1. Introduction

The dosimetry of small field sizes is considerably unlike from that for standard radiotherapy field sizes. In low density medium similar to the lung, small fields have important perturbations that are energy and density dependent. According to the abovementioned challenges, selecting a detector with good performance in small fields is difficult. The necessary properties of a desired detector are high spatial resolution, high signal, low energy dependence, water equivalence, high stability and easy to use clinically. Certainly, there is no standard dosimeter for a small field because no detector has all the aforementioned properties. Radiotherapy plays a leading role for the treatment of cancer patients in all over the world. External beam radiotherapy includes high energy photon and electron beam linear accelerator, tele-cobalt therapy, cyclotron-based proton beam therapy, reactor-based boron neutron capture therapy, low energy X-ray therapy, etc. The outcome of the radiotherapy is highly dependent on how pre visional dose deliver to the tumor which should not exceed 5% of the prescribed dose including all types uncertainties involved in the treatment procedure such as dosimetry, treatment planning and dose stability of the treatment unit. This implies that dose measurement should be accurate which lies within ±3.5% as per ICRU recommendation. In conventional radiotherapy with linear accelerator, radiation fields are characterized between 4 cm × 4 cm to 25 cm × 25 cm. The field size below 4 cm × 4 cm is considered as small field. The dosimetry of small-field is a challenging issue. The increased use of small fields in modern radiotherapy has created the demand for more standardization of dosimetry procedure and better understanding of these in non-reference small field conditions. For absorbed dose to water measurement of photon and electron beam above 4 × 4 cm² field sizes are already established and several protocols for dosimetry are already established by various international organizations such as IAEA (TRS-277, TRS-381, TRS-398), AAPM (TG-21, TG 51), DIN-6800-2. The deviation of dose measurements between the protocols lies within ±1.2%. To minimize the uncertainties still many laboratories has emphasized their research regarding the improvement of precision of dosimetry. What value of precision is required in radiotherapy!! Recently, IAEA has declared to revise their latest dosimetry protocol TRS-398. In photon and electron beam dosimetry, ideally a detector (volume: 0.125 cc to 0.6 cc) with high sensitivity, good spatial resolution, low energy dependence, good stability and tissue equivalence should be utilized and reference condition is also established. The absorbed dose to water measurement of aperture less than 4 × 4 cm² is a critical issue. Small cancerous lesions are typically treated with photon and electrons. Depending upon the size, shape and location of the lesion, the field aperture, commonly referred to as cutout, is custom-made using Cerrobend. Accurate dosimetry for such small fields is challenging due to the loss of lateral electronic equilibrium within the field. This can result in a shift of d_max towards the surface and other modifications of the depth dose curve, as well as a change in the beam profile characteristics at depth. Small (narrow or sub-centimeter) beam apertures are used for delivery of prescribed dose to patients by advanced photon beam radiotherapy techniques (intensity modulated radiotherapy, IMRT; image-guided radiotherapy, IGRT; stereotactic radio surgery/radiotherapy, SRS/SRT). Both standard and specialized (e.g. Cyber Knife and Tom therapy) medical electron linear accelerators with high resolution MLCs as well as specialized tele isotope machines (e.g. Gamma Knife) deliver treatments by using small photon fields ([1] [2]). The use of small photon fields is almost a pre-requisite for high precision localized dose delivery to delineated target volume, sparing of organs at risk, and escalating the dose to the tumor for improved control of the disease (e.g. prostate). Thus, small-beam apertures are important for fulfilling the clinical goal of radiotherapy. However, such fields have inherent characteristics of charge particle disequilibrium and high-dose gradient, making dosimetry measurements challenging. A small photon field is generally defined as the one having dimensions smaller than the lateral range of the charged particles that contribute to the dose deposited at a point along the central axis of the beam. The main limitations of most dosimeters are insufficient spatial resolution, water nonequivalence, and energy dependence. In megavoltage (MV) photon beams from linear accelerators, small field conditions occur as a result of two scenarios. Firstly, as the collimator opening is made smaller the entire photon source is no longer visible from the measurement point and, secondly, the size of the radiation field becomes small in comparison to the maximum lateral range of secondary electrons. As a consequence, in a small field, a reduction in beam output is observed on the beam central axis, as well as a widening of the penumbra in the transverse direction as a result of overlapping penumbrae [3]. In recent years, with the growth in prescribed use of high doses per fraction in radio surgery and dose escalation cancer treatments, the dosimetry of high dose-rate FFF beams from modern study [4]. Hence, the dosimetry of small photon field presents many challenges, which are related to source occlusion, lateral electronic disequilibrium, and the choice of the detector. Radiation detectors such as diamond, pencil, diode detectors and thermos-luminescence dosimeter could be a leading device the leading devices to meet the current desirable characteristics.

The micro-Diamond has shown excellent characteristics, such as the small spatial resolution and stable response against the beam energy, dose rate and temperature ([5] [6]), although the cost is higher than for diodes ([7] [8]).

2. Design and Methods

The research program will thus include the following major phases of investigation:

• Selection of ionization chamber and its characteristics (semi-flex chamber, diode, pinpoint, and diamond chambers)
• Selection of phantom such as blue phantom or Alderson Rando Phantom or slab phantom for absorbed dose determinations
• Determination of absorbed dose to water for various field sizes (0.1 cm × 0.1 cm to 25 cm × 25 cm) using TRS-398
• Calculation of the total scattering factors for various field sizes
• Effect FB and FFF in treatment planning system
• Calibration of TL dosimeter
• Measurement of irradiated TLD by Harshaw TLD reader
• Analysis of experimental data for various measurement
• Theoretical analysis with Geant4/Fluke/MCNP code

The outline of the research program is more or less provided in the above. The final details or essential modifications etc. could be worked out as per advice made by supervisors and according to the facilities available (see Figure 1).

 

 

Figure 1. Diagram of medical radiation dosimetry and calibration equipment.

3. Purpose of the Design

This study clarified coplanar and non-coplanar beams using different types of detectors with the effect of angular dependence on small-field dosimetry. (Kohei kawata, Tomohiro ono, Hideaki hirashima {..}Manabu Nakata) ([9] [10]).

4. Types of Radiation and Range of Beam Qualities

This code of practice provides a methodology for the determination of absorbed dose to water in the low, medium and high energy photon beams, electron beams, proton beams and heavy ion beams used for external radiation therapy. Actually, the range of radiation qualities covered in this report are given below (for a description of the beam quality index see the corresponding sections):

1) Low energy X-rays with generating potentials up to 100 kV ana HVL of 3 mm A1 (the lower limit is determined by the availability of standards); 4. The boundary between the two ranges for KV X-rays is not strict and has an overlap between 80 kV, 2 mm A1. In terms of overlap region, the various for absorbed dose determination given in site 8 or 9 are equally satisfactory, and whichever is more convenient should be used.

2) Medium energy X-rays with generating potentials above 80 kV and HVL of 2 mm A1.

3) ⁶⁰Co gamma radiation.

4) High energy photons generated by electrons with energies in the interval 1 – 50 Mev, with TPR20, 10 values between 0.50 and 0.84.

5) Electrons in the energy interval 3 – 50 Mev, with a half-value depth, R50, between 1 and 20 g/cm².

6) Protons in the energy interval 50 – 250 Mev, with a practical range, Rp, between 0.25 and 25 g/cm².

7) Heavy ions with Z between 2 (He) and 18 (Ar) having a practical range in water, Rp, of 2 to 30 g/cm² (for carbon ions this corresponds to an energy range of 100 Mev/u to 450 Mev/u, when u is the atomic mass unit.

5. Quantities and Symbols

Almost of the symbols utilized in this code of practice are identical to those used in major advance radiotherapy over the last few years has been the climb up use of proton and heavy ion irradiation convenient for radiation therapy. Practical dosimetry in this fields is also based on the use of ionization chambers that may be provided with calibrations each according to Air kerma and in terms of absorbed dose to water, therefore the dosimetry methods developed for high energy photon and electrons can also be applicable to heavy ions and protons. On the other extreme of the range available teletherapy beams lie KV X-ray beams, and for these the utilize of standards of absorbed dose to water was introduced in IAEA technical reports series no 277 (TRS-277), and only a few are new in the context of standards of absorbed dose to water. For completeness, a summary is provided here for all quantities of relevance to the different criteria used in this code of practice. There are a lot of symbols utilizes these methods. For instance,

C_pl: Material dependent scaling factor to convert ranges and depths measured in plastic phantoms into the equivalent values in water. Basically, this applies to electron, proton and heavy ion beams.

C_sda: Continious slowing down approximation.

Eo, Ez: An electron beam of energy at the pantom surface and at depth z respectively. Unit: Mev.

H_pl: Difference between electron fluence in plastic and water equivalent depth.

H_VL: Half-value layer, used as abeam quality index for low and medium energy X-ray beams.

K_elec: Calibration factor of an electrometer.

K_pol: Factor to correct the response of an ionization chamber for the effect of a change polarity of the polarizing voltage applied to the chamber.

K_s: Factor to correct the response of an ionization chamber for the lack of complete charge collection (due to ion recombination).

M_Q: Reading of dosimeter at quality Corrected for influence quantities other than beam quality. Unit: C or rdg.

N_k,Qo: Calibration factor in terms of air kerma for a dosimeter at a reference beam quality Qo. Unit: Gy/C or Gy/rdg.

P_cav: Factor that corrects the response of an ionization chamber for effects related to air cavity, predominantly the in -scattering of electrons that makes the electron fluence inside a cavity different from that in the medium in the absence of the cavity.

P_DD: percentage Depth Dose.

P_wall: Factor that corrects the response of an ionization chamber for the non-medium equivalence of the chamber wall and any waterproofing material.

Q: General symbol indicates the quality of a radiation beam. A subscript “O”, i.e. Qo, indicates the reference quality used for the calibration of an ionization chamber or a dosimeter.

R_dg: Value, in arbitrary units, used for the reading of a dosimeter.

R₅₀: Half-value depth in water (in g/cm²), used as the beam quality index for electron beams.

R_p: Practical range (in g/cm²) for proton, electron and heavy ion beams.

R_res: Residual range (in g/cm²) for proton beams.

R_cy1: Cavity radius of a cylindrical ionization chamber.

SAD: Source axis distance.

SCD: Source chamber distance.

SOBP: Spread out brag peak.

SSD: Source-surface distance.

TMR: Tissue maximum ratio.

TPR_20,10: Tissue phantom ratio in water at depths of 20 and 10 g/cm², for a field size of (10 × 10) cm and SCD of 100 cm, utilized as the beam quality index for high energy photon radiation.

u_c: Combined standard uncertainty of a quantity.

W_air: The mean energy expended in air per ion pair formed.

Z_max: Depth of maximum dose (in g/cm²).

Z_ref: Reference depth (in g/cm²) for pantom measurements. When specified at Zref, the absorbed dose to water refers to Dw, Q at the inter section of the beam central axix with plane defined by Zref.

And also has Dw, Q, Ki, Kh, KQ, Q, KTP, Mem, ND, air, ND, W, Qo, Pcel, Pdis, Peff, PQ.

6. Temperature Dependence

By changing water temperature with a flattening filter (WFF), which temperature was evaluated by irradiating a 6 MV Photon beam. Actually, the micro silicon and diode E detectors were located at a 10 cm below from the entrance window of the water phantom model GRI-7670A (Toyo-Medic, Tokyo, Japan) [10] that was scheme to measure lateral beams. According to the field size, gantry angle and source to surface difference (SSD) were 100 × 100 mm², 270 degree and 90 cm respectively. At first, measurements were repeated by replacing little bit cold water with hot water, cold water was poured into the phantom. In terms of the course of the experiments, from 11.5 to 31.3-degree Celsius water temperature was boosted. Because diode E was much lower than the response of the microsilicon, the detector readings were normalized at 21.5 degree Celsius and relative methods were compared.

1) Saini AS, Zhu TC (temperature dependence of commercially available diode detectors. Med Phy 2002; 29: 622-630

2) Scherf C, Peter C, Moog J et al., Silicon diodes as an alternative to diamond detectors for depth dose curves and profile measurements of photon and electron radiation. Strahlenther Onkol 2009; 185: 530-536

7. Expected Results and Impact

Figure 2 illustrates the temperature dependence of the Diode E and micro-Silicon detectors. The Diode E exhibited a positive correlation of the response to temperature, and the difference between 11.5˚C and 31.3˚C was 5.06%. When calculating the linear regression, the variation of the readings was 0.26%/˚C. In contrast, the micro-Silicon diode yielded very small variations within the range from −0.33% to 0.06%. Figure 3(a) depicts the dose-response linearity for various detectors. Values were normalized at 100 MU. All detectors showed good linearity (R₂ = 1.000 for all detectors). Figure 3(b) shows the readings of each detector divided by the irradiated dose. Most detectors, including the Farmer-type ionization chamber, showed large variations for irradiation < 10 cGy, likely due to the instability of the beam output. For irradiation ≥ 10 cGy, all detectors showed excellent linearity within 0.5%.