How is 3-dimensional conformal radiotherapy (3D-CRT) delivered for 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

Answer

The process of 3D-CRT requires the acquisition of imaging data. The initial step is immobilization of the patient in either the supine or the prone position.

Theoretic advantages of supine immobilization include the ease of daily setup for the patient and staff, the ability to fuse treatment-planning images with previously obtained diagnostic images (ie, MRI), and the relative ease of use when performing daily setup localization with ultrasonographic assistance. However, many centers prefer prone positioning. Theoretic advantages of prone positioning relate to relative sparing of small bowel from the radiation portals and reproducibility of patient positioning on daily setup.

Once the patient has been appropriately positioned, fabrication of the immobilization device is performed. This step becomes increasingly critical as the margin around the target is decreased (eg, when a 1.5-cm margin around the prostate is decreased to a 0.5-cm margin).

Numerous materials have been used to immobilize patients before CT or MRI data are acquired for treatment planning. Commercially available products include thermoplastic casts, vacuum-shaping bags, and self-contained thermochemicals. Regardless of which type of device is chosen, the goal of immobilization is to reproduce the position in which the patient is treated each day.

Upon fabrication of the device or devices, axial images of the area of interest are obtained. Consecutive CT scans or MRIs are obtained, starting from 3 cm below the prostate and extending superiorly to 3 cm above the superior tip of the seminal vesicles. Additional CT imaging or MRI data can be obtained above or below the areas of interest. However, this information has minimal impact on patient treatment or dose computation.

The targets (CTV, PTV, or both) are identified on each relevant axial CT slice. Similarly, normal structures, including the bladder wall, rectum, small bowel, bony structures, and skin surface, are outlined on each relevant CT slice.

The target volume and normal structures are then digitally reconstructed in 3 dimensions and displayed with the beam’s-eye view (BEV) technique. The adequacy of target coverage and normal tissue doses can be viewed by using dose-volume histograms (DVHs) or serial 2-dimensional images with isodose curves superimposed on them.

Compared with conventional EBRT, 3D-CRT techniques implement a larger number of beams daily to improve the tumor–to–normal tissue dose ratio. Implementation of 3D-CRT requires the use of newer treatment machines capable of rapidly delivering a large number of precisely shaped fields under automated computer control (ie, multileaf collimators [MLCs]; see the image below).

Conformal radiation therapy. A linear accelerator Conformal radiation therapy. A linear accelerator equipped with a multileaf collimator is a device that can decrease the time a patient spends in the treatment room and one that improves treatment accuracy.

MLCs are capable of automatically shaping the apertures of each treatment field in rapid succession under computer control. Treatment times are shortened, individual treatment blocks need not be fabricated, and more complex beam shaping can be attempted.


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