ACOTACIÓN A LAS SINGULARIDADES HISPÁNICAS

For assessing the quality of the beam irradiation plan, during optimization, cu- mulative dose-volume frequency distribution, in the form of histograms, usually known as dose-volume histograms (DVH) can be plotted by the TPS. It graphi- cally summarizes the simulated radiation distribution within a volume of interest of a patient which would result from a proposed radiation treatment plan. These DVH permit comparing rival treatment plans for a specific patient by clearly quan- tifying the uniformity of the dose in the target volume and hot spots in adjacent normal tissues or organs. However, because of loss of positional information in the volume(s) under consideration, it should not be the sole criterion for plan evaluation.

From the DVH it is also possible to evaluate several dosimetric parameters, in order to quantitatively analyze the dose distributions, in what the coverage and homogeneity inside the target volume and OAR are concerned. The dose values were extracted from the DVH, analyzing each structure, according to the volume that receives a certain dose, or the dose that reaches at a percentage of volume, relating a posteriori with the tolerance estimated dose.

4.5.1 Evaluation of the PTV doses

• V95% - PTV volume that receives, at least 95% of the prescribed dose;

• V107% - PTV volume that receives, more than 107% of the prescribed dose;

• Dmax - maximum dose the PTV receives;

• HI - homogeneity index;

• CI - conformity index;

• CN - conformation number.

Coverage of the target volume by the 95% isodose curve, V95%

For all the patients and techniques, the PTV percentage volume, that was covered by the 95% isodose was analyzed, according to ICRU [34, 35] recommendations. The ultimate goal is that the 95% isodose encompasses the total PTV, which is difficult to achieve in WBI due to the shape of the breast.

Evaluation of higher doses in the dose distribution

High doses or hot spots, are doses above 107% of the prescribed dose, often ob- tained in breast dose distributions. Dmax, the point of maximum dose in the

distribution is also determined. One of the goals of the dosimetrist is to decrease these doses as much as possible, being sometimes not possible to have no doses higher than 107%, as recommended by ICRU50. Having such high doses also means the dose distribution is not homogeneous.

Heterogeneity and Conformity Indexes

To evaluate the quality of the dose distribution within the PTV, the following indexes were assessed:

Heterogeneity Index, HI It is a measure of the degree of heterogeneity of the prescribed dose (95% isodose) of the plan, defined as the ratio between the minimum dose in the hottest 5% and 95% of the total PTV volume:

HI = V5%

V95%

(4.3)

Conformity Index The conformity index (CI) is a measure of how the pre- scribed dose fits the contoured PTV: the closest to 1, the most conformal the dose distribution is. It is determined as the ratio between the VRI (volume covered by

the reference isodose) and the target volume (TV).

CI = VRI

T V (4.4)

This conformity index does not take into account whether the VRI is surrounding

the PTV and its efficacy is therefore limited.

Conformation Number Due to the limitations of the CI, another conformity index was calculated according to the definition provided by van’t Riet et al [243]:

CN = T VRI T V

T VRI

VRI

(4.5)

, where T VRI is the target volume covered by the reference (95%) isodose. The

closest to 1, the most conformal is the dose distribution.

4.5.2 Evaluation of the OAR doses

In all the patients, the contour of both lungs, heart and CLB were performed and were used to assess the doses that reach these specific OAR. The following dose parameters in the OAR were analyzed:

Ipsilateral lung

The lung right beneath the irradiated breast is a volume that can easily receive high doses, due to the breast shape and its localization. The following doses were extracted from the DVH:

• Dmean, the mean dose in the lung;

• V10%, the lung volume that receives at least 5 Gy (10% of the prescribed

dose);

• V20%, the lung volume that receives more than 10 Gy (20% of the prescribed

• V40%, the lung volume that receives at least 20 Gy (40% of the prescribed

dose);

• V80%, the lung volume that receives more than 40 Gy (80% of the prescribed

dose).

Contralateral lung

The right lung, contralateral to the PTV being treated is expected to only receive residual dose. The following parameters were assessed:

• Dmean, the mean dose in the right lung;

• D5%, the maximum dose that is in 5% of the lung;

• V5%, the lung volume that receives at least 2.5 Gy (5% of the prescribed

dose).

Heart

The heart is a sensitive organ and high doses should be avoided. The following parameters were investigated:

• Dmean, the mean dose in the heart;

• V20%, the heart volume that receives more than 10 Gy (20% of the prescribed

dose);

• V50%, the heart volume that receives more than 25 Gy (50% of the prescribed

dose).

Contralateral breast

The contralateral breast is often in the beam path of medial tangential fields and it is required that these receive as low dose as possible to avoid second breast cancer in the other breast. The parameters assessed for the CLB were:

• V10%, the CLB volume that receives more than 5 Gy (10% of the prescribed

dose);

• V4%, the CLB volume that receives more than 2 Gy (4% of the prescribed

dose).

Body without PTV

The external contour of the body was considered and the PTV was subtracted. The goal is that this (Body-PTV) volume receives as low dose as possible, meaning that the dose should be confined to the PTV. The parameters assessed for this volume were:

• Dmean, the mean dose in the Body-PTV;

• V80%, the Body-PTV volume that receives more than 40 Gy (80% of the

prescribed dose);

• V40%, the Body-PTV volume that receives more than 20 Gy (40% of the

prescribed dose);

• V4%, the Body-PTV volume that receives more than 2 Gy (4% of the pre-

scribed dose).