Dose efficiency measurement phantom in radiotherapy

Abstract

The invention relates to a quality control phantom designed for dosimetric measurements in radiotherapy. Said phantom can perform both dose efficiency and energy determination measurements in high energy photon beams.

Description

[0001] DOSE EFFICIENCY MEASUREMENT PHANTOM IN RADIOTHERAPY

[0002] Technical Field of the Invention

[0003] The invention relates to a quality control phantom designed for dosimetric measurements in radiotherapy. Said phantom can perform both dose efficiency and energy determination measurements in high energy photon beams.

[0004] State of the Art

[0005] Radiation therapy plays an important role in the treatment of cancer. Radiation therapy is a treatment method that uses high-energy rays to destroy the tumour, stop its proliferation, prepare the person for surgery, or prevent tumour formation in cancer treatment. The success of radiation therapy depends on the dosimetric controls of the radiation-emitting treatment devices being performed as required and at the required time intervals. For this reason, it is necessary to verify that the radiation doses given to the patient in radiotherapy centres are within the permitted limits. It is recommended that a standard dosimetric measurement protocol be used by the responsible institutions in these verifications. Quality control tests are performed on the treatment devices in radiotherapy clinics before patients enter treatment. Some of these tests involve mechanical tests and some involve dosimetric tests. Because the accuracy of the dose to be given to the patient is important for the quality of treatment. Radiation dose measurement is performed using phantoms and medical linear accelerators. Medical linear accelerators used in radiation oncology are usually dual energy. In other words, there are more than two photon (x-ray) energies in the treatment. Radiation dose efficiency measurements in linear accelerators are made by placing the ion chamber at different depths in the solid water phantom for each energy. Photon energy determinations are performed by calculating the ratio of the radiation reading value of the ion chamber placed at a depth of 10 cm to the radiation reading value of the ion chamber placed at a depth of 20 cm in a solid water phantom, with measurement conditions kept constant for each energy. For each photon energy, the measurement conditions (phantom thickness) of the solid water phantom and ion chamber must be changed for energy determination and radiation dose efficiency measurements. During this change, as many layers of solid water phantoms with a thickness of 1 cm are added as needed. During this addition, there must be no gap between the phantoms and the position of the ion chamber must not change. Phantoms used for quality control in radiotherapy must have features such as being practically usable, being able to perform many tests with one assembly and being able to perform these tests precisely. The quality of radiotherapy is proportional to the accuracy of the radiation dose given to the patient. Therefore, the quality control tests of these devices are performed separately, mechanically and dosimetrically.

[0006] In the state of the art, the quality of high-energy photon beams is determined by the TPR2010 (tissue phantom ratio) ratio. In this measurement, the dose measured at 20 cm depth is expressed as a ratio to the dose at 10 cm depth. This value gives the energy of the radiation and must be kept at the same value continuously. Daily radiation dose measurements are made with a 0.6 cc cylindrical ion chamber at a depth of 5 cm or 10 cm solid water phantom, depending on the energy to be controlled by the device. For this measurement, a set-up mechanism is established in the solid water phantom with a 10x10 cm2area, ensuring that the source-to- chamber distance (SCD) is 100 cm. After the ion chamber is placed, the ion chamber is connected to the electrometer with a suitable cable in order to take readings. Without changing the set-up, a 10 cm thick phantom is placed on the ion chamber (SCD=90 cm) and 100 monitor units (MU) are irradiated, and the electrometer is read. In the second stage, without changing the set-up, another phantom is placed on the phantom (SCD=80 cm) and 100 MU is irradiated, and another reading is taken from the electrometer. The value obtained by expressing the ratio of these two reading values with each other gives the photon beam quality, that is, the photon energy. It is very important to test the accuracy of the energy in absorbed dose determination measurements in photon beams. Because the beam quality values vary depending on the ion chamber used. This directly affects the absorbed dose calculations. Radiotherapy quality is proportional to the correct determination of the absorbed dose. In the state of the art, water is preferred as a phantom in dosimetric measurements since more than 70% of the human body is water. However, using a water phantom as a phantom in daily quality control dosimetric measurements in clinics is not effective in terms of time. Instead of a water phantom, solid water phantoms such as polymethyl methacrylate (PMMA), RW3 or acrylic, the densities of which are water equivalent or close to water equivalent, can be used.

[0007] In another known case of in the state of the art, during the daily radiation measurement performed every day, the measurement set-up is established with solid water phantoms found in clinics. In particular, the appropriate thicknesses of the solid water phantoms must be brought together, counted and adjusted so that there is no air gap between them. There is a high probability that the thicknesses will be calculated incorrectly during these operations. Although care is taken, there is an air gap between worn phantoms. All of these factors affect the correct measurement of the radiation dose. Changes in the air gap and ion chamber position can increase the maximum error limit of 2% for energy determination and dose efficiency measurements. The increase in this error margin directly affects patient treatment.

[0008] Due to the limitations and inadequacies of the solutions in the state of the art, the air gaps between the phantoms used for quality control in radiotherapy because of their non-monolithic nature, and the incorrect calculation of thicknesses in cases where changes in the ion chamber position are required, and therefore the increase in the margin of error in energy determination and dose efficiency measurements, it has become necessary to make a development in this field.

[0009] Brief Description and Aims of the Invention

[0010] The invention relates to a quality control phantom designed for dosimetric measurements in radiotherapy. Said phantom can perform both dose efficiency and energy determination measurements in high energy photon beams.

[0011] An aim of the invention is to minimise the set-up error in the quality control phantom used in measuring radiation dose and to ensure that radiation dose can be determined more accurately. In said phantom, with the same set-up, energy determinations and dose efficiency measurements of all photon energies in the linear accelerator device can be taken without changing the locations of the solid water phantoms. Said phantom is designed as a single block in a monolithic form without changing the locations of the solid water phantoms.

[0012] Another aim of the invention is to minimise the margin of error resulting from the failure to adjust the phantom parallel to the ground plane in radiation dose measurements. For this reason, 4 adjustable feet are placed under the phantom so that the phantom can be adjusted parallel to the ground plane, and spirit levels are placed at the corners of the phantom to show two different planes.

[0013] Another aim of the invention is to be able to measure backscatter in the phantom during radiation measurement. In the phantom that is the subject of the invention, there is an additional 5 cm phantom thickness under the depth of 20 cm in order to measure backscatter.

[0014] Another aim of the invention is to make the quality control phantom used in radiation dose measurements easily portable. The phantom that is the subject of the invention is designed as a monolithic structure with dimensions of 20 cm (width) x 20 cm (length) x 25 cm (height) and a weight of 10 kg, so it can be easily carried.

[0015] Another aim of the invention is to prevent the problem of ineffectiveness in terms of time, encountered when a water phantom is used as a phantom in dosimetric measurements. For this reason, in the phantom that is the subject of the invention, RW3 is used as a solid water phantom material, which is close to the water phantom equivalent.

[0016] Another aim of the invention is to be able to determine the energy in high-energy photons with the phantom. The quality control phantom in the form of a monolithic block produced from the RW3 (2.1% ± 0.2% T1O2 doped polystyrene (CsHs); density: 1.04 gr / cm3) material subject to the invention can determine the energy in high- energy photons in dosimetric measurements in radiotherapy. Description of Drawings

[0017] Figure 1. View of the phantom (1)

[0018] Figure 2. View of the phantom with the ion chamber (7) and plugs (6) installed

[0019] Definition of Elements / Parts Composing the Invention

[0020] 1. phantom

[0021] 2. holes where the ion chamber is placed

[0022] 3. adjustable feet

[0023] 4. spirit level

[0024] 5. x and y axes marker (centre)

[0025] 6. plugs that close the other two holes except the hole where the ion chamber is located during the measurement

[0026] 7. ion chamber

[0027] Detailed Description of the Invention

[0028] The invention relates to a quality control phantom for use in dosimetric measurements in radiotherapy. Said phantom can perform relative dose measurement and determine the energy in high energy photons.

[0029] Quality control phantom (1) produced from RW3 (2.1% ± 0.2% TiO2 doped polystyrene (CsHs); density: 1.04 gr / cm3) material for use in both absorbed dose measurement and dose efficiency determination for high energy photon beams in radiotherapy comprises:

[0030] - 3 holes (2) where the ion chamber (7) will be placed,

[0031] - 4 adjustable feet (3) placed under the phantom (1) to ensure parallelism with the ground plane,

[0032] - 2 spirit levels (4) placed at the corners of the phantom (1) to show two different axes, - x and y axis markers (5) positioned on the 20 cm x 20 cm area of the phantom (1),

[0033] - two plugs (6) that close the other two holes except the hole where the ion chamber is located during the measurement, and

[0034] - the ion chamber (7) that is placed at a depth suitable for the measurement purpose through 3 holes (2) during the measurement.

[0035] Said phantom is 20 cm (width) x 20 cm (length) x 25 cm (height).

[0036] Said phantom is made of a material with a density equivalent to the density of water (1 gr / cm3) and has a monolithic structure. The phantom weight is 10 kg. Said phantom is designed for use with a 0.6 cc ion chamber.

[0037] There are 3 holes (2) in said phantom where the ion chamber will be placed. One ion chamber (7) is placed at a depth appropriate for the measurement purpose and the other two holes are closed with plugs during the measurement.

[0038] Said phantom is made of RW3 (2.1% ± 0.2% TiO2 doped polystyrene (CsHs); density: 1.04 gr / cm3). Radiation dose measurements are made in water or water-equivalent solid water phantoms. Therefore, the density of the material used should also be close to the density of water. For this reason, RW3, which is the closest material to the density of water, is used in the phantom of the invention. The x and y axes are marked on the 20 cm x 20 cm area of the phantom to match the radiation field. The ion chamber holes used in the phantom of the invention are opened in accordance with the shape of a 0.6 cc cylindrical ion chamber up to the phantom centre level, corresponding to the effective measurement point of the ion chamber. The holes where the ion chamber will be placed (at depths of 5 cm, 10 cm, and 20 cm) are opened along the z axis and also coincide with the centre of the x and y axes. The depths not used in the measurement in the phantom of the invention (two of the three holes) are closed with plugs that is of the same material as the phantom. In other words, the measurement is made in the hole where the ion chamber is placed during the radiation measurement and the other holes are closed with plugs during the measurement. In addition, since only the ion chamber will be replaced on a single phantom, time is saved in radiation measurements. Said phantom can practically control the daily radiation dose efficiency for each high-energy photon energy and can also perform monthly TPR2010 measurements with the same phantom.

[0039] Since said phantom is made of a material with the same density as water, is monolithic and easily portable, it is possible to measure absorbed doses at all energies in the treatment device with the same set-up. The presence of the phantom water level and adjustment screws in said phantom also easily enables the phantom surface to be adjusted parallel to the ground. In addition, with said phantom, without changing the measurement mechanism, the energy accuracy can be determined by simply placing the ion chamber in the appropriate hole in all photon energies in the radiotherapy device without disturbing the set-up mechanism. Said phantom has holes in both 10 cm and 20 cm where the ion chamber can be placed appropriately. In addition, its thickness is designed as 25 cm to account for backscattering. With this phantom, the set-up is adjusted practically, therefore, readings are taken by changing only the location of the ion chamber (after taking the reading at 10 cm, it is placed at 20 cm). After the energy determination of the photon energy is made, the absorbed dose determination can be easily made for photons at both 6 MV and 18 MV energies with the ion chamber placed separately at 5 cm and 10 cm depths in the phantom. Said phantom is designed for dosimetric measurement in absorbed and relative dose measurements of the device. According to the TRS-398 dose measurement protocol, absorbed dose measurement can be made for different high- energy photon beams at both 5 cm and 10 cm depths for different energies. In addition, it can also measure dose at a depth of 20 cm, suitable for TPR2010 measurements for energy quality determination. In addition, there is an additional 5 cm phantom thickness below the 20 cm depth in order to measure backscatter in the phantom during radiation measurement.

Claims

CLAIMS1. Quality control phantom (1) produced from RW3 (2.1% ± 0.2% TiO2 doped polystyrene (CsHs); density: 1.04 gr / cm3) material for use in both absorbed dose measurement and dose efficiency determination for high energy photon beams in radiotherapy, comprising:- 3 holes (2) where the ion chamber (7) will be placed,- 4 adjustable feet (3) placed under the phantom (1) to ensure parallelism with the ground plane,- 2 spirit levels (4) placed at the corners of the phantom (1) to show two different axes,- x and y axis markers (5) positioned on the 20 cm x 20 cm area of the phantom (1),- two plugs (6) that close the other two holes except the hole where the ion chamber is located during the measurement, and- ion chamber (7) that is placed at a depth suitable for the measurement purpose through 3 holes (2) during the measurement.

2. A phantom according to claim 1, wherein It has dimensions of 20 cm (width) x 20 cm (length) x 25 cm (height) and has a monolithic structure.

3. A phantom according to claim 1 or 2, wherein said ion chamber (7) is 0.6 cc.

4. A phantom according to claim 3, wherein the ion chamber holes (2) are positioned at depths of 5 cm, 10 cm and 20 cm.

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