Ten left-sided breast cancer patients underwent a Computed Tomography (CT) scan with 2 mm slice thickness. All sets of CT cuts were transferred to Focal system. Tumor volumes such as gross target volume (GTV), clinical target volume (CTV), and planning target volume (PTV) [9] as well as Organs at Risk (OARs) had been delineated by the physician on each axial slice, then CT slices were transferred to the treatment planning system (TPS).

Three-dimensional treatment inverse planning system (CMS) XiO software (release 4.64) was used to carry out dose calculation for all patients under study by Super position algorithm.

XiO employs a conjugate gradient algorithm to search for the minimum value of the cost function, which was a member of the gradient descent family of optimization methods, where variables were changed systematically, as opposed to stochastic methods where variables were changed randomly. The beam parameters were adjusted by changing the intensity distribution, which was physically achieved by creating a series of multileaf collimator (MLC) field segments and assigning a monitor unit (MU) weighting to each. The series of field segments that make up a beam were called a leaf sequence. To control this conversion process the acceptable percentage difference between the desired and the actual dose distribution can be specified. Currently a 2% match was specified, and this typically leads to the generation of 10 to 20 segments per field. The minimum segment Monitor Units (MUs) and field size can also be specified.

The beam arrangement was determined by the size and location of the tumor. Major five fields with equal values of dose were used to plan each patient. IMRT plans were generated using commercial inverse planning software [10] [11] . The beams are spread around the target with equispace and to avoid the opposing fields an odd numbers of the treatment fields were used. IMRT dose constraints for both target and OARs were entered to Xio TPS [12] -[14] . Also some conditions were given to the inverse TPS which were the minimum, goal and maxi- mum radiation doses for the target volume, the power and weight of each structure (Figure 1). Several trials

were taken place by Inverse TPS to achieve the IMRT dose constraints and a homogeneous dose distribution [11] [12] .

The treatment plans can be qualitatively and quantitatively evaluated by many tools which are already included into Xio TPS. In our study, we used two main treatment plan evaluation tools; the visual slice-by slice review of the treatment plans using isodose lines distribution as a qualitative evaluation for the treatment plans which is important to know the location of the hot and cold areas and review dose distribution to both of the clinical target (in our study is the left breast) and OARs (in our study are ipsilateral lung, heart, contralateral breast ) and the Dose Volume Histogram (DVH) was generated to evaluate the dose to the different structures in different treatment plans and it can be used as a quantitative evaluation for the treatment plans [15] -[17] . However, DVHs do not provide spatial information such as the location of the high- and low-dose regions (“hot” and “cold” spots) inside the volume of interest (VOI) [18] [19] .

That unusually delineated normal volume and additional portion beside it are delineated as a single structure. We called that as (The Non-Taken Into-Account Volume (NTIAV)) and considered it as an OAR so we referred to it by (OAR4) (where the number 4 refers to that that volume is the organ at risk number 4 after the three usually contoured organs at risk (ipsilateral lung, heart, and contralateral breast).

According to the next two axial cuts groups (a) and (b) (Figure 2) for ten patients respectively and as compared in the two methods of being the NTIAV was non-taken into dose constraints account and when it was taken, a large partition of it was covered with some of high value isodose lines like the 90%, 80%, and 70% of the prescribed dose in the case of being it not taken into account as it is shown in Figure 2(a). But the same volume was significantly spared in the case of being it taken into dose constraint account as it is shown in Figure 2(b).

The axial cuts in groups (a) and (b) for patients no.4, 5 and 6, showed a very important reason for that significant difference between the two plans. It is noticed in (a) that isodose lines passed away sparing the left lung but distributed across the NTIAV. This means that the treatment plan didn’t consider with that Non-Taken Into- Account Volume and distributed the dose across it in order to achieve the other given dose constraints before. But in (b), it is noticed that the plan took that volume into its dose calculation considerations so it spared it to be not covered with high isodose values.

High values isodose lines can also cover a portion of the liver, a portion of colon in addition to that one of the

stomach (as it is showed in Figure 2(a) patients no.7, 8, 9 and 10). These three portions in addition to other covered surrounded normal tissue volume were contained into the NTIAV delineation to be taken into consider- ation in plan dose calculation process to give those result showed in Figure 2(b) patients no.7, 8, 9 and 10.

Figures 3(a)-(c): showed the Mean and Standard Deviation (± SD) of minimum, maximum and mean doses re- ceived by PTV in Method 1 when OAR2 was taken into consideration in IMRT dose constraints and in Method 2 when it wasn’t, are (43.06 ± 12.6, 43.09 ± 13), (104 ± 4.22, 102.9 ± 3.20) and (92.3 ± 0.69, 91.9 ± 0.84) re- spectively. It is noticed that the all values are approximately equal which means that there is no significant dif- ference between the two methods with respect to PTV. But for OAR4, there is a very significant difference be- tween the two methods which is shown in Figures 3(d)-(f) where the Mean and ± SD of minimum, maximum and mean doses received by OAR4 in the two methods are (7.75 ± 3.58, 13 ± 5.12), (72.46 ± 12.22, 94.52 ± 2.86) and (28 ± 7.29, 55 ± 6.96) respectively.

Also, in all DVHs (Figure 4(a)), OAR4 (NTIAV) received lower dose in Method 1 than in Method 2 al- though of being the dose received by PTV not significantly affected in the two methods and Figure 4(b), showed that all OARs also not significantly affected in both two methods.

Also, we noticed that when the NTIAV is delineated but not taken into account in IMRT dose constraints it gave the same dose distribution as when that volume is not delineated (see Figure 5(a), Figure 5(b)). So, the delineation of that volume does not has any effect as long as it is not being taking into consideration in IMRT dose constraints or dose calculation process.