Treatment planning in teletherapy

Author: Dr. Csilla Pesznyák

 
The external beam radiotherapy directs the radiation at the tumour from outside the body. The most wide-spread external technique is conformal radiotherapy, but we know several special techniques such as intensity modulated radiation therapy (IMRT), image-guided radiation therapy (IGRT), breath-holding treatment, volumetric arc therapy, heavy ion therapy, stereotaxic and so on.
Three-dimensional (3D) conformal radiation therapy is a technique where the beams of radiation used in treatment are shaped to match the tumour. Conformal radiation therapy uses the target information to focus precisely on the tumour, while minimizing the damage of the healthy surrounding tissue. This exact targeting makes it possible to use higher doses of radiation in treatment, which is more effective in shrinking and killing tumours.
Volume definition is a prerequisite for meaningful 3-D treatment planning and for accurate dose reporting. ICRU Reports No. 50 and 62 define and describe several targets and critical structure volumes that aid in the treatment planning process.
Gross tumour volume (GTV): the gross palpable or visible/demonstrable extent and location of malignant growth. The tumour can be discovered by palpation and application of different medical imaging methods.
Clinical target volume (CTV): The clinical target volume (CTV) is the tissue volume that contains a demonstrable GTV and/or sub-clinical microscopic malignant disease, which has to be eliminated. This volume thus has to be treated adequately in order to achieve the aim of therapy, cure or palliation. The CTV is an anatomical–clinical volume and is usually determined by the radiation oncologist. The CTV is usually stated as a fixed or variable margin around the GTV (e.g. CTV = GTV + 1 cm margin). Margins will be added to the CTV to create the Planning Target Volume (PTV).
Planning target volume (PTV): The planning target volume (PTV) is a geometrical concept, and it is defined to select appropriate beam arrangements, taking into consideration the net effect of all possible geometrical variations, in order to ensure that the prescribed dose is actually absorbed in the CTV. The PTV includes CTV and an additional margin for set-up uncertainties (organ motion, systematic errors caused by inaccuracy of patient positioning and machine tolerances). The PTV is linked to the reference frame of the treatment machine and is often described as the CTV plus a fixed or variable margin (e.g.PTV = CTV + 1 cm). PTV does NOT include margin for dosimetric characteristics of the beam (penumbra, build-up region)
Treated volume (TV): - enclosed by an isodose surface selected by the radiation oncologists as appropriate to achieve purpose of treatment, which is usually 95%. of the prescribed dose. The Planning Target Volume (PTV) should always be fully enclosed by the Treated Volume.
Irradiated volume (IRV) - receives a dose that is significant in relation to normal tissue tolerance. The significance level will depend on the normal tissue type. It is usually the volume surrounded by the 50% isodose surface.
The volumes of TV and IRV are decreasing by using the conformal techniques, reducing the radiation exposure of normal tissues.
Region of interest (ROI): The region of interest is a user defined region, which is commonly abbreviated as ROI. In radiation therapy: normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose.
ICRU 62 gives some new definitions:
Internal margin (IM) - A margin that needs to be added to the Clinical Target Volume (CTV) to form the Planning Target Volume (PTV) to account for any positional errors from the planning information. ICRU Report 62 divided this margin into the Set up Margin (SM) and Internal Margin (IM), in order to separate the contributory sources of positional error into physiological error and set up error, respectively. The IM compensates for physiological variation of the size and shape of the volume (filling of rectum, movements due to respiration).
Setup margin (SM) - The IM incorporates both intra-fraction errors such as that due to respiration, and inter-fraction errors such as that due to weight gain/loss or digestive system changes. SM accounts for all uncertainties in patient-beam positioning and technical factors (patient immobilization, machine stability). The total required margin is SM + IM.
Internal target volume (ITV): accounts for motion of CTV in the patient, does not account for setup uncertainties, ITV = CTV + IM.
The PTV in the case of conformal therapy and IMRT have to be determined with formula:
PTV = ITV+set-up errors (on the base of on-board imaging).
Planning organ at risk volume (PRV): PRV = OAR + margin (accounts for OAR movements). PTV and PRV may overlap.
Conformity index (CI): CI = Treated volume/PTV

Conformal therapy and its types:

Definition: the creation of a radiotherapy dose distribution that closely conforms to the shape of the target volume (= gross tumour volume plus margins for microscopic tumour extension and treatment set-up variations) in three dimensions using customized shielding, while minimizing dose to normal tissues.

1. Static conformal therapy: conformal therapy is delivered with fixed fields (usually defined by individually fashioned shielding blocks or by multileaf collimators), from multiple individual directions (typically four to eight). After each static field is delivered, the therapist goes in and out of the treatment room to manually change the treatment machine setup (angle, field size, and to insert the new blocks and other beam modifiers).
2. Segmental conformal therapy: Individual fixed field portals (or ‘segments’) are treated, as per static conformal therapy, but the multiple segments are treated sequentially and automatically under computer control. The RTTs do not enter the treatment room at any stage during treatment delivery.
3. Dynamic conformal therapy: Rather than delivering treatment by individual static fields, during dynamic conformal therapy one or all of the machine gantry, collimator, MLC leaves or treatment couch are in motion. Movement of the MLC leaves results not only in dynamic variation of the beam shape, but in variation in the intensity of the radiotherapy dose delivered. The introduction of the MLC thus makes individually constructed shielding blocks, wedges and missing-tissue compensators obsolete.

The oncoradiology treatments start with diagnostic investigation, and include patient follow-up after treatment. The radiotherapy team consists of radiation oncologists, medical physicists, dosimetrists and radiation therapy technologists.
The physician’s responsibilities are:
1. inspect, diagnose, and treat cancerous tumors
2. the patient’s awareness
3. contouring the target volume and organs of risk
4. dose prescription
5. on-treatment supervision and evaluations (treatment summary reports, follow-up monitoring and evaluation of treatment outcome and morbidity
6. Development and maintenance of a quality assurance program
7. Continuing education of the radiation oncology staff

The roles of physicists are:
1. performance specification, acceptance testing and commissioning of new equipment,
2. calibration of the sources and maintenance of all information necessary for their appropriate use
3. development and maintenance of a quality assurance program
4. maintenance of all instrumentation required for calibration of sources, measurement of radiation, and calculation of doses
5. acquisition and storage of data for treatment plans,
6. calculation of dose distributions and machine settings for patient treatments
7. in-vivo measurement to verify the dose delivered to a patient
8. continuing education of the radiation oncology staff
9. participation in the institutional Radiation Safety Committee, and other committees

Workload in teletherapy can be seen in figure 1. The whole process begins with patient positioning and body fixation and the creation of individualized 3D digital data sets of patient tumours and normal adjacent anatomy in CT. These data sets are then used to generate 3D computer images. Radiation oncologists make a contouring for tumour and organs of risks. Sometimes it is necessary to fusion the images, combining of MRI and PET-images into CT slices. The next steps are dose planning with a treatment planning system, acceptance of treatment plan and transporting of information into the treatment and simulating equipment through a computer network. After that the patient is placed on the simulator table and the final treatment position of the patient is verified using the fluoroscopic capabilities of the simulator. The images from simulator are compared with digitally reconstructed radiographs (DRRs) from treatment planning system. The clinical aspects of treatment simulation, be it with a conventional or CT simulator, rely on the positioning and immobilization of the patient as well as on the data acquisition and beam geometry determination. Treatment evaluation consists of verifying the treatment portals (through port films or on-line portal imaging methods) and comparing these with simulator radiographs or DRRs and/or performing in vivo dosimetry through the use of diodes, thermoluminescent dosimeters (TLDs) and other detectors.

Three different types of calculation algorithms are used for treatment planning systems:
1. Measurement based algorithm (i.e. Clarkson).
2. Model based algorithms which use a pencil beam convolution model and primarily equivalent path length corrections to account for inhomogeneities. Changes in lateral electron and photon transport are not modelled (no lateral transport).
3. Model based algorithms which primarily use a point kernel convolution/superposition model and account for density variations in 3D. Changes in lateral electron and photon transport are approximately modelled (with lateral transport).

 
All of the systems are commercially available and it is assumed that all of the TPSs and algorithms have been previously evaluated and commissioned for clinical use.

Simultaneously and independently, several investigators turned to the Fermi–Eyges theory of thick target multiple Coulomb scattering (Eyges 1948) for the solution. Perry and Holt (1980) first demonstrated how the mean free path of an electron pencil beam can be approximated by the central pencil-beam axis, accurately predicting dose distributions beneath an irregular surface. The pencil-beam algorithm (PBA) by Hogstrom et al (1981), who first used the Fermi–Eyges pencil-beam theory to calculate dose in patients, was unique in that it showed how to
• input measured square-field central-axis depth dose in water data to accurately calculate dose for any field size,
• utilize CT data on a pixel-by-pixel basis,
• accurately model variable air gap,
• redefine pencil beams at the surface, accurately calculating the effect of irregular surface on dose homogeneity, regardless of the air gap
Due to computer limitations in memory and speed of calculation, early implementations of the PBA were multi-planar; however, as computing technology advanced, Starkschall et al (1991) showed how to implement the Hogstrom PBA in 3D. That implementation presently serves as the basis for electron dose calculations for many commercial 3D systems, e.g., Pinnacle and CMS FOCUS later XIO.
Lax et al (1983) addressed the former, showing how a 3-Gaussian kernel for the scatter distribution improved the accuracy of the calculation. This enhanced their PBA (Brahme 1981), which was also implemented into some commercial treatment-planning systems, e.g. Varian’s CADPLAN.
Keall and Hoban (1996) developed the super Monte Carlo dose algorithm that similarly regenerated electron track kernels. Numerous investigators have reported on differing levels of success in modelling electron beams using Monte Carlo to predict electron beam dose distributions, and the difficulty is best summarized by Antolak et al (2002) who challenged the medical physics community to demonstrate 2% or 0.1 cm agreement between measured and calculated doses over the entire range of clinical treatment parameters.

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