**8. Electron density**

The electron density is the measure of the probability of an electron being present at a specific location. It is calculated from its mass density and its atomic composition.

#### **8.1 Comparison of Hounsfield units and relative electron densities of organs**

The Hounsfield Unit (HU) and relative electron density of bone, fat, air cavity, bladder, and rectum in Computed Tomography (CT) images of a heterogeneous phantom and an actual patient were measured and has been given in **Table 1**. All the measurements were calculated by using Computed Tomography (CT) scanner console in terms of mean and stander deviation due to density variation in different CT slices. For the actual patient, a CT image of one patient was taken.

According to the results obtained from the Computed Tomography (CT) images of a heterogeneous pelvic phantom, relative electron densities for bone, fat (wax), air cavity, bladder (water), and rectum (borax powder) were 1.632, 0.896, 0.159, 1.037, and 1.051, respectively. On the other hand, relative electron densities for bone, fat, air cavity, bladder, and rectum were 1.335, 0.955, 0.158, 1.039, and 1.054, respectively, in an actual patient Computed Tomography (CT) image.

#### **8.2 Radiation treatment plan creation**

Various Intensity Modulated Radiation Therapy (IMRT) plans for prostrate patients were generated on the Monaco planning system. Plans were created with 5, 7, 9, and 12 coplanar 6MV photon beams. Couch and collimator angles were kept


#### **Table 1.**

*Comparison of Hounsfield units and relative electron densities of organs.*

as 0°for all plans. Calculation parameters such as grid spacing, fluence smoothing, and statistical uncertainty were 0.3 cm, medium, and 1% per plan respectively. Furthermore, the Monte Carlo algorithm was used for the plan optimization, and all the plans were generated in step and shoot mode.

#### **8.3 Gamma analysis**

The difference between measured and planned dose distribution is evaluated using quantitative evaluation methods. The Quality Assurance (QA) procedures of Treatment Planning System (TPS) narrated by Van Dyk et al. subdivides the dose distribution comparisons into high and low dose gradients regions, each with a different acceptance standard. In regions of low gradient, planned and measured doses are compared directly, with an acceptance tolerance placed on the difference between the measured and calculated doses. On the other hand, in high dose gradient regions, a small spatial error, either in measurement or calculation, results in a large dose difference between measurement and calculation. Therefore, in the region of high dose gradient, the concept of a Distance-To-Agreement (DTA) distribution is used to determine the acceptability of the dose calculation. The Distance-To-Agreement (DTA) is the distance between a measured data point and the nearest point in the calculated dose distribution exhibiting the same dose. The Dose-Difference (DD) and Distance-To-Agreement (DTA) evaluations complement each other when used as determinants of dose distribution calculation quality.

#### **8.4 Pre-treatment verification**

Two kinds of phantoms were chosen for absolute dosimetry of plans already done for the treatment. First one is heterogeneous pelvic phantom developed for radiotherapy quality assurance. Second one was Delta4 phantom (Scandidos, Uppsala, Sweden). CT scan of both the phantoms was done and images were transferred to the Monaco planning system.

After the complete optimization of the Intensity Modulated Radiation Therapy (IMRT), the plans were exported to a pelvic phantom and Delta4 phantom for a pretreatment verification. After position verification, all Intensity Modulated Radiation Therapy (IMRT) plans were delivered by a linear accelerator.

### **9. Absorbed dose calculation**

There are various methods to achieve accuracy in dosimetry and they are based on International Atomic Energy Agency (IAEA) recommendations published in technical reports series number 277 [17] and 398 [18].

In this study, absorbed dose at reference depth was calculated according to the Technical Reports Series No. 398 (TRS398) of the International Atomic Energy Agency (IAEA) [18] using the relation:

$$\mathbf{D} = \mathbf{M}\_{\mathrm{Q}} \times \mathbf{N}\_{\mathrm{D,W}} \times \mathbf{K}\_{\mathrm{Q},\mathrm{Qo}} \times \mathbf{K}\_{\mathrm{T,P}} \times \mathbf{K}\_{\mathrm{S}} \times \mathbf{K}\_{\mathrm{pol}} \tag{2}$$

where, MQ is the electrometer reading (charge), ND,W is the tor, kQ,Qo chamber specific factor, kT,P temperature–pressure correction factor, Kpol polarity correction factor, KS ion recombination factor.

*Intensity Modulated Radiation Therapy Plan (IMRT) Verification Using Indigenous… DOI: http://dx.doi.org/10.5772/intechopen.102710*

#### **9.1 Chamber calibration factor, ND,W**

The ND,W is the calibration factor in terms of absorbed dose to water for a dosimeter at a reference beam quality *Qo.* The chamber calibration factor ND,W for the ionization chamber (PTW 0.6 cm3 ; TN 30013–006353) is 5.386 x 107 Gy/C as obtained by BARC, Mumbai.

#### **9.2 Chamber specific factor, KQ,Qo**

The KQ,Qois a factor that corrects for the difference between the response of the ion chamber in the reference beam quality Qo used for calibrating the chamber and in the actual user beam quality Q. The subscript *Q*o is omitted when the reference quality is Co-60 gamma radiation i.e. KQ always corresponds to reference quality Co-60. The chamberspecific factor KQ,Qois 0.99 for the ionization chamber (PTW 0.6 cm3 ; TN 30013–006353).

### **9.3 Temperature pressure correction factor, KT,P**

The mass of air in the cavity volume is subject to atmospheric variations. The correction factor to be applied to convert the cavity air mass to the reference conditions is given by:

$$\mathbf{K}\_{\mathbf{r},\mathbf{p}} = \frac{\left(27\mathbf{3}.2 + T\right)}{\left(27\mathbf{3}.2 + T\_{\rm O}\right)P} \frac{P\_{\rm O}}{P} \tag{3}$$

where, P and T are the cavity air pressure and temperature at the time of the measurements, and PO and TO be the reference values (generally 101.3 kPa and 20°C).

#### **9.4 Polarity factor, Kpol**

The polarity factor is used to correct the response of an ionization chamber for the effect of change in polarity of the polarizing voltage applied to the chamber. It can be accounted for, by using a correction factor

$$\mathbf{K}\_{\rm pol} = \frac{\left| \mathbf{M}\_{\rm \ast} \right| + \left| \mathbf{M}\_{-} \right|}{2 \mathbf{M}} \tag{4}$$

where, M+ and M− are the electrometer readings obtained at positive and negative polarity, respectively, and M is the electrometer reading obtained with the polarity used routinely (positive or negative).

#### **9.5 Ion recombination factor ks**

The incomplete collection of charge in an ionization chamber cavity owing to the recombination of ions requires the use of correction factor ks*.*

$$\mathbf{k}\_s = \mathbf{a}\_0 + \mathbf{a}\_1 \left(\frac{\mathbf{M1}}{\mathbf{M2}}\right) + \mathbf{a}\_2 \left(\frac{\mathbf{M1}}{\mathbf{M2}}\right)^2\tag{5}$$

where, ao = 2.337, a1 = −3.636, a2 = 2.299 and M1 and M2 are the electrometer readings at the polarizing voltages V1and V2, respectively, measured using the same irradiation conditions. V1 is the normal operating voltage and V2 is a lower voltage; the ratio V1/V2 is equal to two.

In a pelvic phantom, the dose for each plan was measured by PTW UNIDOS E electrometer connected with 0.6 cm3 ion chambers using Eq. 1 according to International Atomic Energy Agency (IAEA) published, Technical Reports Series-398 (TRS 398) protocol. These measured doses were compared with doses planned on the treatment planning system (TPS).

For Delta4 phantom, TPS calculated dose fluence was compared with measured dose fluence using the gamma evaluation method with critically acceptable criteria of 3 mm Distance-To-Agreement (DTA) and 3% Dose-Difference (DD). Before the evaluation of an Intensity Modulated Radiation Therapy (IMRT) plan, two more measurements were done by delivering 100 cGy with a 10 × 10 cm field at gantry angles of 0° and 90° in order to check the phantom for positional corrections and linear accelerator output constancy.

**Table 2** shows the planning parameters, including number of fields, segments, and monitor units, and the percentage variation between planned doses and measured doses for each test case using pelvic phantom.

The gamma analysis results of each test case, including Dose-difference (DD), Distance-To-Agreement (DTA), and Gamma Index passing rates, are presented in **Table 3**.


#### **Table 2.**

*Percentage variation between planned dose on treatment planning system and measured dose on linear accelerator using heterogeneous pelvic phantom.*


#### **Table 3.**

*Result of dose difference, distance to agreement and gamma index using Delta4 phantom.*

*Intensity Modulated Radiation Therapy Plan (IMRT) Verification Using Indigenous… DOI: http://dx.doi.org/10.5772/intechopen.102710*
