Chapter 12 Kahn: The Physics of Radiotherapy
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What is the best way to acquire body contours and internal structures?
Imaging: Computed Tomography (CT), MRI, etc.
When performing imaging scans, how should the patient be positioned?
The patient should be positioned similarly to his/her treatment position.
When manually contouring the patient (solder wire, lead wire, array of rods, pantograph-type apparatus), there five important points that needs to be considered. What are they?
- 1.) The patient contour must be obtained with the patient in the same position as used in the actual treatment. For this reason, probably the best place for obtaining the contour information is with the patient properly positioned on the treatment simulator couch.
- 2.) A line representing the tabletop must be indicated in the contour so that this horizontal line can be used as a reference for beam angles.
- 3.) Important body landmarks as well as beam entry points, if available, must be indicated on the contour.
- 4.) Checks of body contour are recommended during the treatment course if the contour is expected to change due to a reduction of tumor volume or a change in patient weight.
- 5.) If body thickness varies significantly within the treatment field, contours should be determined in more than one place.
What one form of imaging cannot be used for contouring and localizing internal organs relative to external contour?
Diagnostic radiographs or atlases of cross-sectional anatomy.
What are three imaging modalities that can be used for the contour and localization of internal structures?
- 1.) Computed Tomography (CT).
- 2.) Magnetic Resonance Imaging (MRI).
- 3.) Ultrasound (US).
Knowing that you can extract electron density from CT numbers and that there is linearity between the relationship of the two in a certain range, what range displays non-linearity?
The range is between soft-tissue and bone.
What causes the non-linear relationship between the Hounsfield Numbers and the electron densities of the objects of interest?
The non-linearity is caused by the change in atomic number of tissues, which affects the proportion of beam attenuation by Compton versus photoelectric interactions.
What are two significant advantages of CT image information when planning a treatment?
- 1.) Delineation of target volume and the surrounding structures in relation to the external contour.
- 2.) Providing quantitative data (in the form of CT numbers) for tissue heterogeneity corrections.
Sontag et al made an important quote regarding the causes of severe errors when dealing with CT imaging in treatment planning. Can you paraphase the quote?
"The most severe errors in computing the dose distribution are caused by inaccurate delineation of the geometric outlines of tissue inhomogeneities. Less severe errors in the dose calculation are caused by using an inaccurate relative electron density for the inhomogeneity, provided the outline is correct."
Does pixel-by-pixel dose calculations which includes the CT numbers quantitatively explaining the tissue inhomogeneity significantly improve the dose accuracy calculation?
No. But it may be very important in regions where there are large dose gradients.
What are seven common, important considerations when obtaining treatment planning CT images?
- 1.) A flat tabletop should be used; usually a flat wooden board can be designed to provide a removable insert for the diagnostic CT couch.
- 2.) A large-diameter CT aperture (e.g., >= 70cm) can be used to accommodate unusual arm positions and other body configurations encountered in radiation therapy.
- 3.) Care should be taken to use patient-positioning or immobilization devices that do not cause image artifacts.
- 4.) Patient positions, leveling, and immobilization should be done in accordance with the expected treatment technique or simulation if done before CT.
- 5.) External contour landmarks can be delineated using radiopaque markers such as plastic catheters.
- 6.) Sufficiently magnified images for digitization can be obtained if radiographs on film are to be used for drawing target and other structures.
- 7.) Image scale should be accurrate in both the X and Y directions.
Magnetic Resonance Imaging is another imaging modality that can used to acquire treatment plan information. What are its advantages/disadvantages over CT imaging?
- 1.) MRI can directly acquire sagittal, coronal, transverse, as well as oblique planes (images).
- 2.) No ionizing radiation is used, hence patient exposure is 0.
- 3.) Higher contrast.
- 4.) Better image resolution of soft-tissue.
1.) Lower spatial resolution.
- 2.) Inability to image bone or calcifications.
- 3.) Longer scan acquisition time thereby increasing the possibility of motion artifacts.
- 4.) Technical difficulties due to small hole of magnet and magnetic interference with metallic objects.
- 5.) Current unavailability of many approved MRI contrast agents.
Ultrasound imaging is another modality that is widely becoming recognized as an important tool in radiation therapy. What are its advantages/disadvantages?
- 1.) No ionizing radiation.
- 2.) Less expensive (cheaper).
1.) Image quality or clinical reliability is not as good as that of the CT.
What type of x-ray tube does a radiographic treatment simulator use? Why? (Pg. 210)
A treatment simulator is an aparatus that uses a diagnostic x-ray tube but duplicates a radiation treatment unit in terms of its geometric, mechanical ,and optical properties.
What is the main function of a radiographic simulator? (Pg. 210)
The main function of a simulator is to display the treatment fields so that the target volume may be accurately encompasses without delivering excessive irradiation to surrounding normal tissues.
What is the name of the capability that most commercially available radiographic simulators have that allows one to dynamically visualize before a hard copy is obtained in terms of the simulator radiography? (Pg. 210)
Most commercially available simulators have "fluoroscopic capability".
What are four important reasons why there is a significant need for radiographic simulators? (Pg. 210).
- 1.) Geometric relationship between the radiation beam and the external and internal anatomy of the patient cannot be duplicated by an ordinary diagnostic x-ray unit.
- 2.) Although field localization can be achieved directly with a therapy machine by taking a port film, the radiographic quality is poor because of very high beam energy, and for Co-60, a large source size as well.
- 3.) Field localization is a time-consuming process that, if carried out in the treatment room, could engage a therapy machine for a prohibitive length of time.
- 4.) Unforeseen problems with a patient setup or treatment technique can be solved during simulation, thus conserving time within the treatment room.
Has the simulator room assumed the role of a treatment planning room at most institutions? (Pg. 211)
Yes, but despite that, the practical use of simulators varies widely from institution to institution.
What does a CT simulation system use to localize treatment fields? (Pg. 211).
CT simulation systems use CT scanners to localize the treatment fields on the basis of the patient's CT scans.
What does a computer program, used by CT simulation systems, do when simulating patient treatment? (Pg. 211).
The software provides...
- 1.) outlining of external contours
- 2.) target volumes and critical structures
- 3.) interactive portal displays and placement
- 4.) review of multiple treatment plans
- 5.) a display of isodose distribution
What is the "process" used by CT simulation systems known as? (Pg. 211).
The process is known as "virtual simulation". 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.
What CT simulation systems perform "virtual simulation", what is the simulation film and what is it called? (Pg. 211).
The simulation film in this case is a reconstructed image called the DRR (digitally reconstructed radiograph), which has the appearance of a standard 2D simulation radiograph.
How are the DRRs (Digitally reconstructed radiographs acquired from CT simulation systems) created? (Pg. 212).
DRRs are 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".
How are functional images from PET imaging used? (Pg. 212).
Positron emission tomography (PET) provides functional images that can differentiate between malignant tumors and the surrounding normal tissues.
Why has the idea of simulating radiation treatment plans using PET/CT arise? (Pg. 212).
CT images provided high quality images of the patient's anatomic information. Due to that CT capability, combining both images allows to one to observe the exact anatomical position of the functional (tumor) process within the patient.
What are five advantages acquired from combining PET and CT images when simulating the treatment scenario? (Pg. 213)
- 1.) Superior quality CT images with their geometric accuracy in defining anatomy and tissue density differences are combined with PET images to provide physiologic imaging, thereby differentiating malignant tumors from the normal tissue on the basis of their metabolic differences.
- 2.) PET images may allow differentiation between benign and malignant lesions well enough in some cases to permit tumor staging.
- 3.) PET scanning may be used to follow changes in tumors that occur over time and with therapy.
- 4.) By using the same treatment table for a PET/CT scan, the patient is scanned by both modalities without moving (only the table is moved between scanners). This minimizes positioning errors in the scanned data sets from both units.
- 5.) By fusing PET and CT images, the two modalities become complementary. Although PET provides physiological information about the tumor, it lacks correlative anatomy and is inherently limited in resolution. CT on the other hand, lacks physiological information but provides superior images of anatomy and localization. Therefore, PET/CT provides combined images that are superior to either PET or CT images alone.
What is the primary purpose of "port filming"? (Pg. 214)
The primary purpose of port filming is to verify the treatment volume under actual conditions of treatment. Although the image quality with the megavoltage x-ray beam is poorer than with the diagnostic or the simulator film, a port film is considered mandatory not only as a good clinical practice, but also as a legal record.
What are four reasons why using port films to verify the treatment volume is sometimes not the best option? (Pg. 214).
The overall reason is the port film being of insufficient quality hence the field boundaries cannot be anatomically described.
Four reasons contributing to bad film quality are...
- 1.) high beam energy (10 MV or higher).
- 2.) large source size (s).
- 3.) large patient thickness.
- 4.) poor radiographic technique.
What are three limitations of using port filming for treatment verification and what are the two advantages that "electronic portal images" have over port film? (Pg. 214).
- 1.) viewing is delayed because of the time required for processing.
- 2.) it is impractical to do port films before each treatment.
- 3.) film image is of poor quality especially for photon energies greater than 6 MV.
Electronic portal imaging advantages.
- 1.) overcomes the first two disadvantages above by making it possible to view the portal images instantaneously (i.e., images can be displayed on computer screen before initiating a treatment or in real time during the treatment).
- 2.) Portal images can also be stored on computer discs for later viewing or achiving.
What are three advantages with using the kV-CBCT imaging system? (Pg. 217).
- 1.) produce volumetric CT images with good contrast and submillimeter spatial resolution.
- 2.) acquire images in therapy room coordinates.
- 3.) use 2-D radiographic and fluoroscopic modes to verify portal accuracy, management of patient motion, and making positional and dosimetric adjustments before and during treatment.
Why is the MV-CBCT a great tool for treatment verification? (Pg. 217).
MV-CBCT is a great tool for on-line or pretreatment verification of...
- 1.) patient positioning.
- 2.) anatomic matching of planning CT and pretreatment CT.
- 3.) avoidance of critical structures such as spinal cord.
- 4.) identification of implanted metal markers if used for patient setup.
What are a couple advantages that MV-CBCT has over kV-CBCT despite the fact that the kV-CBCT produces images of better quality?(Pg. 217).
- 1.) Less susceptibility to artifacts due to high-Z objects such as metallic markers in the target, metallic hip implants, and dental fillings.
- 2.)No need for extrapolating attenuation coefficients from kV to megavoltage photon energies for dosimetric corrections.
When dealing with dosimetric calculations and the surface being irregularly shaped or curved, what are three mathematical correction methods that are recommended for angles of incidence of up to 45 degrees for MV beams and up to 30 degrees for orthovoltage beams? (Pg. 217).
- 1.) Effective source to surface distance (SSD) method.
- 2.) Tissue-Air (or Tissue-Maximum) Ratio method.
- 3.) Isodose shift method.
Of the three methods used to correct the PDD for irregular contours, which of the three is the most reliable/accurate? (Pg. 220).
The TAR/TAM method.
The method "Isodose Shift Method" which is used to correct the PDD due to irregular contours, has a constant value 'k'. What is that constant dependent on? (Pg. 220).
The factor 'k' depends on the...
- 1.) radiation beam quality.
- 2.) field size.
- 3.) depth of interest.
- 4.) SSD.
Which of the two methods used to correct the
PDD due to irregular contours/curves are useful in computer treatment planning? (Pg. 220)
- 1.) TAR/TMR Method.
- 2.) Effective SSD method.
Standard isodose charts and depth dose tables that are used to complete treatment planning are based on what important assumption? (Pg. 220).
Applications of standard isodose charts and depth dose tables assume homogeneous unit density medium.
What two general categories can the effects of tissue inhomogeneities be classified into and when are these effects most significant? (Pg. 220).
- 1.) changes in the absorption of the primary beam and the associated pattern of scattered photons.
- 2.) changes in the secondary electron fluence.
The relative importance of these effects depends on the region of interest where alterations in absorbed dose are considered. For points that lie beyond the inhomogeneity, the predominant effect is the attenuation of the primary beam. Changes in the associated photon scatter distribution alters the dose distribution more strongly near the inhomogeneity than farther beyond it. The changes in the second electron fluence, on the other hand, affects the tissues within the inhomogeneity and at the boundaries.
What energy range does the Compton effect become a predominant mode of interaction?
What is the radiation beam (interactions) governed by? (Pg. 221)
The attenuation of the beam in any medium is governed by electron density (number of electrons per cubic cm).
Important Facts!!! Not a question! (Pg. 221)
- An effective depth can be used for calculating transmission through non-water-equivalent materials.
- Close to the boundary or interface, the distriubtion is more complex. For example, for megavoltage beams, there may be loss of electronic equilibrium close to the boundaries of low-density materials or air cavities.
- For orthovoltage and superficial x-rays, the major problem is the bone. Absorbed dose within the bone or in the immediate vicinity of it may be several times higher than the dose in the soft tissue in the absence of bone. This increased energy absorption is caused by the increase in the electron fluence arising from the photoelectric absorption in the mineral contents of the bone.
What are the three mathematical methods used to correct percent depth dose values due to beam attenuation and scattering? (Pg. 221)
- 1.) Tissue-air method.
- 2.) Power law Tissue-air ratio method.
- 3.) Equivalent Tissue-air ratio method.
For the Tissue-air ratio method (correction method for beam attenuation and scattering), what is one external variable excluded from the calculation? (Pg. 221).
The tissue-air correction method does not include the position of the inhomogeneity relative to point P. In other words, the correction factor will not change with d3 as long as d and d' remain constant.
Important facts regarding the three different mathematical methods used to correct dosage due to beam attenuation and inhomogeneity. (Pg. 223).
- 1.) The tissue-air ratio method overestimates the dose for all energies.
- 2.) The equivalent tissue air ratio method is best suited for the lower-energy beams (<= 6 MV).
- 3.) the generalized Batho method is the best in the high-energy range (>= 10 MV).
- Thus the accuracy of different methods depends on the irradiation conditions:
- 1.) energy.
- 2.) field size.
- 3.) location.
- 4.) extent of inhomogeneity.
- 5.) location of point of calculation.
What is the absorbed dose within an inhomogeneity area or in adjacent soft-tissues strongly influenced by? (Pg. 224)
Alterations in the secondary electron fluence.
For example, for x-rays generated at potentials less than 250 kVp, there is a substantial increase in absorbed dose inside bone because of increased electron fluence arising from photoelectric absorption.
Assuming there is an electronic equilibrium between different media, what are two variables that can be used to calculate the absorbed dose ratio? (Pg. 224)
f-factors and mass attenuation coefficients.
What soft tissue elements exists within bone? (Pg. 224)
The soft tissue elements in bone may include
- 1.) blood vessels.
- 2.) living cells called osteocytes.
- 3.) bone marrow.
If the thickness of soft tissue within the bone is small compared with the range of the electrons traversing it, can the soft tissue be considered as a Bragg-Gray cavity? (Pg. 224)
Yes. Under these conditions photon interactions in the cavity can be ignored and the ionization in the cavity is considered entirely due to electrons originating from the surrounding material.
For the same photon energy fluence, soft tissue inside the bone will receive higher dose than the dose to the bone mineral or the dose to soft tissue in the absence of the bone. What are the two reasons for this increase in dose? (Pg. 226)
- 1.) The mass energy absorption coefficient is greater for bone than soft tissue in the very-low-energy range because of the photoelectric process and in the very-high-energy range because of the pair-production. However, in the Compton range of energys, the mass energy absorption coefficient for bone is slightly less than that for soft tissue.
- 2.) The average mass collisional stopping power is greater for soft tissue at all energies because it contains a greater number of electrons per unit mass than the bone.
When soft tissue surrounds bone, there are two cases: dose in tissue when beam travels from tissue to bone, and dose in tissue when beam travels from bone to tissue. What are the reasons for the dose in both cases? (Pg. 226)
- 1.) On the entrance side of the photon beam, there is a dose enhancement in the soft tissue adjacent to the bone because of the electron backscattering caused by the bone.
- 2.) On the transmission side of the photon beam, the forward scatter of electrons from bone and the buildup of electrons in soft tissue give rise to a dose perturbation effect, which depends on the photon beam quality. Low energies cause dose buildup while high energies initially start high (due to pair production) then decrease with distance.
When using parallel opposed beams in a situation where bone is in the center of the patient, are dose distributions normally corrected for the presence of the bone?
When using megavoltage beams, no, but when using lower energy beams, yes.
What is the dose within lung tissue primarily governed by? (Pg. 228)
It is primarily governed by its density.
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