Nov - Dez - English

reatment

with

intensity

modulated

radiation

therapy

(IMRT)

for

breast

cancer

ssoc

ras

2014; 60(6):508-511

509

ced disease (tumor size > 5 cm, more than three lymph

nodes affected, and more).

From the standpoint of the existing RT techniques,

conventional planning (2D) is performed based on two

dimensions (planar images such as X-ray) with dose dis-

tribution calculation in a single plane (contour) of the pa-

tient. The total volume irradiated is not considered, that

is, the dose variation outside this contour is ignored (no

accurate volumetric measurement of dose distribution).

The combinations of the radiation beams are simple and

there is a need for good margin coverage, in order to en-

sure complete coverage of the treatment target volume.

Thus, critical structures (organs at risk) are often inclu-

ded in the treatment field, which contributes greatly to

increased toxicity.

In the late 1980s, with the improvement of hard-

ware and planning systems, it was possible to provide

tools capable of assisting the evaluation of treatments

that have become fully three-dimensional (conformal

technique - 3D).

In 3D RT for the breast, dose distri-

bution is calculated over the total volume irradiated,

with tissue inhomogeneity corrections, considering

the contour of the patient’s chest at different levels.

The measurements of heart volume and lung volume,

which are irradiated through field entries, are known

for the calculation of dose-volume histograms. This

is an important tool because it helps to assess the ho-

mogeneity of planning, as well as measures the dose

to be received by each organ during treatment delivery.

Ultimately, it is the tool used to determine whether a

treatment is prohibitive due to the high chance of side

effects, or if it is acceptable on account of greater spa-

ring effect for the organs at risk.

As previously said, 3D RT offers better distribution of

the prescribed dose at the target volume. However, dose

deposition in tissues using the 3D technique is practically

the same as in 2D RT. Thus, in the scope of treatment, the

intensity of radiation is the same in each treatment beam,

that is, the dose is uniformly delivered to both the tumor

and normal adjacent tissues in the treatment target area.

In order to improve the intensity of non-uniform

beams, a technique of beam intensity-modulated RT

(IMRT) was developed. For this, an inverse planning is

used, consisting in first defining the organs at risk and

target volumes, and the dose that each structure should

receive. As a result, a non-uniform deposition of dose

is obtained within treatment area, which is achieved by

dividing the field into several sub-fields. In the case of

breast cancer, in which the incidence of treatment beams

is arranged tangentially to the chest wall, conventio-

nal beams end up generating inhomogeneous dose dis-

tributions in the breast, particularly in areas of lower

thickness. Therefore, the treated area lacks dose homo-

genization, while IMRT, which usually allows a ‘non-

-uniform deposition’ of the dose at the treatment tar-

get, offers exactly the opposite: the homogenization of

the dose in the breast.

Importantly, in practice, for the treatment of breast

cancer with radiotherapy, the so-called IMRT involves the

adoption of two distinct strategies:

the use of inverse planning, as mentioned above;

the use of a computerized planning system capable

of evaluating the dose distribution in the overall vo-

lume. Through visualization of high-dose regions,

field segmentation is carried out manually using col-

limation blocks or a multileaf collimator, resulting

in a more homogeneous dose distribution (no need

to use reverse planning).

dose

distribution

for

breast

irradiation

superior with

intensity

modulated

radiation

therapy

(IMRT)

compared

conventional

and

conformal

radiotherapy

IMRT is the modality that provides best dose coverage at

the treatment target (breast) compared to the conformal

and conventional techniques (

3,4

In addition, IMRT significantly reduces the dose de-

livered to organs at risk. The dose reduction in the con-

tralateral breast can reach 50%, which reduces the like-

lihood of radiation-induced breast cancer especially in

young women (

5,6

The same was noted with regard

to other structures such as the heart and lungs (ipsila-

teral and contralateral), which can be associated with a

reduced risk for chronic lung and heart disease (

7-14

This benefit was also demonstrated in patients under-

going mastectomy and adjuvant radiation therapy for

plastron (

Recommendation

Intensity-modulated radiation therapy (IMRT) is a mo-

dality that provides better dose distribution in breast ir-

radiation.