Intensity Modulated Radiotherapy

Question 1

What kind of system is Linac?

Answer 1

Linac stands for linear accelerator. It is a radiotherapy X-ray irradiation device usually combined with an MRI machine (MR-Linac). The gantry of the system holds radiation detectors and radiation souces, which makes irradiation from different angles possible and at the same time imaging is possible.

Question 2

So Linac is a special system that makes simultaneous imaging and radiotherapy possible. In this way, the pre-treatment planning can be real-time adjusted based on what is imaged and targeted at that specific time.
How is the system able to adjust the pre-treatment plan?

Answer 2

The X-ray beams that send out the X-rays contain a multi-leaf collimator (MLC). The MLC is made of individual pieces that can move independently in and out of the path fof a radiotherapy beam in order to shape and vary its intensity.

Question 3

What important structures are delineated during the treatment planning procedure?

Answer 3

• Planning Target Volume (PTV)
• Clinical Target Volume (CTV)
• Gross Tumor Volume (GTV)
• Organ at Risk (OAR)

Question 4

Describe the treatment planning procedure in short.

Answer 4

• Import of CT scan in treatment planning system.
• Delineation of important structures.
• Prescribe the dose

Question 5

• What is important to take into consideration when prescribing the dose for radiotherapy?

Answer 5

• The PTV should receive 95-107% of the prescribed dose.
• The prescribed dose should be constrained to the organ(s) at risk (as low as reasonably achievable)

Question 6

What is important to take into consideration when generating the treatment plan (set of instructions for the linac) itself?

Answer 6

• Irradiation directions
• Field shapes
• Dose rate

Question 7

Describe what the computer calculates if it has received its recipe i.e. if all the important structures are delineated, the dose is determined and the treatment plan for the Linac is available

Answer 7

• So the list of planning objectives (the recipe) is entered into the computer.
• The computer is then able to generate a treatment plan and allocates a penalty to certain features of the treatment plan if certain objectives are not met. For example, the penalty/cost will increase if the kidneys are defined as the organ at risk and still get quite a high dose of radiation.

Question 8

Just take a look at the difference between a semi-automatic treatment plan and a fully-automated plan.

Question 9

What is Intensity-Modulated Radiation Therapy (IMRT)?

Answer 9

An advanced type of radiation therapy that uses computer-controlled Linac to safely deliver precise radiation to a tumor while minimizing the dose to surrounding healthy tissue. IMRT allows for the radiation dose to conform more precisely to the three-dimensional (3-D) shape of the tumor by modulating—or controlling—the intensity of the radiation beam in multiple small volumes.

Question 10

What is a dose-volume histogram (DVH)?

Answer 10

A graphical representation of the radiation dose delivered to any defined volume. The volume of interest can be either the target or the organ at risk (i.e. the delineated structures during the planning of the treatment).
- The horizontal axis is the dose expressed in absolute values (Gy) (can also be in relative values).
- The vertical axis is the volume expressed in absolute or relative values.

So when the treatment plan has been generated, a DVH can be generated by the computer. The treatment plan has defined how much radiation are received by certain delineated structures (the volume of the organ that receives radiation). The DVH is then able to tell the corresponding radiation dose to this volume of the organ that receives radiation.

Question 11

Use the picture to answer the questions.

• if the treatment plan has confirmed that 8% of the lungs will receive radiation, how much cGY will the lungs then receive?
• if the treatment plan has confirmed that 100% of the PTV will receive radiation, how much cGY will the PTV then receive?
• if the treatment plan has confirmed that 60% of the PTV will receive radiation, how much cGY will the lungs then receive?

Answer 11

• Around 520 cGY
• Around 2080 cGY
• Around 2600 cGY

Question 12

What is Volumetric Modulated Arc Therapy (VMAT)?

Answer 12

The gantry rotates around the patient with continuously changing of beam direction, beam shape and dose rate. This makes VMAT faster than IMRT.

Question 13

Just have a look at the differences between VMAT and IMRT.

Question 14

Choose the right words.

• IMRT has less/more number of beam shapes than VMAT.
• IMRT’s delivery time is slower/faster than VMAT.
• The dose distribution in IMRT is inferior/superior compared to VMAT.

Answer 14

• IMRT has less number of beam shapes than VMAT.
• IMRT’s delivery time is slower than VMAT.
• The dose distribution in IMRT is inferior compared to VMAT.