiation beam intensity, the field shape and the treatment angle for a patient's fractionated treatment. This is referred to as Intensity Modulated Radiation Therapy (IMRT). This requires significant increases in precision (dose rate and mechanical) and reliability if the excellent potential for this technology is to be utilised to patient benefit.

St. Luke's Hospital is at the forefront of this technology, with the recent installation of new linacs, networking of the treatment machines and the installation of a Record and Verify System. IMRT is currently being implemented and developed in the hospital on two of the linacs. In addition the provision of a Stereotactic Radiosurgery Service has been established in conjunction with Beaumont Hospital, the first of its kind in the country.

John McGivney and Pat Cooney
St. Luke's Hospital, Dublin