At the position At the position ddmaxmax of maximum energy loss of radiation, the of maximum energy loss of radiation, the number of secondary ionizations products peaks which in turn number of secondary ionizations products peaks which in turn maximizes the dose at that location.maximizes the dose at that location.

The dose is denned (see section on dosimetry) as total energy loss of The dose is denned (see section on dosimetry) as total energy loss of radiation per mass. It can be formulated in terms of the activity radiation per mass. It can be formulated in terms of the activity A(t)A(t) (number of (number of incident particles/second in cases of external beam treatment incident particles/second in cases of external beam treatment N(t)N(t))) and energy loss and energy loss or stopping power or stopping power dE/dx. dE/dx. The total absorbed dose The total absorbed dose D(t) D(t) after a period after a period t t of irradiation of irradiation is expressed in terms of number of particles N(t), total amount of energy lost is expressed in terms of number of particles N(t), total amount of energy lost EERR, , and and irradiated area A:irradiated area A:

with m, V, and with m, V, and as mass, volume, and density of the exposed organs. This as mass, volume, and density of the exposed organs. This results in a absorbed dose results in a absorbed dose D(t,d) D(t,d) after an irradiation period after an irradiation period t t at a certain depth at a certain depth d:d:

Within the area Within the area A A each point at a certain depth each point at a certain depth d d receives receives the same dose the same dose ISODOSE.ISODOSE.

The absorbed dose at a certain depth The absorbed dose at a certain depth d d is directly is directly proportional to the stopping power proportional to the stopping power 1/1/ dE/dx dE/dx !!

AA 75%75%

100%100%

[cm[cm22/g]/g]

The average dose due to energy loss of The average dose due to energy loss of radiation within radiation within a depth a depth d d over a period over a period t t is:is:

The dose is directly proportional to the transfer The dose is directly proportional to the transfer and absorption coefficients which change with depth.and absorption coefficients which change with depth.

The dose distribution is less well defined compared to particle beams.The dose distribution is less well defined compared to particle beams.

Within the area Within the area A A each point at a certain depth each point at a certain depth d d receives the receives the same dose same dose ISODOSE.ISODOSE. Isodose profiles are plotted in terms of the Isodose profiles are plotted in terms of the percentage depthpercentage depth dose %DD dose %DD because absolute dose measurements because absolute dose measurements are difficult. The percentage depth dose is the absorbed dose at a given are difficult. The percentage depth dose is the absorbed dose at a given depth depth d d expressed as a percentage of the absorbed dose at a reference expressed as a percentage of the absorbed dose at a reference depth depth ddmax max along the central axis of the beam.along the central axis of the beam.

In figure above the percentage depth dose at point A is 75 %.In figure above the percentage depth dose at point A is 75 %.

Isodose charts are usually plotted in increments of 10 %. They depend on Isodose charts are usually plotted in increments of 10 %. They depend on the beam geometry and the various absorption effects within the body tissue.the beam geometry and the various absorption effects within the body tissue.

Examples of isodose Examples of isodose profilesprofiles

The isodose profile widens The isodose profile widens rapidly due to wide angle scattering.rapidly due to wide angle scattering.

For electron beam the percentage depth dose increases with depth, For electron beam the percentage depth dose increases with depth, the maximum range depends on the initial energy of the electron beam.the maximum range depends on the initial energy of the electron beam.

For heavy ion beam the profile remains well defined but the For heavy ion beam the profile remains well defined but the percentage depth dose increases rapidly at well localized position due to percentage depth dose increases rapidly at well localized position due to Bragg curve behavior plus decay radiation from on-line produced activities.Bragg curve behavior plus decay radiation from on-line produced activities.

For For -radiation the percentage depth dose peaks at -radiation the percentage depth dose peaks at small depths but ranges deeply into the tissue proportional to the small depths but ranges deeply into the tissue proportional to the absorption coefficient.absorption coefficient.

Cobalt 60Cobalt 60

6 MeV6 MeV 15 MeV15 MeV

4 MeV4 MeV

The angle scattering is small, the beam profile and The angle scattering is small, the beam profile and therefore the isodose profile remains well defined.therefore the isodose profile remains well defined.

A carefully designed treatment plan is necessary to maximize A carefully designed treatment plan is necessary to maximize the dose at the tumor location while minimizing the dose in the the dose at the tumor location while minimizing the dose in the surrounding body tissue! Notice, while tumor might get maximum surrounding body tissue! Notice, while tumor might get maximum dose, the surrounding tissue may be exposed to at least 50 % of it dose, the surrounding tissue may be exposed to at least 50 % of it which may cause problems.which may cause problems.

Dose calculation should consider the following aspectsDose calculation should consider the following aspects

1.1. geometry of treatmentgeometry of treatment

2.2. energy loss effectsenergy loss effects

3.3. straggling and widening of beamstraggling and widening of beam

4.4. backscatter backscatter

Treatment plan needs to be carefully designed, should rely Treatment plan needs to be carefully designed, should rely on careful localization of tumor with modern imaging techniques on careful localization of tumor with modern imaging techniques (CT, MRI). Dose and dose losses should be simulated (three (CT, MRI). Dose and dose losses should be simulated (three dimensional simulation).dimensional simulation).

AA BB

CC DD

Typically, the prescribed dose depends on the size of the Typically, the prescribed dose depends on the size of the tumor and the specific organ which has been effected. The tumor and the specific organ which has been effected. The prescribed total doses range between 40 Gy to 70 Gy.prescribed total doses range between 40 Gy to 70 Gy.

For external beam therapy For external beam therapy the dose will be administered the dose will be administered over a period of five to six weeks with a daily dose ranging between over a period of five to six weeks with a daily dose ranging between 1.9 and 2.2 Gy/day (five days a week).1.9 and 2.2 Gy/day (five days a week).

The treatment time depends on the intensity of the radiation source!The treatment time depends on the intensity of the radiation source!

For brachytherapy For brachytherapy a radioactive source is implanted in a a radioactive source is implanted in a location near the tumor. Therefore the radiation is constant until location near the tumor. Therefore the radiation is constant until the desired dose has been reached. the desired dose has been reached.

ExampleExample

For calculating the dose to be delivered geometrical aspects For calculating the dose to be delivered geometrical aspects and backscattering have to be taken into account. Critical is the source-and backscattering have to be taken into account. Critical is the source-surface distance surface distance SSD SSD which determines intensity losses between which determines intensity losses between source and body.source and body.

d is the depth of the tumor location!d is the depth of the tumor location!

A dose rate of A dose rate of DRDR11 = = 1.17 Gy/min delivered over a distance of 1.17 Gy/min delivered over a distance of SSDSSD11+ + d = d = 80.5 cm reduces over a distance of 80.5 cm reduces over a distance of SSDSSD22 + + d = d = 100 cm to:100 cm to:

Substantial losses can occur by back scattering, the Substantial losses can occur by back scattering, the backscattered radiation will increase the dosage in the surrounding backscattered radiation will increase the dosage in the surrounding body tissue. Therefore a further modification has to be introduced by body tissue. Therefore a further modification has to be introduced by subtracting the amount of backscattered radiation subtracting the amount of backscattered radiation BS BS in the body tissue.in the body tissue.

The backscatter is defined as the ratio of The backscatter is defined as the ratio of scattered dose at depth scattered dose at depth d d of body tissue to the scattered of body tissue to the scattered dose in air at the same length dose in air at the same length d.d.

To optimize treatment often multiple beam treatment is applied.To optimize treatment often multiple beam treatment is applied.

This approach maximizes the dose at the location of the This approach maximizes the dose at the location of the tumor and minimizes the dose in the surrounding body tissue.tumor and minimizes the dose in the surrounding body tissue.

Alternative options are the introduction of wedges which allow Alternative options are the introduction of wedges which allow beam attenuation and absorption to shape the radiation field for optimal beam attenuation and absorption to shape the radiation field for optimal treat ment.treat ment.