Devita, Hellman, and Rosenberg's Cancer

LWBK1006-07 LWW-Govindan-Review November 22, 2011 13:54

94 DeVita, Hellman, and Rosenberg’s CANCER: Principles and Practice of Oncology Review

or within body or surgical cavities (intracavitary brachytherapy), either

permanently or temporarily. Relative to external beam therapy energies,

the emitted spectra are of low energy, but high doses can be delivered

within few centimeters of the source. The ability to irradiate tumors from

close range can lead to conformal treatments with potentially lower nor-

mal tissue doses.

Answer 7.17. The answer is A.

Radiation related adverse effects can be divided into acute, subacute and

chronic (late) effects. Acute effects tend to occur in organs (typically

within the field of radiation) that depend on rapid self renewal. Acute

effects commonly occurs, and typically are self limiting. Examples include

mucositis, esophagitis, diarrhea, and skin reaction. Subacute toxicities

typically occurs 2 weeks to 3 months after radiation has been completed.

Radiation-induced pneumonitis is usually a subacute toxicity. Late effects

from radiation therapy are usually observed after 6 months from comple-

tion of therapy. Some examples include radiation myelitis, brain necrosis,

and bowel obstruction.

Answer 7.18. The answer is A.

Immobilization is critically important because it adds to the accuracy of

daily setup and treatment. A planning computed tomography (CT) scan is

obtained after immobilization has been completed. At the time of image

acquisition for planning, tumor motion caused by respiration must be

determined. The planning volume must account for respiratory movement

and uncertainty of tumor position. Three-dimensional dose distribution

in each patient is not easily measured, and it must be predicted from

computer calculation. Intensity-modulated radiation therapy can have

a high degree of control on the shaping of the dose distribution. The

computer determines the intensity profiles to achieve the desired dose

distribution.

Answer 7.19. The answer is B.

Charged particles include protons and carbons. These particles differ from

photons in that they interact only modestly with tissue until they reach

the end of their path where they then deposit majority of their energy

and stops (Bragg Peak). This ability to stop at given depth gives them the

potential advantage of treating tumors that are close to critical structures.

While proton therapy may have potential use in clinical settings demand-

ing such characteristics, it’s widespread use for all cancer is not yet war-

ranted. Furthermore, in the era of IMRT (Intensity Modulated Radiation

Therapy) and IGRT (Image Guided Radiation Therapy), despite theoret-

ical and dosimetric advantages conferred by charged particles, whether

this will allow higher dose to be delivered more safely, and yield a clinical

advantage over highly conformal photon techniques is not yet fully deter-

mined. Cost of development and operation of charged particle therapy

facilities has also limited its widespread development and use.