Diagnostic Radiology and Radiation Therapy

Types of radiation therapy

Globally, radiation therapy is divided into external beam radiation therapy (External beam), where the radiation source is outside the body, and brachytherapy (Brachytherapy), in which the source is placed inside the affected organ.

External beam therapy uses photons (X-rays and $\gamma$-rays) or corpuscular radiation (electrons, protons, carbon ions, neutrons). Penetration depth depends on energy:

• Low-energy rays (60–300 keV) are used for superficial tumors (e.g., skin cancer).
• Megavoltage rays (6–18 MeV) are generated by a linear accelerator and are needed to treat deep-seated neoplasms.

The evolution of the method has led to a transition from traditional therapy (with simple "box" fields, mostly for palliation) to three-dimensional conformal radiation therapy (3D-CRT). The computer evaluates beam attenuation voxel by voxel using Hounsfield units on CT, while multileaf collimators adjust the beam shape precisely to the target contour.

Planning: how to target the tumor

To deliver the dose with submillimeter precision, radiation oncologists need to perfectly contour the borders of cancer spread. The problem is that the gross tumor volume (Gross Tumour Volume, GTV) visible on scans does not always coincide with the true pathomorphological borders.

Therefore, in standard practice, a safety margin (safety margin) is always added to the GTV. For maximum contouring precision, deformable fusion (deformable fusion) of data from different modalities (CT, MRI, and PET) is used.

A clinical example of supreme precision is the CyberKnife system. For recurrent glioblastoma, it delivers radiation in the form of multiple narrow beams (pencil beams). The device uses image guidance (image guidance) and automatically compensates for patient movement (including breathing) so that the dose perfectly matches the tumor contours.

Anatomical and functional imaging

Modern scanners not only show anatomy but also assess tumor biology. CT remains the baseline method due to its availability and isotropic submillimeter resolution.

Functional methods make it possible to integrate data on oxygenation, cellular proliferation, and blood flow directly into the radiation plan.

Radiation and chemotherapy complications (Toxicity)

Therapy also affects healthy tissues, which is clearly visible on follow-up scans.

• Bones and bone marrow: MRI changes are noticeable even at doses of 15–20 Gy. Hematopoietic marrow transforms into fatty marrow, producing a characteristic high signal on T1. In the late period (2+ years), sclerosis and trabecular coarsening develop, and spontaneous fractures are possible.
• Lungs: On CT, acute toxicity (e.g., from bleomycin) appears as extensive basal interstitial changes.
• Nervous system: Irradiation of the brain and spinal cord can cause deep white matter ischemia (high signal on T2 or FLAIR). Breast irradiation sometimes leads to radiation plexopathy—thickening of the brachial plexus, which on MRI must be differentiated from cancer recurrence.

Class and exam questions

What else to study on this topic

• Intensity-modulated radiation therapy (IMRT)
• Features of proton therapy and carbon ion therapy
• Posterior reversible encephalopathy syndrome (PRES) on imaging
• Differential diagnosis of radiation plexopathy and cancer recurrence

Other topics in the "Diagnostic Radiology in Oncology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.