Can’t Hit What You Can’t See
Increasing anatomic precision in radiation therapy requires a strong knowledge of cross-sectional imaging anatomy.
Image-guided radiotherapy is the newest treatment modality radiation oncologists have to improve survival and limit toxicity for patients with a variety of malignancies. Advanced imaging techniques, like PET and MRI scans, help to even more accurately delineate tumor extent than CT simulation mapping of radiotherapy fields alone. Such precision allows contemporary radiation oncologists to escalate radiation dose delivered to tumors, thus increasing the probability of cure rates. At the same time, that radiation can be delivered within smaller anatomic margins, thereby sparing normal tissue and ultimately limiting toxicity.
I searched Pub Med recently and found over 4,200 unique references to image-guided radiotherapy, with the numbers of citations increasing from 2012 to the present.
Greater risk to normal tissues can be seen as radiotherapy dose is increased. Therefore, accurate identification and contouring of organs at risk is imperative to achieve a positive therapeutic window. Increasing radiotherapy dose, however, is not a guarantee for improved outcomes. Radiation Therapy Oncology Group (RTOG) trial 0617 compared standard chemoradiotherapy using a dose of 60 Gy to a dose of 74 Gy with similar chemotherapy in patients with non-small cell lung cancer. Overall survival was not improved with high-dose radiotherapy. In fact, a decrement in survival was seen in patients who have greater radiation dose to the heart. In addition, greater toxicity was seen in patients treated with the higher radiotherapy dose. These patients presented with increased esophagitis and lower quality-of-life scores.
Accurate identification has become increasingly important as radiotherapy doses have been escalated and treatment techniques changed. Gondi et al reported 24 percent of patients with brain metastases enrolled on the RTOG 0933 trial of hippocampal avoidance during whole brain radiotherapy had unacceptable deviations after an initial review of contouring of the hippocampus prior to starting any trial treatment. In addition, 23 percent unacceptable deviations were found in patient scans reviewed after treatment.
The symbiotic relationship between therapy and imaging, however, is not new. Early in the 20th century, general radiologists not only interpreted images but also delivered treatment, first with ortho and kilovoltage machines and then cobalt-60 units. The first therapeutic radiology certificate was offered by the American Board of Radiology in 1934, and the last general radiology certificate was offered in 1982. The field of therapeutic radiology became radiation oncology in 1987.
Therefore, should radiation oncology residents spend mandatory time in radiology rotations learning normal radiographic anatomy? Identification of normal anatomy is already included in part 1 of the radiation oncology written board exam. Most training programs have moved away from formal radiology rotations to incorporating radiology training during case conferences. The ACGME states, however, "The program must educate resident physicians in adult and pediatric medical oncology, oncologic pathology, and diagnostic imaging in a way that is applicable to the practice of radiation oncology."
Accurate identification of normal anatomy will be even more critical for radiation oncology trainees as new treatment techniques are developed, especially MRI-guided linear accelerators, where MRI scanners on linear accelerators allow MRI imaging during the course of a radiotherapy treatment. A return to formal training in diagnostic imaging may be a way of ensuring radiation oncologists can confidently identify tumor and normal tissue so they see what they are trying to hit.