Chapters from Clinical Oncology Lecture notes University of Szeged Faculty of Medicine Department of Oncotherapy 2012. 1
RADIOTHERAPY Technical aspects Dr. Elemér Szil Introduction There are three possibilities for the treatment of cancer patients, namely surgery, radiotherapy and chemotherapy. These may be applied by themselves or in combination (any two of them or possibly all three). Treatments are performed according to well-defined protocols. In this chapter we will analyse (primarily) the technical aspects of radiotherapy. It is generally true that radiotherapy can be applied in case of localised tumors. The ultimate goal of radiotherapy is to kill all tumor cells by radiation. What usually causes difficulties is that there are healthy tissues and organs at risk in the immediate vicinity of the tumor, and it is important to protect them in order to prevent the different early and late side effects, or at least decrease the probability of their development. Thus today s modern radiotherapy has a double goal: - to increase dose to the target volume, - to decrease dose to the surrounding healthy tissues. The first goal means a better tumor control, the second results in the decrease of the probability of side effects. The two together increase the chance of cure. The probability functions TCP (Tumor Control Probability) and NTCP (Normal Tissue Complication Probability) allow for a more exact analysis. The course of radiotherapy The course of radiotherapy consists of the following important steps: Immobilization Imaging Tumor Localization Treatment Planning Patient Positioning Treatment. We specify these steps as follows. I. Patient Immobilization High TU (tumor) dose and low OAR (Organ At Risk, risk organ) dose means in case of an adjacent tumor and risk organ that there is a steep gradient within a small distance. Thus, patient setup and immobilisation is very important, because a small error (movement, displacement) may cause underdosage in the tumor or overdosage in the healthy tissues. General Considerations 2
To understand the importance of immobilisation, we must be aware of the definition of GTV (Gross Tumor Volume), CTV (Clinical Target Volume) and PTV (Planning Target Volume). It is the CTV that must be treated, but the radiotherapy plan is made for the PTV and is realised by the treatment device field by field. The PTV is static, while CTV moves together with the patient. In case of a correct treatment, the PTV contains the CTV during the whole treatment. This is promoted by immobilisation. Immobilisation techniques We apply invasive fixation in radiosurgery, when the total dose is delivered in a single fraction. (Stereotactic head frame.) In case of fractionated radiotherapies fixation is non-invasive. (masks, vacuum pillows, thermoplastic plates, adhesive tapes,...) II. Imaging Imaging for therapy planning serves the following purposes: 1. We create a 3D patient model with the help of an image series made by an imaging examination (usually CT). In order to be able to choose the optimal beam directions we must know the relation of the tumor and the risk organs and their localisation in relation to the body surface. 2. CT slices contain electron density, the knowledge of this is needed for radiation planning systems to be able to model the interaction of the beam and the material and to be able to count energy absorption (absorbed dose). Evaluation and analysis of treatment plans is based on dose distribution (eg. DVH dose volume histogram for tumor and risk organs). 3. The 3D anatomical model is also a basis for the patient s positioning before treatment. The basis for the 3D anatomical model is usually CT but other imaging examinations (MRI, PET, SPECT) also provide important supplementary information, primarily for a more accurate definition of the tumor. III. Tumor localisation On the images provided by the imaging examination before planning different structures must be marked. The following questions come up: 1. Which structures are important? 2. How can they be delineated? 3. How can different modalities be combined? 3
1. For planning, the knowledge of two structures is important: - target volume (TU tumor) - Organs At Risk (OARs) These are discussed in ICRU Report 50 (1993) and ICRU Report 62 (1999) in details. We must also be aware of the definitions of GTV, CTV and PTV. 2. Delineation of structures is otherwise called segmentation. Segmentation is the process that distinguishes the relevant structure (volume) from its environment. Segmentation for radiation treatment planning means the delineation of the PTV, the organs at risk and the surface contour of the body; after the completion of that the 3D anatomic model can be created. Segmentation may be manual, semiautomatic or automatic. 3. If we want to correctly use the information of the different imaging modalities (eg. CT, MRI, PET, ) simultaneously, a definite relation is necessary between the picture elements (pixels). The methods that are able to calculate these relations are called registration. This is in fact a mathematical transformation during which we create the correlation between the two sequences. Registration may be manual, semiautomatic or automatic. The simultaneous display of the registered sequences is called image fusion. IV. 3D treatment planning The goal of planning is to create an optimal treatment plan. Planning is based on the 3D model of patient anatomy. During the preparation of the planning cycle, CT (and/or MR, PET ) image series are acquired and the tumor volume and the organs at risk are defined within them. The actual planning starts with the definition of treatment parameters, which is followed by virtual therapy simulation and dose calculation. After the evaluation of dose distribution if the results are satisfactory the treatment can begin. If the plan does not fulfil the prescriptions completely, we step back to the level of the definition of treatment parameters and we change or delete fields or define new ones etc. until an adequate dose distribution is reached. During the treatment cycle the dose should be concentrated to the target volume which can be provided by applying more fields (beams). The doses provided by the individual fields sum up within the tumor, while the dose of the healthy tissues can be kept below the tolerance level. During planning, with the help of the 3D model: 1. We define the optimal beam directions. The main criterion is that the beam (field) should include the target volume completely without including any organs 4
at risk. If the latter cannot be accomplished which is usually the case the volume of the organ at risk covered by the field should be minimised. Graphic tools: Beam s Eye View (BEV): The planner views the 3D model from the position of the radiation source. Observer s View: we view the beams from an arbitrary direction. This helps us minimise the subvolume where the single beams overlap outside of the tumor. 2. Beam shapes are formed to correspond to the tumor shape exactly (BEV). We approximate the tumor shape with a fraction line that is formed by a multileaf collimator (MLC) in practice. 3. Further treatment parameters: definition of the type and energy of radiation, beam modifying devices etc. 4. A dose calculation algorithm calculates the expected dose distribution. 5. Evaluation of the treatment plan(s): Evaluation of dose distribution: - 3D dose distribution, isodose surfaces, - dose distribution from slice to slice, dose of critical points, - Dose-Volume Histograms. V. Patient positioning Positioning means giving the exact treatment position of the patient before the first treatment. It usually consists of three well separable steps. 1. Definition of a patient-fixed coordinate system. 2. Definition of a target point in the above coordinate system. The target point is the point fixed during planning where the axes of the beams intersect. 3. Positioning at the treatment device or the simulator. During that the target point is moved into the isocenter of the treatment device. The isocenter of the treatment device is the point where the axes of the beams administered by the device coming from arbitrary directions intersect. VI. Treatment 1. Linear Accelerators (Linacs) Linear accelerators are the treatment devices most commonly used in radiotherapy. 5
The basis of their operation is the following: electrons are accelerated in the field of an electromagnetic wave travelling in a waveguide. Instead of high-voltage a series of smaller voltages are applied (These fields are produced by microwaves.) Types of accelerators: Travelling-wave accelerator. Standing-wave accelerator. Shortened standing-wave accelerator. 2. Treatment procedures 1. Conventional (classic) conformal RT. Conformity: the 3D dose distribution should follow the tumor shape while sparing the organs at risk. 2. Intensity Modulated Radiation Therapy (IMRT). VII. Clinical dosimetry 1. Basic principles Absorbed dose is the energy absorbed in the dm mass element divided by the dm. Unit: J/kg (Gy Gray) Radiation types: - photons _electrons Energy transfer by photons and electrons: Photons: - photoelectric effect, - Compton effect, - Pair production. These interaction processes release secondary electrons that again interact with the matter. Electrons - collisions with the atoms and electrons. - radiative processes (Bremsstrahlung production). Collisions with the electrons in the atomic shell lead to excitation and ionization of the atom. 2. Measurement of dose 3. Phantoms 6