Dose Distributions

Chapter 22 W/L

Dose Distributions

Treatment planning can be defined as the process by which dose delivery is optimized for a given patient and clinical situation.

Dose distributions are spatial representations of the magnitude of the dose produced by a source of radiation.

·  Percentage depth dose (PDD) curve is a one-dimensional representation of the variation of dose. Describe dose variation with depth along the central axis of a beam

·  Isodose distributions are two-dimensional spatial representations of dose. Illustrate dose variation both along as well as across the direction of a beam of radiation.

Treatment fields are used in ways that produce adequate tumor dose and minimal normal tissue dose

Profiles: describes radiation intensity as a function of position across the beam at a given depth.

·  Depicts the beams intensity in a direction perpendicular to the beams direction.

·  Usually measured at a depth of 10 cm.

·  The shape of a beams profile is a function of depth.

§  At shallow depths characterized the beams primary intensity- because scatter is less

§  The dmax profile shows influence of the accelerators flattening filter on the beam.

Flatness: the difference in the maximum and minimum intensity of the central 80% of the profile and specifying this defferece as a percentage of the central axis intensity. The degree of evenness of dose across a beam profile.

Symmetry: the maximum point to point distance in the central 80% of the profile.

Flattening filter designed to produce a flat intensity pattern at some predefined depth (normally 10 cm)

Isodose curve is a 2D representation of how dose varies with position within a beam along directions both parallel and perpendicular to the beams direction- all having the same dose.

Isodose distributions combine both the depth dose and off axis profile characteristics of the beam

·  Will vary with beam energy, source skin distance and field

Wedge: a variable thickness attenuator

·  Less dose exists under thicker portions of the wedge than exists along less thick portions

Wedge angle: the angle between the slanted isodose line and a line perpendicular to the central axis of the beam.

·  Tilt of isodose lines varies slightly with depth – because of the relative influence of scatter- either the depth of the 80% depth dose or the depth of 10 cm is often chosen for wedge angle measurement.

·  The energy spectrum of wedged fields differs slightly from that of its nonwedged counterpart. Slight differences in PDD- more pronounced at lower beam energies and larger wedge angles.

Dynamic wedge: the use of a moving collimator jaw to produce a wedged field.

Parallel opposed fields two treatment fields share common central axes, 180 degrees apart.

·  Used to deliver reasonably uniform doses

·  Lower energy beams because of their decreased penetrating ability, produce higher doses at their respective entrance depths producing a less uniform dose distribution as a function of depth.

·  When thickness increases the entrance to midline dose ratio becomes unacceptably high.

Multiple convergent beams are often used in situations in which parallel opposed beams cannon produce acceptable dose distributions or when it may be desirable to further restrict the high dose region of the dose distribution.

Hinge angle: the angle between the beams central axes

·  HA= 180- 2(WA)

Contour corrections: corrections for beam incidence onto surfaces other than flat surfaces and for angles of incidence other than 90 degrees.

Heterogeneity corrections account for the presence of irradiated media other than water

·  Relative density of water: 1.0

·  Relative density for lung: 0.25

·  Relative density for bone: 1.65

3D conformal radiation therapy: 3D image visualization and treatment planning tools are used to conform isodose distributions to only target volumes while excluding normal tissues as much as possible.

3D treatment planning: planning is image based and patient anatomy is represented by computed tomography data sets, tumor volumes and critical structured are identified using image contouring tools that enhance their visualization

beams are arranged to target tumor volumes and avoid critical structured

3DCRT & IMRT: target volumes and critical structured are defined and beams are arranged in an attempt to maximize the dose to targets while minimizing the dose to critical structures

IMRT: intensity modulation techniques allow for the modification of the distribution of intensity within a treatment bean to achieve the stated goal.

Types of IMRT:

·  Step and shoot: the gantry in a fixed position with an initial MLC pattern, a portion of the dose is delivered through this leaf pattern or segment (beam is delivered after the MLC has achieved the shape corresponding to it appropriate segment) with the gantry in the same position the beam is automatically interrupted and the leaf pattern changed to that corresponding to the second segment.

·  Dynamic MLC: the leaves of the MLC move during the delivery of the dose (sliding window technique)

·  Tomotherapy: uses arc therapy

·  Robotic IMRT: similar to radiosurgery, small beams are delivered to the tumor volume through multiple angles.

Inverse planning (IMRT): calculate dose distributions and create MLC patters based on initial dose delivery and avoidance parameters, the intensity of the beams are then altered, by opening and closing the MLC, to achieve the requested doses. Beam arrangements are set up by the planner.

·  Uses treatment planning algorithms: consist of a series of mathematical equations, and their associated input parameters that produce values of dose as a function of position within the dose calculation matrix- a grid of points at which dose is computed and subsequently displayed.

**Steps:

·  Physician dictates dose delivery and avoidance parameters

·  This info is put into the TP computer

·  The MLC patterns change intensity to get the prescribed dose and lack of dose

·  The planner sets the beams and PTV margins

·  The computer changes the intensity map until it arrives at the best plan

Forward planning (3DCRT ): requires that dose altering parameters and beam modifiers be entered into the treatment plan by the planner, after the initial dose calculation is completed, the planner evaluated the dose distribution and edits the modifiers or other parameters to produce an improved plan. Repeated until an acceptable plan is achieved.

**Steps:

·  Dose prescription determined by DR.; dose limitations are known by planner

·  Beams arranged

·  Weights changed; normalization changed

·  Dose distribution evaluated

·  Beam angles modified, wedges may be added

·  Dose distribution reevaluated

·  Plan edited until acceptable by planner

·  MD approval

The International Commission on Radiation Units and Measurements (ICRU) has recommended the use of specifically defined volumes

·  Gross tumor volume: the gross palpable, visible, and or demonstrable extent and location of malignant growth, it is the volume of known disease

·  Clinical target volume: containing the GTV and or subclinical microscopic malignant disease.

·  Planning target volume is a geometrical volume, it has dimensions believed to always contain the CTV taking into account all possible geometric uncertainty such as setup or patient motion.

·  Treated volume; is the volume enclosed by the isodose surface selected as being appropriate to achieve the purpose of treatment.

·  Irradiated volume: the volume of tissue receiving a significant dose (50%) of the specified target dose.

Beams eye view: images reconstructed from CT data that represent the patient’s anatomy and defined volumes from the perspective of the treatment beam.

Virtual simulation: a process by which treatment fields are defined using patient CT image data and treatment unit geometric information. Most of the simulation process is completed without the presence of the patient.

·  The computer makes a virtual patient out of the CT contours

·  Gives the ability to see data in different planes

·  A DRR can be generated

**Both MRI and PET incorporate physiology in the production of images, this can be used to better differentiate between diseased and normal tissues.

Image fusion: combining the images from the different modalities with the CT image

·  Images need to be registered- matched up at common points, such as bony landmarks or implanted fiducials

·  Properly fused images combine the enhanced image capabilities of PET and MRI and the special accuracy of CT.

Digitally reconstructed radiograph: based on acquired CT information. These are images that render a beams eye view display of the treatment field anatomy and areas of treatment interest. These images resemble conventional radiographs.

Room’s eye view: demonstrated the geometric relationship of the treatment machine to the patient. The room’s eye view allows clear visualization of the entrance and exit of the beam through the patient.

·  Help prevent possible orientations of the equipment that could result in collisions with the patient.

Dose volume histogram is a plot of target or normal structure volume as a function of dose.

**The characteristics of an optimal target volume DVH are:

·  High percentage volume at prescribed target dose (adequate target volume coverage)

·  Rapid decrease in volume beyond the prescribed dose (dose uniformity within target)