Assessments and Dosimatric Evaluation of Matching Fields Technique in the Treatment Cranio-Spinal Cancers
Ali Ahmed Abulmajeed1, Fezaa Shalal Neda2, Hayder Hamza Alabidi3
1M.Sc. Medical Physics Al-Mustansiriya University, College of Medicine/Iraq.
2Assist. Professor Ph.D in Medical Physics, Dept. of Physiology, Medical Physics Unit, Al-Mustansiriya University, Medical College, Iraq.
3Clinical Oncologist, Director Baghdad Center of Radiotherapy and Nuclear Medicine/Iraq
*Corresponding Author E-mail: 4modern@gmail.com, Fezabaghdad@yahoo.co.uk, Hydr_hamza@yhaoo.com
ABSTRACT:
Radiotherapy is one of the main methods of cancer treatment (often with chemotherapy and surgery) and is generally assumed to be 50 to 60% of cancer patients will benefit from radiotherapy, where radiation therapy is done using the Linear accelerator device. medulloblastoma, a primitive tumor growing in the cerebellum, is one of the most sensitive tumors in the childhood brain to radiotherapy. The most common malignant tumor in children is cancerous tumor with an overall infection among children aged 0-19 from 16 to 20 percent of all brain tumors in hospitals. Radiotherapy is an essential means of treating these tumors and, after surgery, radiation therapy has a significant impact, This study was conducted to understand the impact of the possibilities that could occur during the process of radiotherapy through systemic and irregular errors when set-up the patient for treatment on the occurrence of cold spots and hot spots and the risk of dose at both the craniospinal at treatment. Objectives: Aim of The Work To evaluate the dosimetry homogeneity abutting junction between photon-photon fields in treatment cranio-spinal cancers. Methodology: The study included ten of patients with medulloblastoma cancer, where the radiation treatment planning do for the medulloblastoma using two adjacent beams photon-photon and take the possibility of error (0, + 1, + 2,-1,-2 mm) in area between the fields which can Occur during the treatment process and caused of overlap or gap between this beams (hot and cold areas), Statistical analysis was performed on; maximum dose percentage received by the planning target volume (PTV max dose %) and matching beams dose percentage and mean volume of the hot area and the cold area. Results and Discussion: Disclose results of radiotherapy planning technique that two matching beams Photon-Photon, the overlap by 1 mm has been causing significant change in dosage in matching area more increase than the dose prescribed by oncologist this increase up to 10% of the prescribed dose and the volume of this hot area more than 8 cubic cm, with increased fields by 2 mm in the dose has increased by 20% of the dose prescribed. The opposite happens when the fields is reduced to (-1, -2 mm) where we find that dose decrease occurring in the region and there didn’t complete coverage of the area to be treated, causing what is called a gap (the region receives a dose of less than 95%). It reveals the result of adjacent of two beams photon-electron in radiation treatment planning for patients with nasopharynx, that an electron might enable us to reduce the dose to organ at risk (OAR) such as the spinal cord but the overlap between the photon and electron by (1, 2 mm) may cause an increase Hot spot in a dose up to 130% and the volume of hot spot is big relative to the size of the PTV. As to reduce the area by (-1, -2 mm) may result in reduced hot area or may cause a gap or cause a gap with stayed the hot area while because of the use of two adjacent photon and electron
KEYWORDS: Three-Dimensional Conformal Radiation Therapy (3DCRT); medulloblastoma cancer; radiotherapy techniques; matching field; adjacent beams; cranio-spinal cancer treatment.
INTRODUCTION:
Radiation therapy is to provide a specific dose of radiation to the size of the tumor specified with the least possible harm to surrounding healthy tissue [1], Radiotherapy treatment is done by use of x-ray beams between 4 MV and 25 MV, although the linear accelerators which can produce both photon and electron beams [2]. Medulloblastoma, comprising 20% cases of childhood brain tumors [3,4], Cranial-spinal irradiation (CSI) is a fundamental challenge The technique of radiation therapy is fraught with difficulties caused by need to ensure adequate and full coverage of the brain [5]. The medulloblastoma treatments are carried out with adjacent radiation fields. where the length of the target volume is greater than the maximum field size of linac or fields need to different orientations to treat different parts of the patient [6]. In the matching fields should produce a uniform dose across the junction of 2 adjacent fields, the matching is made relatively simple by using an asymmetric jaw positioned at the beam central axis. [7], Examples of the application of asymmetric collimation for abutting fields are the 3-field head and neck treatments, (using 2 parallel opposed laterals to brain fields and a posterior spinal cord field). These misalignments of systematic jaw cause error throughout the course of treatment and can yield underdoes or overdoses in the treatment volume [8]. The American Association of Physicists in Medicine (AAPM) Task Group 40 in their report number 46 recommends a tolerance of 2 mm for symmetric jaw positioning [9], While the American Association of Physicists in Medicine (AAPM) Task Group 142 in their report recommends a tolerance of 1 mm for symmetric jaw positioning [10].
Aim of This study is to evaluate the dosimetry homogeneity abutting junction between photon-photon fields in treatment cranio-spinal cancers, the comparative plans are evaluated by study dose at the junction as result of overlap or underlapping of matching fields to evaluate the tolerance of digital display of our asymmetric collimators.
This study included ten craniospinal patients will be delineated and the radiation dose will be optimized using XIO (CMS version 5.00) treatment planning system, 3D conformal plans will be performed utilizing beams matching technique, The Patients were placed in the supine position lies on a carbon-fibre couch top with neck extended with a vacuum-form body immobilizer and thermoplastic mask to support the head and shoulders.
Anterior and lateral tattoos are placed [11], CT scanners supply anatomical images related to the local electron density, measured in the so-called Hounsfield units, with a typical spacing between 2 and 5 mm. these images, which are essential for accurate dose calculation, allow to delineate contours of the patient for both tumor volumes and normal tissues. This step, called ‘volume definition, is necessary for 3D treatment planning [6].
craniospinal irradiated CSI consists of isocentric, parallel-opposed, lateral cranial fields of the 6-MV, with a collimator rotation to match the divergence of the posterior spinal beam, and one or two direct, posterior spine fields depending on the patient’s height, the 10-MV vertical beam at gantry angle 180°, set at 100 cm (SAD), If two posterior spinal fields are required, a thoracic (T) spine and a lumbar (L) spine they are matched on the spinal cord [11] [12] [13].
This study included applying five different matching fields arrangement, in all the five arrangement the cranial fields were fixed by technique half-beam-block and the change done by the matching posterior field size depending on the jaw Y of collimator (0 mm, ± 1 mm, and ± 2 mm) position [14].
Statistical Analysis:
Analysis of data was carried out using the available statistical package of SPSS-22 (Statistical Packages for Social Sciences- version 22). Data were presented in simple measures of frequency, percentage, mean, standard deviation, and range (minimum-maximum values). The significance of difference of different means (quantitative data) were tested using Students-t-test Statistical significance was considered whenever the P value was equal or less than 0.05.
RESULTS AND DISCUSSION:
a. Dose distribution
Figure (1): The mean of the percentage dose in the five plans with diffiernts field size (-1,-2,0,+1,+2)
The result of this study showed that the value of dose percentage at the match line which was recorded through three readings in the match area of the two beams and took out the mean of it for all five potential and the maximum dose of plans with filed size (0, +1, +2), Where we find through (fig 1)
It was clear that the mean value of PTV maximum dose percentage show a significant difference among the five plans by the changed of filed size. Where at the no gap or overlap plan the dose of the matching, when the two asymmetric fields are prefect abutted the dose distribution in the match line was uniform and the dose percentage at the match line was 99% of the prescribed dose and the maximum dose of PTV was under 107% of the prescribed dose.
Where the jaw of extended by 1 mm over the other beam (increase the field size of the beam 1mm) causing an overlap at the field junction, the over dose is more than the previous readings and the rate of increase varies between 8%-10%. This result of our study is agreement with the study of [14] and [15] who found that at the 1 mm overlap causing a dose increase of up to 109% and noted that the dose increased by 10% from the dose that was read in case there is no gap or overlap. While the decrease the field size by (-1mm) the underdose or gap between the two fields causing reduced dose percentage approximately to about -12% of the prescribed dose [7]
The increased the size of the domain by +2 mm; we saw increase in dose in the matching and maximum dose, which indicates that we have exceeded the prescribed and permissible dose to a large extent, which determines that the hot spot does not exceed the dose of 107% of the prescribed dose, at the 2 mm overlap causing a dose increase of up to 8% of the prescribed dose [16] ,while the decrease the field size to the (-2mm)
The dose in the matching line was about 76% of the dose [14]
b. Volume of hot and cold area
The volume of the hotspot or cold spot as result of the changing field size (fig 2)
It show there was no any of hot or cold spot at the zero case where there no overlap or gap but by the change of the filed size to (+1mm) the hot area become more than 1cm of volume for the 110% of the prescribed dose, while the change to (+2mm) of filed size the dose reach to 120 with volume of hot area (0.84mm) the International Commission on Radiation Units and Measurements in the report ( icru report 62 and 50) that hot spot any area received more than 107% of the prescribed dose and more than 15 mm in diameter [17] , while the volume of cold area (-1mm) gap under V95% more than 9 cm3 and at the (-2mm) the volume of cold area under V95% is 16 cm3 [18]
CONCLUSION:
The radiation treatment with matching field technique may increase or decrease of the dose in the matching area if not make sure the safety and efficiency of the machines and taking readings of field size of jaw, multileaf collimator or correctly as well as couch movement and angles to both gantry and collimator and templates for electron so we should do quality assurance and quality control to ensure that treatment has been properly
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