The demands of modern radiotherapy planning are quite different from those 20 years ago. Clinicians now require to define the target volume more precisely, not just in two dimensions, but also in three dimensions. It has therefore become necessary to visualize anatomy in three dimensions to enable planning to conform the dose around the target volume in order to irradiate the tumour to as high a dose as possible, and saving the normal tissues. In order to achieve this, the following tools are necessary:
1. Identification of critical structures using advanced anatomical and functional imaging methods(CT Simulation).
2. Visualization of treatment targets with respect to other structures in three dimensions.
3. Efficient and accurate outlining of tumour using contouring tools.
4. Addition of symmetrical or asymmetrical volumetric margins.
5. Beam’s eye view (BEV) of targets and structures.
6. Shaping fields around the target.
7. Adding beams together.
8. Dose volume histogram (DVH) generation.
9. Tools for optimizing plans using forward or inverse interactive techniques.
10. Export of plan to linear accelerator.
11. Monitor unit calculations.
12. Export of digitally reconstructed radiographs, (DDRs — see below) to an image database for on-line assessment of treatment accuracy.
An exciting development in the area of simulation is that of converting a CT scanner into a simulator (figure (a)). CT simulation uses a CT scanner to localize the treatment fields on the basis of the patient's CT scans. A computer program, specifically written for simulation, automatically positions the patient couch and the laser cross-hairs to define the scans and the treatment fields. The software (as part of CT scanner or a stand-alone treatment-planning system) provides outlining of external contours, target volumes and critical structures, interactive portal displays and placement, review of multiple treatment plans, and a display of isodose distribution. This process is known as virtual simulation.
Figure (a) shows picture of the CT Simulator
The nomenclature of virtual simulation arises out of the fact that both the patient and the treatment machine are virtual—the patient is represented by CT images and the treatment machine is modeled by its beam geometry and expected dose distribution. The simulation film in this case is a reconstructed image called the DRR (digitally reconstructed radiograph), which has the appearance of a standard 2-D simulation radiograph but is actually generated from CT scan data by mapping average CT values computed along ray lines drawn from a “virtual source” of radiation to the location of a “virtual film.” DRR is essentially a calculated (i.e., computer-generated) port film that serves as a simulation film. The quality of the anatomic image is not as good as the simulation radiograph but it contains additional useful information such as the outlined target area, critical structures, and beam aperture defined by blocks or MLC. A DRR can substitute for a simulator radiograph by itself but it is always preferable to obtain final verification by comparing it with a radiographic simulation film.
1. CT simulation leads to
a) Accurate delineation of targets
b) Three dimensional planning
c) Increase in conformality of dose to the target
1. d) All
1. CT simulation for radiotherapy treatment planning
1E G A AIRD, MSc, PhD, FIPEM and 2J CONWAY, BSc, PhD, MIPEM
1Medical Physics Department, Mount Vernon Hospital, Rickmansworth Road, Northwood, Middlesex HA6 2RN and 2Department of Radiotherapy Physics, Weston Park Hospital, Whitham Road, Sheffield S10 2SJ, UK
2. The Physics of Radiation Therapy by F. M. Khan