What is the role of intensity-modulated radiation therapy (IMRT) in the treatment of prostate cancer?

Updated: Nov 29, 2018
  • Author: Isamettin Andrew Aral, MD, MS; Chief Editor: Edward David Kim, MD, FACS  more...
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Answer

Intensity-modulated radiation therapy (IMRT) can achieve tightly conformal dose distributions with the use of nonuniform radiation beams. The intent of this form of therapy is to create highly conformal fields by treating the patient with multiple static portals (so-called step-and-shoot IMRT) or dynamic fields. In dynamic IMRT, a series of arcs are administered through the area of interest. Multileaf collimators (MLCs) are reshaped many times as the machine performs a series of arc rotations around the target (see the image below).

Multileaf intensity-modulating collimator (MIMiC) Multileaf intensity-modulating collimator (MIMiC) unit. This is used to deliver intensity-modulated radiotherapy.

Complex treatment-planning software algorithms allow exceedingly high doses of radiation to be delivered to the target while significantly smaller doses of radiation are delivered to the adjacent normal tissue. In contrast to the traditional method of radiation planning, inverse treatment planning is commonly used for the calculation of doses during IMRT.

IMRT establishes a treatment plan after the establishment of acceptable doses to regional (normal) anatomy. For instance, the maximum tolerable dose to be delivered to the involved segments of the bladder, bowel, and rectum is specified. The desired target dose is then prescribed to the planning target volume (PTV).

The computer, through a series of complex iterations, designs a treatment that maximizes the dose delivered to the target and minimizes the dose delivered to adjacent normal tissue. Implicit in the name of this form of therapy is the concept that the intensity of the radiation beam changes throughout the course of therapy.

IMRT has been successfully used to treat tumors when the target area is readily identifiable at the initiation of daily treatments and the desired dose for optimum tumor control is significantly higher than the acceptable dose limits for adjacent normal tissue. Tumors of the head and neck and tumors of the breast are clinical sites where this treatment has been successfully used.

IMRT is no longer considered an investigational technique in the management of prostate cancer. Rather, it has rapidly become a highly precise method of delivering increasing doses of radiotherapy to the prostate and the immediate periprostatic tissues.

To date, however, no multicenter, phase III, prospective, randomized trial has been performed to establish whether IMRT is superior to well-designed 3-dimensional conformal radiotherapy (3D-CRT). Data from the Memorial Sloan-Kettering Cancer Center have demonstrated that doses of more than 80 Gy can be safely delivered by means of IMRT. The value of dose escalation when additional adjuvant treatments are being considered (eg, hormonal blockade, chemotherapy) remains unclear.

IMRT in the treatment of prostate cancer continues to evolve; however, reproducible identification of the target (on daily treatments) remains challenging. The use of implantable fiducial markers and ultrasonographic localization devices has become increasingly popular. Both techniques allow the treating therapists to identify the desired target immediately before each day’s treatment. Without such specificity, the logic of using IMRT is questionable.


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