Analyzing Planning Techniques for Whole Brain Radiotherapy

Aims: This study compares data between the Field in Field planning and Wedge planning techniques to figure out which technique has better dose coverage and distribution for PTV, and, if using FiF technique for whole brain treatment, how many beams will have better plan. Methods: 56 patients, who need to radiate whole brain with 30 Gy/10 fractions, have been selected for this study. Four plans have been made for each patient (FiF1—one subfield per field plan, FiF2—two subfields per field plan, FiF3—three subfields per field plan, and a Wedge plan). Results: The results of Field in Field plans including Compare dose distribution on the transverse CT slice, plan evaluation using DVH, number MU of plan, D max , HI, HTCI, D maxPTV , D meanPTV . Volume of PTV with the dose over 105% prescribed dose, dose of organ at risk, and Quality Assurance (QA) plan, are better than those of Wedge plan. Con-clusions: Plans using Field in Field technique has better coverage, is more homogeneous in dose distribution than plan using Wedge technique. When using Field in Field technique for whole brain radiotherapy, using three subfields per field has better result than two subfields per field and one subfield per field.

scribed dose, and minimum doses, which affect organs at risk. The aim of this research is to figure out: 1) Whether using planning technique in whole brain treatment Field in Field or using Wedges planning technique is better for the patients and 2) When using Field in Field technique for whole brain treatment, how many subfields in field are better for patients [1]- [8].
The wedge, which is used in radiotherapy, is a wedge-shaped piece of lead or lead alloy. It is placed in the field to create the slope of the radiation intensity in the field. The thick end of the wedge is called the wedge heel; the thin end of the wedge is the wedge tip. The angle of the wedge is defined as the angle between the 50% isodose line and the line perpendicular to the field axis. The wedge angle is usually in the range from 10˚ to 60˚ [2].
There are three types of wedges currently in use: physics wedge, automatic wedge, and dynamic wedges. The physics wedge is the one removable by the LINAC operator. The automatic wedge is inserted and removed from the field by the remote control. The dynamic wedge is used to create a change in the intensity of the photon beam, which is similar to using the wedge by moving the accelerator's jaw while LINAC emits radiation. The wedge we used in this research is dynamic wedge [2]. Wedges are used for two main purposes: 1) To compensate the slope of the patient's body surface or density of tissue; 2) To reduce hotpot when using nonparallel field pair [2]. Field in Field planning technique is one of the most popular 3D treatments planning technique all over the world. This planning technique also has become more and more popular in Vietnam. Regarding research on Field in Field planning technique, Zoran Stefanovski and his colleague have published the article "Advantages of the technique with segmented fields for tangential breast irradiation [4], and G. Yanas and other researchers have researched on comparing the chest wall dose between Field in Field plan and the traditional 3D planning technique [2].
The Field in Field planning technique is used to create one or more subfield in the primary field. These subfields have the same parameters: Field size, gantry angle, collimator angle, and couch angle. The subfield differs from primary field in the MLC position, and the MU numbers that are shot in the subfield, are usually smaller than the MU number LINAC fires in the primary field. Therefore, in order to apply the Field in Field technique, required facilities must include an accelerator with MLC and a treatment planning software that has the Field in Field planning feature. The Field in Field planning technique is also used for two main purposes: 1) To reduce the dose at the hotpot and 2) To increase the dose at the coldpot shaped due to the slope on the patient's body surface and changes in tissue density in the patients' body by creating subfield with MLC that mask hotpot and open MLC at coldpot in the primary field or due to not using parallel pair. The use of subfield in the primary field therefore will help to decrease dose in hotpot and increase the dose for coldpot in the plan without the need to use wedges or bolus [2].

Materials and Methods
All patients of Hanoi Oncology Hospital receiving whole brain radiotherapy from February 2017 to September 2018 were selected for this study. There were totally 54 selected patients including 38 males and 16 females, age ranging from 18 years old to 75 year old. They were all diagnosed with lung cancer plus brain metastasis. All CT images of these patients required no scattering due to metal objects; the slice thickness is 5 mm; all have contoured PTV, left eye, right eye, left len, and right len. Plus, all patients have been prescribed 30 Gy per ten fractions.

Research Methodology
First step: Each whole brain treatment patient is made four plans: One Field in Field plan that has one subfield in a main field (FiF1), one Field in Field plan that has two subfields in a main field (FiF2), one Field in Field plan that has three subfields in a main field (FiF3), and one plan using Wedge technique (W). After that the verification plan is made for every plan as shown in Figure 1.
Second step: Using Dose Volume Histograms (DVH) lines, 2D (two directions) dose distribution, Homogeneity Index (HI), Healthy Tissues Conformity Index (HTCI), dose parameter of PTV, and dose parameter of organ at risk to compare four plans.
Third step: Quality Assurance (QA) is made for all plans using the Delta 4 Scandidos equipment. In the software of the QA equipment we set 3% Dose Dif- The QA results of four plans of each patient then are compared with each other.  left eye (D meanLE ), max dose of the right eye (D maxRE ), and mean dose of the right eye (D meanRE ) as shown in Figure 3. The mean value and standard deviation of each parameter are plotted in the same graph.

Data Collection
HI is the parameter used to evaluate the homogeneity of dose distribution.
The ideal value of HI is zero. Therefore, the smaller HI value of plan is, the more homogenous dose plan is (8).  95%

Results
a) Compare dose distribution on the transverse CT slice By observing and comparing the dose distribution on each transverse CT image of four plans of the same patient, Figure 4 showed that 95% isodose line of plan using Field in Field technique covers PTV better than 95% isodose line of the plan using Wedge technique.
b) Plan evaluation using DVH When comparing DVH of four plans of the same patient, the authors realize that the volume of whole brain, that has the dose higher than the prescribed dose, decreases orderly W, FiF1, FiF2, FiF3; and the volume of whole brain, that has the dose lower than the prescribed dose, also decreases orderly W, FiF1, FiF2, and FiF3 as shown in Figure 5.
c) Number MU of plan [9].            Figure 11. Volume of PTV with the dose over 105% prescribed dose of four plan types FiF1, FiF2, FiF3, and W.

Discussion
Since human's head has a globular shape, if the 3D whole brain radiotherapy does not use the Field in Field planning technique or the Wedge planning technique, there will be coldpot in the center of the head, and some hotpots in the skull bones. When using the Wedge planning technique, these hotpots can be eliminated, but the coldpot is still an unsolved issue. However, when using the Field in Field planning technique, both the coldpot and the hotpot are solved with the subfield. It is because the PTV of Field in Field plan is covered with the 95% isodose line better than of the Wedge plan as we saw in the Results section part a) Compare dose distribution on the transverse CT slice and b) Plan evaluation using DVH. In addition, the Field in Field plans have less hotpots than the Wedge plans does, as proved in d) Results of plan D max , g) D maxPTV , D meanPTV results and h) Volume of PTV with the dose over 105% prescribed dose. According to those results, the dose distribution of the Field in Field plans is more homogeneous, covers better than the one of the Wedge plan. It can be seen in b) Plan evaluation using DVH, e) HI results, f) HTCI results, and j) The QA results of plan.  Regarding the organs at risk, even though they are outside of the beam field size, they may be affected by the radiation leakage from the head of LINAC and by the scattering radiation. That explains the similar doses of the organs at risk of the Field in Field plans and the Wedge plan.

Conclusion
The collected results and above data analysis prove that in the whole brain treatment, using the Field in Field planning technique ensures more homogenous and better covered dose distribution than using the Wedge planning technique. Therefore, it is recommended for medical physicists to choose the Field in Field planning technique over the Wedge planning technique for whole brain treatment. Plus, when using Field in Field technique planning for the whole brain treatment, using three subfields in a main field is better for patients than using two subfields or one subfield in a main field.