Chapter 12 Kahn: The Physics of Radiotherapy

Imaging: Computed Tomography (CT), MRI, etc.

The patient should be positioned similarly to his/her treatment position.

• 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.

Diagnostic radiographs or atlases of cross-sectional anatomy.

• 1.) Computed Tomography (CT).
• 2.) Magnetic Resonance Imaging (MRI).
• 3.) Ultrasound (US).

The range is between soft-tissue and bone.

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.

• 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.

"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."

No. But it may be very important in regions where there are large dose gradients.

• 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.

• ADVANTAGES
• 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.

DISADVANTAGES

• 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.

ADVANTAGES 

• 1.) No ionizing radiation.
• 2.) Less expensive (cheaper).

DISADVANTAGES 

1.) Image quality or clinical reliability is not as good as that of the CT.

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.

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.

Most commercially available simulators have "fluoroscopic capability".

• 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.

Yes, but despite that, the practical use of simulators varies widely from institution to institution.

CT simulation systems use CT scanners to localize the treatment fields on the basis of the patient's CT scans.

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

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.

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.

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".

Positron emission tomography (PET) provides functional images that can differentiate between malignant tumors and the surrounding normal tissues.

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.

• 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.

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.

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.

• 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.

• 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.

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.

• 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.

• 1.) Effective source to surface distance (SSD) method.
• 2.) Tissue-Air (or Tissue-Maximum) Ratio method.
• 3.) Isodose shift method.

The TAR/TAM method.

The factor 'k' depends on the...

• 1.) radiation beam quality.
• 2.) field size.
• 3.) depth of interest.
• 4.) SSD.

• 1.) TAR/TMR Method.
• 2.) Effective SSD method.

Applications of standard isodose charts and depth dose tables assume homogeneous unit density medium.

• 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.

Megavoltage Range.

The attenuation of the beam in any medium is governed by electron density (number of electrons per cubic cm).

- 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.

• 1.) Tissue-air method.
• 2.) Power law Tissue-air ratio method.
• 3.) Equivalent Tissue-air ratio method.

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.

• 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.

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.

f-factors and mass attenuation coefficients.

The soft tissue elements in bone may include

• 1.) blood vessels.
• 2.) living cells called osteocytes.
• 3.) bone marrow.

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.

• 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.

• 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 megavoltage beams, no, but when using lower energy beams, yes.

It is primarily governed by its density.