Verification is the process by which the accuracy of radiotherapy is assessed. It is achieved by comparing the images of the treatment with that planned. (RCR, 2008.)
In order to identify structures correctly, it’s vital that the images used, illustrate a clear contrast so that bony structures and soft tissues can be differentiated. Due to the photoelectric effect, kilovoltage (Kv) x rays produce images with better contrast than Mv images. Within this process, an incident photon interacts with an atomic electron from the inner shell of the atom. The photon transfers its energy to the electron causing the electron to be emitted from the atom. Absorption of the photon is dependent upon Z3 .Soft tissue consists of atoms with lower atomic numbers which absorb fewer x rays, hence they look darker on radiographs. Contrastingly in bones, more x rays are absorbed, as atoms have higher atomic numbers. Consequently, lighter images are produced. In MV beams, the Compton Effect is an overriding concept. It involves an incident photon not being fully absorbed and being partially scattered, leading it to have less contrast than Kv images. This explains why bones and soft tissue are not easily differentiated in Mv images. (Symonds 2012.)
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Our critical match points are located within the pelvis and are used as internal landmarks to accurately verify the patient’s positon and isocentre. At a gantry angle of 0.0, an anterior view of the pelvis has the following landmarks: obturator foramen, ischium, acetabulum, pubic symphysis and pelvic brim. These match points were used over other bony anatomy present like the femoral heads. These structures cannot be used as constant match points as they are unstable. At a right lateral view of the pelvis with a gantry position of 270, we identified the obturator foramen, acetabulum and pubic symphysis as strong match points. Without verifying this, the patient position may contradict the position planned for treatment. Hence, organs nearby like the urinary bladder, may be over irradiated resulting in symptoms like haematuria. To avoid this, we must use digitally construct radiographs (DRR) to compare the critical match points to the planned image before treatment (LO, S. S. (2012).
Orthogonal imaging consists of two images being taken at 90 degree angles to each other. It produces a 3D image that can produce a frontal, sagittal and transverse plane (Baker, C.2012). Due to this, it allows accurate treatment planning, which enables higher doses to be given at smaller volumes to radical patients safely. Therefore, the probability of risk to organs at risk, is reduced. However, ANT/POST images, are mainly used for palliative patients because they don’t require high levels of accuracy due to the aim of their treatment being to relieve symptoms rather than cure the patient.
Offline imaging analyses the accuracy of the setup, after the treatment has been delivered by comparing it to reference images. Hence this is suitable for palliative patients, as it is a faster treatment that doesn’t require the patients to be imaged before every treatment. This is suitable for patients experiencing trouble with immobilisation on the couch. Online imaging caters better towards radical patients, as they can receive higher doses at better degrees of accuracy. This method is effective in reducing the risk to organs in close vicinity.
Online image matching compares the images taken in the treatment room with the reference images. Before the treatment is given, corrections are applied. In order to reduce the variation as a result of patient mobility, the time duration between treatment delivery and online verification should be reduced. If the duration is too long the information may not embody the patient’s true position during treatment (RCR, 2008.)
Percentage depth dose (PDD) is the ratio of dose at depth to surface dose expressed as percentage for a constant Skin to surface distance. Percentage depth dose changes when field size, depth, beam energy and SSD change. Percentage depth dose increases with SSD due to the effects of the inverse square law. The maximum dose (dmax) is where the dose is 100%. As the depth increases, the radiation penetrating through the tissues decreases. The dmax value is dependent on the photon energy (SEMMLER, W. (2008).
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Increasing field sizes has notable effects on the percentage depth dose graphs (figure 1) even if the beam remains consistent, the dmax build up and fall of region are all impacted even if the beam energy is the same. When increasing the field size from 10 x 10 cm to 20 x 20 cm for a 6mv beam, it results in the dose being deposited on the surface to increase from 48% to 58% at a depth of 0 mm. This is due to the fact that at larger field sizes there’s less shielding of low energy electrons from the collimator, which hence increase the chances of geometric penumbra being produced (Massey, J. 1974.) As the low energy scattered radiation accumulates, the surface receives a larger dose. Consequently, at larger doses the graph levels off at a higher percentage depth dose, due to a large amount of secondary radiation being produced at a deeper depth by the electrons and photons with lower energy. This is evident on the graph, when the 20 x 20 levels off at 18% whereas the 10 x 10 levels off at 17%.
Dose distribution is affected by the quality of the beam. This explains why there is a difference in the PDD and the maximum dose, even when the field size remains the same. For example, a 15Mv beam with a field size of 20 x 20, has a surface PDD of 58% with a DMax of 17 mm, whereas a 10 x 10 has a surface PDD of 48% and a Dmax of 18mm. This is because the beam is more penetrating which causes the scattered electrons to have a higher energy. This results in the build-up region occurring at deeper depths due to the increase in penetrating power of the beam. This in turn increases the value of the PDD subsequently decreasing the Dmax value at 0 mm.
To conclude it is important to acknowledge that variation in field size has an impact on dose distribution, scatter and dmax value. Radiographers must be cautious of this change as it has an impact on the dose the organs receive.
- Royal College of Radiologists, Society and College of Radiographers, Institute of Physics and Engineering in Medicine, 2008. ‘Principles of Geometric Verification’ In: Royal College of Radiologists, Society and College of Radiographers, Institute of Physics and Engineering in Medicine. On Target: Ensuring Geometric Accuracy in Radiotherapy, London, RCR, pg.10, pg.19.
- Baker, C.2012. ‘Radiation Interaction With Matter’ In: Symonds, P., Deehan, C., Mills, J. and Meredith, C. Walter and Miller’s textbook of radiotherapy. Edinburgh: Elsevier Churchhill Livingstone, pg.27.
- Khan F.M., 2012. ‘The Physics of radiation therapy, 4th edition.’ Minneapolis, Minnesota: Emeritus, pg 143.
- Meredith, W. and Massey, J. 1974. ‘Teletherapy Dosage Data for Clinical Use’. In. Fundamental physics of radiology. Bristol: Wright, pg. 389-394.
- LO, S. S. (2012). Stereotactic body radiation therapy. Berlin, Springer, Pg 91.
- REISER, M. F., HRICAK, H., & SEMMLER, W. (2008). Magnetic Resonance Tomography. http://dx.doi.org/10.1007/978-3-540-29355-2. Pg, 176.
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