Pediatric radiation oncology

1

How does radiation therapy work?

In the realm of external beam radiation (the most commonly used radiotherapy), a linear accelerator produces a beam of high-energy electrons, which hits a metal target at the head of the machine. Upon striking the target, electron energies are subsequently converted to x-ray (photon) energies. Filters and multileaf collimators (MLCs) are placed distal to the target in the head of the machine to selectively block and shape the x-ray radiation as it exits the linear accelerator and enters the patient. When these uncharged photons collide with a charged electron in the body, this excited electron can cause damage either directly or indirectly:

• Direct Action: The ionizing event leads to DNA damage and triggers cell death.

• Indirect Action: The ionizing event forms free radicals (most commonly hydroxyl) that can then cause DNA damage and cell death.

2

Describe the simulation process.

The simulation session allows the radiation therapist to place patients in a reproducible position that is used each day during treatment. A variety of immobilization devices are available to minimize patient motion and maximize reproducibility for the duration of treatment. Sometimes permanent tattoos are placed to help align the patient for a reproducible setup. Once the computed tomography (CT) scan is complete with the customized immobilization devices, other imaging modalities such as positron emission tomography (PET) and magnetic resonance imaging (MRI) can be used to help identify the tumor target and normal organs at risk (OARs).

3

Which structures are contoured and how?

The most common target volumes used for radiation planning are gross tumor volume (GTV), clinical target volume (CTV), and planning target volume (PTV):

• GTV: Represents tumor that is visible on CT scan (MRI or PET information can be used).

• CTV: Represents GTV + microscopic disease. This typically includes a margin around the GTV plus any regions concerning for lymphatic spread.

• PTV: Represents an expansion around the CTV that takes into account patient motion and setup variation.

Additionally, OARs near the target volumes are delineated to generate a plan that maximizes dose to the tumor while minimizing toxicity to nearby structures.

4

What are the different techniques used to deliver external beam radiation?

There are several factors that influence radiation technique:

• The type and frequency of imaging of the patient before radiation delivery

• The complexity of the type of radiation to be delivered (i.e., physicians manually placing the beams versus using computer-based algorithms to customize radiation beam delivery to spare OARs)

• How quickly the radiation is delivered

The different techniques include three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), standard fractionated radiotherapy, stereotactic radiotherapy, and hypofractionated radiotherapy:

• 3D-CRT: This technique is for simple palliative treatments, such as for Wilms tumors. Using CT scans to visualize the tumor, the radiation oncologist and planners can design the radiation plan without software algorithm assistance.

• IMRT: This is a more advanced form of CT-based radiation planning. Radiation delivery is customized with different intensities of radiation coming at different angles to target the tumor and avoid normal tissue. By using a computer algorithm, the tumor target volume, and the OARs, the computer generates a customized plan for treatment.

• Standard Fractionated Radiotherapy: The treatment is typically 1.5 to 2 Gy per treatment delivered over several weeks. One can have standard fractionated 3D-CRT or standard fractionated IMRT.

• Stereotactic Radiotherapy: Using the same techniques as in IMRT, stereotactic radiotherapy refers to the delivery of high doses of radiation (6 Gy or more) in five or fewer fractions. Stereotactic radiosurgery (SRS) refers to treatment of intracranial lesions, whereas extracranial stereotactic radiation therapy is referred to as stereotactic body radiotherapy (SBRT).

• Hypofractionated Radiotherapy: Treatment courses that are shorter than the standard fractionated treatments but do not fit criteria for stereotactic radiotherapy are considered hypofractionated radiotherapy. One can be treated with 3D-CRT or IMRT with hypofractionation.