Imaging in radiation oncology: a perspective

Abstract

An inherent goal of radiation therapy is to deliver enough dose to the tumor to eradicate all cancer cells or to palliate symptoms, while avoiding normal tissue injury. Imaging for cancer diagnosis, staging, treatment planning, and radiation targeting has been integrated in various ways to improve the chance of this occurring. A large spectrum of imaging strategies and technologies has evolved in parallel to advances in radiation delivery. The types of imaging can be categorized into offline imaging (outside the treatment room) and online imaging (inside the treatment room, conventionally termed image-guided radiation therapy). The direct integration of images in the radiotherapy planning process (physically or computationally) often entails trade-offs in imaging performance. Although such compromises may be acceptable given specific clinical objectives, general requirements for imaging performance are expected to increase as paradigms for radiation delivery evolve to address underlying biology and adapt to radiation responses. This paper reviews the integration of imaging and radiation oncology, and discusses challenges and opportunities for improving the practice of radiation oncology with imaging.

Figure 1.

Radiation therapy (RT) dose distribution, demonstrating high doses tightly conforming to the target liver metastases volumes (shown in thick blue contours).

Figure 2.

Intensity-modulated radiation therapy plan for a patient with a T4N1M0 nasopharyngeal carcinoma, adjacent to critical normal tissues (e.g., brainstem and optic tissues), treated to 70 Gy in 35 fractions.

Figure 3.

Diagram depicting the importance of optimizing imaging performance based on the fundamental objectives of radiotherapy (outer circle). Trade-offs among geometric integrity, tissue contrast, and spatial resolution must be considered when designing time-efficient image acquisition protocols.

Figure 4.

Multimodality contrast imaging to aid in liver metastasis delineation. Both computed tomography (CT) and magnetic resonance (MR) imaging are done in an exhale breathhold to minimize differences in the liver shape and aid in fusion. The liver contour from the planning CT (in blue) is overlaid on the MR image, demonstrating excellent liver-to-liver registration.

Figure 5.

Imaging for motion assessment of liver cancer to determine the margin around the tumor required to be irradiated to account for breathing motion. (A): Respiratory-correlated computed tomography (CT) scan showing tumor (red contour) in the exhale and inhale phases of CT. The blue contour shows the volume to be irradiated to ensure the tumor is irradiated in all phases of respiration. (B): Coronal cine magnetic resonance (MR) images showing the change in liver position from the exhale to the inhale phases.

Figure 6.

Example of verification imaging (kilovoltage cone beam computed tomography [CT]) obtained in the radiation therapy treatment room, used to position a patient with liver cancer prior to conformal radiation therapy delivery. The pink contour, representing where the liver should be positioned (obtained from the planning CT), is overlaid on the verification cone beam CT image.

Figure 7.

Images acquired after the placement of high-dose rate (HDR) brachytherapy catheters and prior to radiation delivery in one patient demonstrating the superiority of magnetic resonance imaging (MRI) in depicting both the catheters and prostatic anatomy. (A): Transrectal ultrasound. (B): Computed tomography. (C): MRI. The tumor visualized in the left peripheral zone (arrow) can be specifically targeted for dose intensification during brachytherapy planning (D).