Treatment planning in palliative radiotherapy

Clinical setup

In circumstances where treatment must begin urgently—particularly at times in which all usual members of the clinical team such as dosimetry may not be available—clinical setup offers an expedited means to initiate simple palliative treatments. While protocols for clinical setup vary across facilities, this treatment technique involves the following steps:

• 1.

  The patient is placed in the optimal treatment position, with immobilization devices selected according to the target location.

• 2.

  The radiation oncologist selects an appropriate position for treatment isocenter.

• 3.

  Optimal beam and collimator angles, beam energy, and couch position are selected.

• 4.

  Megavoltage portal imaging is acquired to confirm the treatment field parameters are appropriate for the target. Anatomic landmarks are often used as proxy for the tumor target if not well-visualized on portal imaging.

• 5.

  Monitor unit calculations (and often subsequent hand calculations) are performed based on field size, beam energy, and source to surface distance measurements, specific to the treatment machine setup parameters.

At our institution, patients receiving multifraction treatments often undergo CT simulation during the next available simulation session. In general, the case then undergoes three-dimensional conformal radiotherapy (3DCRT) replanning to optimize target coverage, dose homogeneity, and avoidance of OAR for the remainder of the treatment prescription.

Conventional simulation

Conventional simulation uses a simulator equipped with kilovoltage imaging and fluoroscopy, in addition to couch and gantry components similar to standard linear accelerators. As with clinical setup, this approach is often reserved for cases requiring rapid treatment delivery. Treatment preparation is similar to clinical setup; however, orthogonal fluoroscopic imaging is used to select isocenter after the radiation oncologist outlines the anticipated field borders using radiopaque markers at the patient surface. Isocenter is then confirmed using orthogonal kilovoltage images. On the linear accelerator, megavoltage portal images may also be taken to verify the selected patient position, target, field size, and couch position.

CT simulation

CT simulation, or virtual simulation, is the most commonly used technique for radiation planning and affords the ability to perform dose calculations based on three-dimensional (3D) volumetric imaging. ^, As compared with clinical setup and conventional simulation, CT simulation with subsequent 3D planning is associated with a reduction in treatment-related adverse effects. This treatment technique involves the following steps:

• 1.

  The patient is placed in the optimal treatment position on a flat-top table that approximates the linear accelerator couch.

• 2.

  Immobilization devices and motion management strategies are selected according to the target location. Note that for dose escalation strategies such as stereotactic radiotherapy, margins are generally minimized to reduce receipt of high doses by adjacent OAR. As such, immobilization and motion management strategies can be increasingly important to use. For brain and cervical spine targets, rigid immobilization using stereotactic head frames or thermoplastic masks that attach to the treatment couch ensure setup accuracy within a few millimeters. For treatment sites not amenable to rigid immobilization, kinematic positioning systems such as HexaPOD (Elekta) can minimize patient setup error to within sub-millimeter levels.

• 3.

  Radiopaque markers can be used to delineate scars, sites of superficial bleeding or pain, or other anatomic locations that may be useful during treatment plan.

• 4.

  Tissue equivalent bolus, testicular shielding, and other dose-modifying devices can be applied as indicated.

• 5.

  The position of the patient and/or immobilization devices are indexed to the couch. The CT simulation system should match the indexing system used on the linear accelerator couch.

• 6.

  A two-dimensional (2D) topogram, or scout view, is taken to confirm patient alignment and to set the superior-inferior, lateral-medial, and anterior-posterior borders for final CT imaging acquisition.

• 7.

  Intravenous and/or oral contrast is administered if indicated.

• 8.

  The final CT image set is captured, according to site-specific imaging protocols. For stereotactic treatments, a thin-slice CT (slice thickness 1 to 2 mm) is generally used.

• 9.

  The radiation oncologist can elect to set the isocenter at the time of CT simulation or at the time of treatment planning. Either the isocenter or virtual isocenter (“premarks”), respectively, are marked at the patient surface or immobilization device using a laser coordinate system, which is also present in the linear accelerator vault.