Question 2

What are the differences between intensity modulated radiation therapy (IMRT) and three-dimensional (3D) conformal radiation therapy?

Question 4

For a step-and-shoot intensity modulated radiation therapy (IMRT) delivery what are control points and how do they relate to the number of segments?

Answer 2

3D conformal radiation therapy uses multiple beams each with a single aperture that either conforms to the target volume or purposely protects the critical structures to produce a conformal isodose distribution. IMRT uses beams with multiple beam apertures. These apertures (or segments) are determined by a computer optimization process, called inverse planning. 3D planning is also referred to as forward planning as the planner shapes the apertures to acheive the target goals, rather than specifying the intended dose for the target, and the computer determining the apertures that best achieve this dose in IMRT inverse planning.

Answer 4

An IMRT beam is delivered using multiple control points. Each control point specifies a multileaf collimator (MLC) shape and a dose to be delivered between control points. Therefore, each IMRT beam segment contains two control points—one specifying that segment’s leaf shape and delivering zero dose, and a subsequent control point with the same leaf shape delivering that segment’s dose, thus the term step and shoot. The machine then moves to the next control point with the new segment shape.

Question 6

What is the difference between gradient search and simulated annealing optimization?

Question 8

How are intensity modulated radiation therapy (IMRT) deliveries verified prior to treatment?

Answer 6

A gradient search method optimizes the intensity modulated radiation therapy (IMRT) fluence by calculating the gradient of the objective function (or cost function), but this method may result in choosing a plan that resides in a local minimum—that is, the best overall plan may not be achieved. Simulated annealing uses a probabilistic approach, which may choose an optimization path with a higher cost function value in the hopes of finding one later with the global minimum cost value: the best optimized plan.

Answer 8

Patient specific IMRT quality assurance (QA) is performed by delivering the IMRT treatment to a phantom to verify plan deliverability, point dose, and two dimensional (2D) dose distribution accuracy. Measurements are taken using an ion chamber and film, an electronic detector array, or using electronic portal imagers.

Question 10

How is the over-travel distance for a multileaf collimator (MLC) defined? How does this affect intensity modulated radiation therapy (IMRT) field size?

Question 12

What precautions should be taken when using intensity modulated radiation therapy (IMRT) to treat a moving target?

Answer 10

The over-travel distance is the maximum distance that a leaf tip can extend beyond the isocenter. A typical distance is 10 cm. If this distance is smaller than half the maximum field size, then this will limit the usable field size available for an IMRT beam.

Answer 12

The motion of a target may have an interplay effect with the motion of the multileaf collimator (MLC) leaves. This effect can be alleviated by using a smaller number of apertures and a larger aperture size. Motion management techniques can also be employed to minimize target motion. These may include abdominal compression, voluntary or active breathing control, or respiratory gating.

Question 14

How is intensity modulation achieved in a proton beam?

Question 16

Which treatment modality typically uses more monitor units (MUs) to deliver the same dose: three-dimensional (3D) conformal or intensity modulated radiation therapy (IMRT)?

Answer 14

Proton beams are modulated using specially milled compensators that modulate beam intensity by using varying thickness. Photon intensity modulated radiation therapy (IMRT) can also be compensator based, but this is no longer common, as it requires the therapist to enter the treatment vault for each beam to change the compensator. This is also a limiting factor in the number of beams used for proton therapy. More recent proton units are using scanning pencil-beam techniques to deliver intensity modulation.

Answer 16

IMRT typically requires more MUs to deliver the same dose than 3D conformal. IMRT beams are comprised of several apertures, each smaller than a single, large aperture used for a 3D conformal plan. In order to deliver the same dose across the entire field using smaller apertures, more total MUs are typically required. For some 3D conformal plans, especially when a 60° physical wedge is used, the 3D conformal plan may have higher MUs than the IMRT plan.

Question 18

What is a downside of intensity modulated radiation therapy (IMRT) delivery compared with conventional radiation therapy?