Absorbed dose measurement methods

is the best technique for absolute and direct measurement of absorbed dose.

It is based on the measurement of the temperature rise of a given volume element of this instrument when it is exposed to a source of radiation.

also called ferrous sulfate dosimetry, is one of the most useful chemical dosimeters in existence. This dosimetry technique depends on the oxidation of ferrous ions (Fe2+) to ferric ions (Fe3+) by ionizing radiation

Fricke dosimetry also called as

as been applied to measure relative dose distributions in an anthropomorphic polystyrene breast phantom having cork lungs, simulating a radiation therapy treatment with 6 MV opposing tangential beams. ... These results demonstrate that film dosimetry is a useful tool for treatment planning verification.

provides a real advantage to radiation therapy departments in that it allows rapid and easy calibration of output at the end of treatment cones.

Dosimetry Parameters

Define as the ratio of the dose rate at the depth maximum dose for the reference field size to the reference field size.

It represents points of equal dose.

It is define as the ratio of the dose at a specific point in tissue or in a phantom to the dose at the same distance in the beam at a reference depth, usually 5 cm.

It is define as the ratio of the dose at a specific point in tissue or in a phantom to the dose at the same distance in the beam at a reference depth, usually HOW MUCH

It is the ratio of the dose at a given point in a medium to the dose at the same point in the free space

Is a special case of TPR where the reference depth is chosen to be at Dmax.

It is the absorbed dose at a given depth expressed as a percentage of the absorbed dose at reference depth on the central axis of the field.

It is the ratio of the scattered dose at a given point in a medium to the dose in air at the same point.

It is really just a variation of SAR and, like SAR, is mainly used in calculation of scattered dose in phantom or tissue.

Is defined as the ratio of dose on the central axis at Dmax to the dose at the same point in air.

Output measured without a scattering phantom, sometimes referred to as in-air, increases with increase in field size.

Typical Field Arrangement

Superficial targets

Important factors to consider in choosing single field

Probably the most commonly used field of arrangement.

Often used to treat metastatic disease in various sites.

Frequently employed in treating large fields.

Also often used during a portion of disease of the brain, pelvis, head and neck, lungs, pancreas, and extremities.

Often used when the depth of the target is uncertain.

The target volume extends throughout a particular area treated.

Multiple angled fields are possible to set up.

More reproducible

Risks of tumor miss are reduced.

The incidental irradiation of large volume of normal tissues situated within the fields

High dose areas can be avoided by using a multiple field arrangement in which all beams encompass the target include different areas of surrounding normal tissues.

Certain beam angles are prohibited due to the presence of critical organs in those direction

The setup accuracy of a treatment may be better with what?

The acceptability of treatment plan depends not only on the dose distribution but also on:

Can be designed to produce uniform dose distributions.

What field is box technique

Box technique is for what organ

Three field technique is for what organs

The used of these is use to alter the dose distribution in beam.

Originally designed to be used in the treatment of small shallow tumors..

Rotation therapy

Arc Treatment

Combining Fixed and Moving-beam Technique.

Treatment preparation

The gross palpable or visible/demonstrable extent and location of malignant growth.

Is a tissue volume a demonstrable GTV and/or subclinical microscopic malignant disease must be eliminated.

Contains the tissues that are to be irradiated to a specific dose according to a specified time-dose pattern.

Is the volume enclosed by an isodose surface, selected and specified by radiation oncologist as being appropriate to achieve the purpose of treatment.

Is the volume of tissue that receive a dose considered significant in relation to tissue tolerance.

Types of beam modification

Types of beam modification devices

Field blocking shaping filters

Refers to the material whose interactions with radiation beam mimic those tissue.

Bolus devices

Bolus of a thickness equal to the depth of maximum dose eliminates WHAT of megavoltage photon beams.

It can also be used to compensate variations in contour surface contour to eliminate air gaps in cavities.

Skewed dose distribution.

Placed in the head of treatment unit, thus modifying the radiation beam to accommodate the contour of the patient.

Compensator devices

The dimension and shape of the compensator must be adjusted to account for

As the distance between the skin and compensator increase the thickness ratio what?

As the distance between the skin and compensator what? the thickness ratio decreases

Compensator consideration

It is designed to change the isodose curve relative to the beam axis at specified depth to the patient. Wedge reduce the dose in areas of overlap between fields that have hinge angles. Less than 180 degrees.

Sizes of wedges

Wedges are made of

Wedge Placed how much from the skin surface

Wedge Placed 15 cm from what

Individual wedges - used in what?

used in cobalt

Wedges

In wedge x and y is what

Special beam modifiers where shadow trays made from lucite are kept at a certain distance from the skin

Allow to transmit of predetermined percentage of the treatment beam to a portion of the treatment field and may be used throughout the course of the treatment.

Used to shape photon or electron fields, take several forms. Materials range from spent uranium to lead to lead alloys.

Used to shape photon or electron fields, take several forms. Materials range from spent what

Full shielding blocks are constructed to transmit how much of the original beam

Cerrobend (lipowitz metal) melting point is what?

Its melting point 165 degrees F (74 degrees Celsius)

Bismuth transmit how many percentage

Leadtransmit how many percentage

Tin transmit how many percentage

Cadnium transmit how many percentage

Shielding blocks thickness

Shielding blocks 9.4 g/cm3 at how many degrees

Shielding blocks how much at 20 degrees

Two types of shielding blocks