Three-Dimensional Conformal Radiation Therapy

Traditional three-dimensional conformal radiation therapy utilizes photons of nominal energy from 6 to 25 MV delivered through a gantry-mounted linear accelerator or linac. For treatment of skull base tumors, lower photon energies are typically utilized, and a bolus may be employed in cases in which the disease is close to the surface. Although the gantry-mounted accelerator permits delivery of X-rays from any angle, lateral and posterior beams are predominantly used for base-of-skull treatments. This radiation modality delivers uniform-intensity beams targeting a field of constant size and shape. The field shape is typically formed by cerrobend castings or a multileaf collimator (MLC).

The patient is placed on a treatment couch moveable with four degrees of freedom, which allow delivery of noncoplanar beams; the treatment couch, collimator, and gantry remain stationary during radiation delivery. Prior to treatment, proper patient alignment is confirmed with either kilovolt or megavolt two-dimensional (2D) planar images, and additional treatment imaging is obtained only in the event of known patient motion. Motion tracking and three-dimensional (3D) alignment imaging are not typically used.

Intensity-Modulated Radiation Therapy

The advent of the MLC allowed for dynamic delivery of photon treatments and the clinical implementation of intensity-modulated radiation therapy (IMRT) using a gantry-mounted linac. IMRT allows modification of the irradiation field and radiation intensity during beam delivery. Historically, the MLC has delivered dose using two distinct methods:

• 
  

  Step and shoot : the MLC leaves move to positions defining the appropriate field size/shape and remain fixed as the radiation is delivered; after that delivery has been completed, the MLC leaves move to the next position.

  
  
• 
  

  Sliding window : the MLC leaves sweep across the treatment field as radiation is being delivered. This method is potentially more efficient than step and shoot, as beam delivery is not paused during MLC motion.

Both of these IMRT methods initially used a static couch, gantry, and collimator. Current IMRT is arc-based, and multiple parameters of delivery (beam intensity, MLC leaf positions, collimator rotation, gantry angle and motion, and treatment couch position) can be modulated during beam delivery so as to improve dose conformity to the target volume and minimize the dose to the surrounding normal structures. Inverse planning radiation protocols are used to optimize all possible machine parameters.

Treatment couch technology varies among vendors and specific products. Patients may be placed on a simple four-degrees-of-freedom couch or in a six-degrees-of-freedom robotic patient positioner whose positioning error is less than 0.5 mm. Patient alignment may use 2D (kilo- or megavolt) or, more typically, 3D kilovolt cone beam computed tomographic (CT) images captured by on-board imaging systems in a single gantry rotation. These 3D image sets fully assess the patient’s position and permit correction prior to commencement of treatment. Additional imaging or, more typically, optical patient monitoring (i.e., vision radiation therapy) can be used to check patient position during treatment.

Helical Tomotherapy

In helical tomotherapy (HT), a form of IMRT, the beam is delivered in helical arcs similar to those of CT imaging. A 6-MV linac is mounted to a CT-ring-style gantry platform that allows for rapid (approximately 15 s per rotation) continuous rotation around the patient. As the radiation source rotates, the patient is slowly moved through the machine’s bore to produce a helical pattern of dose delivery. This minimizes problems with interslice match lines, an issue with early tomotherapy units.

HT uses a fan beam radiation source collimated by a 64-leaf high-speed pneumatic binary MLC. The fan beam is essentially segmented into 64 individual beamlets that can either be open (i.e., on) or closed (i.e., off) at any stage during beam delivery. This collimator system permits conforming of dose to the target volume and sparing of nontargeted structures. The megavolt fan beam is also used for patient imaging and alignment. The 3D megavolt image is compared with kilovolt CT data sets taken during initial patient setup to identify in 3D needed couch corrections in 3D.

CyberKnife

The CyberKnife incorporates a compact 6-MV linac that delivers a monoenergetic beam of photons to the target volume. The accelerator itself is mounted to a six-axis robotic positioner, allowing multiple individual beams to be delivered from different positions (at 100 during a single 1-h treatment) about the patient’s head. Unlike a conventional linac, the CyberKnife beam is shaped by standard circular cones that deliver a radiation field 5–60 mm diameter. With the new variable aperture system (Iris), cones no longer need to be changed to vary field size. Use of multiple beams and multiple robotic positions of beam origination allows treatment of large targets despite these relatively small radiation fields.

The standard CyberKnife treatment couch has five degrees of freedom; an optional robotic positioner has six. Use of multiple noncoplanar beams and the robotic beam delivery system offers considerable advantage in targeting skull base tumors. For imaging for patient alignment, the CyberKnife system has two ceiling-mounted X-ray tubes (40–150 kVp dynamic range) with floor-mounted amorphous silicon panels (1024 × 1024 pixels and a total area of 40 × 40 cm ² ). The imaging X-ray sources are aligned orthogonal to one another and permit multiple imaging assessments of alignment during treatment. Alignment is checked in reference to bony landmarks or implanted fiducials. This system allows automatic correction of misalignment throughout treatment.