A1.2 External photone radiation therapy

Topic

The linear accelerator generates the high-energy photon beam—equivalent to an accelerating voltage of between six and eighteen megavolts—that initiates all external photon radiotherapy; this photon beam passes through the patient's body, depositing a dose within the target tumor volume as it traverses it.

How that dose is distributed within the patient's body depends not only on the photon beam's energy but also on how the radiation travels through matter: during this process, each photon in the beam can interact with tissue electrons in three ways—transferring part of its energy to them, scattering with partial energy loss (known as Compton scattering), or being completely absorbed (the photoelectric effect)—and the combination of these photon-electron interactions determines, point by point, how the dose is distributed within the patient.

This dose distribution within the patient is precisely what the technological evolution of external photon radiotherapy has increasingly refined: starting with simple rectangular fields that irradiated a broad region without clearly distinguishing the tumor volume from surrounding healthy tissue, the advent of intensity modulation allowed the photon beam's intensity to be varied point by point within each field; subsequently, the technique of dynamic rotational arcs enabled the photon beam to rotate around the patient while being modulated; and the technique of delivering very high doses in few fractions with sub-millimeter precision took this principle to its extreme. Each step in this technological evolution succeeds in concentrating more dose within the tumor volume while reducing it in the surrounding healthy tissue. A central principle makes it possible to calculate—rather than merely describe—this dose distribution: the dose received at any given point in the tissue results from combining the photon flux reaching that point with a fixed pattern describing how energy is distributed around each interacting photon—a combination known in physics as convolution. Thus, knowing the photon flux and this deposition pattern is sufficient to predict the dose at any point within the patient. Treatment planning reverses this process: instead of starting with the photon flux to derive the dose, inverse planning begins with the desired probabilities for tumor control and healthy tissue complication; based on these two objectives, it calculates the photon flux—distributed segment by segment across the multileaf collimator shaping the beam—required to produce that dose distribution, thereby closing the loop between the photon flux and the resulting dose.

Taken together, this entire sequence—from the linear accelerator to the photon beam, through radiation transport within the patient's body, to the calculated and subsequently optimized dose distribution—enables external photon radiotherapy to fulfill its purpose: concentrating the photon beam's dose on the tumor volume with increasing precision (reaching the sub-millimeter level with the latest techniques) while minimizing the dose received by the healthy tissue surrounding the tumor.

ID:2305