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Quality Assurance of Superficial Radiotherapy X-Ray Machine

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Study the Quality Assurance of Superficial Radiotherapy X-ray Machine using some Techniques

Abstract

The aim of the present paper is to investigate some factors affecting on quality assurance of superficial radiotherapy x-ray such as reproducibility of dose out put, linearity, reference, entrance and some skin cancer doses were measured for hand, face and nose. Examinations of these factors are studied using Farmer dose master connected with 0.6 cc ionization chamber which was placed on the couch inside the selected field and contacted with an applicator to avoid any air gap inside a field size and 10 exposures were made. The measurements were carried out using an operating condition such as 50kV, 90mAs, 15mA 0.10 min, 6 sec. The tolerance limit for the reproducibility is equal to 0.2% , which is lower than the tolerance limit as mentioned. in American Association of physics in Medicine, AAPM . The relative Error for entrance skin dose equals 3%. The quality assurance tests of superficial radiotherapy x-ray machine are measured and compared with the international tolerance.

Key words: Quality Assurance, Farmer dosemaster. .0.6 cc ionization chamber

Introduction

The principle goal of quality assurance of superficial x-ray machine is obtained accurately and timely assessment. The secondary goal is to maximize of radiation exposure and obtain high radiation quality. This can be assess by performance the x-ray machine by optimum operating parameters such as reproducibility of dose out put, linearity, Reference , Entrance and Exit dose measurements according to code safe of practice for use x-ray in medical diagnosis (NRL, 1994). [1].

Many associations reported work concerning quality assurance of superficial x-ray machine like , AAPM, [2,3] that describes quality assurance protocol for superficial radiotherapy x-ray equipment at the therapeutic technologist level. American Association of Medical Physicists in Medicine ~AAPM, [4] presents a new protocol, developed by the Radiation Therapy Committee Task Group 61, for reference dosimetry of low- and medium energy x rays for radiotherapy and radiobiology (40 kV<tube potential<300 kV). It is based on ionization chambers calibrated in air in terms of air kerma.(4) . Austerlitz C, et.al, 2008 mentioned that the beam outputs presented percent differences ranging from -13 to +25%.[5]. Mehran Goharian et.al , 2010 mentioned that The measured output showed an increase of up to 7.3% relative to the neutral position (0° in-plane and 0° cross-plane) over the possible range of angles of in-plane rotation for 75 kVp (half-value layer, HVL = 1.84 mm Al)[6].

Modern research indicated that the continued successful use of kilovoltage X-ray units for superficial therapy and their different design compared with linear accelerators indicates that specific advice for this type of equipment is required. Useful guidance on quality control of kilo-voltage units is also given in Klevenhagen.et.al [7] and Williams.et.al [8]. Paevans .et.al [9] mentioned that the applicator, system interlocks and dose monitor performance that used in check of Gulmay D3300 kilovoltage X-ray therapy unit were satisfactory and the tube leakage was below the UK recommended maximum (air kerma rate 300 mGy/ h at 5 cm from the tube head). The quality assurances of therapeutic x-ray are based on the Basic Safety Standard –BSS[10] and International Commission of Radiological Protection where they mentioned that there is no limit for medical exposure but they ensure that medical exposure have to decide by consultation between medical authorities and professional bodies.

The aim of the present paper is to investigate some factors affecting on quality assurance of superficial radiotherapy x-ray such as reproducibility of dose out put, linearity, Reference, Entrance and Exit dose measurements.

Material and Method

The X-strahl superficial x-ray machine was investigated for some factors of quality assurance such as reproducibility and linearity . Reproducibility is one component of the precision of a test and reported as a standard deviation. Reproducibility of dose out put of X-strahl machine was measured with Farmer Dose master connected with 0.6 cc ionization chamber, which was placed on the couch inside the selected field and contacted with an applicator to avoid any air gap inside a field size and 10 exposures were made. The measurements were carried out using same operating condition such as 50kV, 90mAs, 15mA , 6 sec. The reproducibility Pz was calculated based on NEWZELAND Radiation Laboratory, NRL protocol (J.L.Plotti , 1995).[11].

Where: SD is the estimator of standard deviation of a series of measurements dose [mGy], Zap is the means value of the measured [dose [mGy]. Extensive measurements were made to assess of changes in mAs on reproducibility and linearity of radiation output. It formed over a range of clinical settings. Calibrated ionization chamber used to measure output expressed as μGy per mAs, at a set distance, without backscatter.

Linearity

The linearity of superficial x-ray machine was studied using farmer dose master that connected with 0.6 cc ionization chamber above Perspex phantom.

The linearity was checked using the next equation that stated by NEWZELAND Radiation Laboratory, NRL protocol (J.L.Plotti , 1995) .[11].

X1-X2/X1+X2 0.1

Where X1 and X2 are two successive readings.

Applications of superficial x-ray machine

Measurement the Surface Radiotherapy Dose for Hands, Face and Nose were conducted using suitable applicator for each case. The applicator for hand treatment was 5 cm diameter, 15 FSD , the applicator for face treatment was 3 cm diameter , 15 FSD and the applicator for nose treatment was 1.5 cm diameter, 15 FSD.

Results and Discussions

Reproducibility

Dose reproducibility for repeated doses of superficial x-ray machine was measured using farmer dosemaster connected with 0.6cc ionization chamber on the surface of Perspex phantom as shown in table 1. In the present work the maximum value was 4.404 , the minimum value was 4.374 and mean doses and standard deviation of the measurements was 4.391 ± 0.008 . The machine reproducibility was found to be 0.2% which is lower than the tolerance limit as mentioned. in NEWZELAND Radiation Laboratory, NRL protocol (J.L.Plotti , 1995) .[11].

Table 1 the Dose reproducibility for superficial radiotherapy machine using an operating condition of 50kV, 90mAs, 15mA , 6 sec. For 0.6 cc Ionization chamber.

Dose, cGy

0.6 cc

Run No.

4.374

1

4.386

2

4.388

3

4.394

4

4.394

5

4.386

6

4.390

7

4.391

8

4.404

9

4.401

10

4.391 ± 0.008

Mean ± Standard Deviation

 

 
 

Linearity of X-ray Machine

The dose output was measured as a function in miliamper second. These measurements were recorded at 25 cm source to detector distance using special applicator of diameter 25 x 25 cm2 as shown in table.2. The linearity of the superficial x-ray machine was given a result lower than 0.1 that means it lower than the tolerance level of the American Association of physics in Medicine AAPM[3] and NEWZELAND Radiation Laboratory, NRL protocol (J.L.Plotti , 1995) .[11]. Linearity was accepted that means the x-ray tube of a machine is calibrated.

Table 2. Linearity of superficial X-ray Machine

Dose, , cGy

mAs

Time, sec

Setting

23

30

6

50 kV, 5 Ma

30

60

12

45

150

30

72

240

48

90

300

60

.

Measurement the Surface Radiotherapy Dose for Hands, Face and Nose

The X-ray tube of the X-strahl machine was located in middle of the room surrounded by one meter from each side. The focus to skin distance, FSD for each applicator under use was adjusted for surface radiotherapy treatment such as hand, face and nose of tissue equivalent phantom. The Ionization chamber was adjusted for operating conditions of the X-ray machine as filter 1, 50kV, 5 mA, 0.2 min, and 0.2 mmAl for different applicators. The Surface dose for skin, nose and hand were recorded as shown in table 4. X-strahl -100 x-ray system low energy X-ray for treatment a wide range of superficial dermatological condition including: basal cell carcinoma, Squamous cell carcinoma and dermatological conditions including psoriasis. Orthovoltage units continue to be used today to treat superficial lesions; in fact, they were practically the only machines treating skin lesions before the recent emergence of electron therapy. The maximum dose from any of these low-energy units is found on the surface of patients; thus, skin becomes the dose-limiting structure when treating patients at these energies. The depth at which the dose is 50% of the maximum is about 7 cm. Therefore the physician can be calculate the treatment dose for superficial x-ray therapy for each case. Since the total dose is equal to dose rate times the time , so, the time of treatment can calculate easily .

Table 4. Entrance patient dose measurements for face, nose and hand..

ESD, (cGy)

Case

Applicator dimensions

 

56.68 ± 0.307

Face cancer

3 cm diameter , 15 FSD

241.2 ± 2.15

Nose cancer

1.5 cm diameter, 15 FSD

58.8 ± 0.669

Hand cancer

5 cm diameter, 15 FSD

Conclusion

This work concluded that The machine reproducibility dose out put was found to be 0.2% so patient risk will decrease. Linearity was lower than 0.1 which is lower than the tolerance limit of the international publications. The maximum dose from any of these low-energy units is found on the surface of patients; thus, skin.X-strahl machine treating skin lesions, hand, face and nose. The treatment time for hand, face, and nose cancer calculated via the surface patient dose measurement.


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