Dose distributions verification for High dose rate brachytherapy plans by using ionization chambers 2 D array

6618 | P a g e J a n u a r y 2 3 , 2 0 1 6 Dose distributions verification for High dose rate brachytherapy plans by using ionization chambers 2D array R. Moustafa Abdelfattah, N. Ahmed Deiab , M.Hassan Elnaggar, M. Hany Khedr, R. Abdelmoneim Rizk 1 Medical Physicist , Radiotherapy and Nuclear Medicine Department, National Cancer Institute of Cairo University rasha_Amylee@hotmail.com 2 Assistant professror of medical physics Physicist , Radiotherapy and Nuclear Medicine Department, National Cancer Institute of Cairo University nashaatad@hotmail.com 3 Professor of Radiation Oncology, Radiation Oncology department, National cancer institute of Cairo University. mervatelnaggar@hotmail.com 4 Lecturer of Biomedical Physics, Physics Department,Faculty of Science of Helwan University mhkdahawy@yahoo.com 5 Professor of Radiation physics, Physics Department,Faculty of Science of Helwan University rizk1953@live.com ABSTRACT


INTRODUCTION
The efficacy of radiation therapy relies on the accuracy of dose delivery to patients. Proper implementation of a treatment planning system for accurate treatment dose calculations and quality assurance procedures to detect dosimetric errors are of critical importance. Since the Brachytherapy treatment planning systems use simplified algorithmsin their dose calculations, verification of the dose distributions presented by the systems may be of clinical importance. Evaluation of the treatment planning systems in terms of their presented dose distribution can be a part of quality assurance in the clinical practice of brachytherapy.The dose distribution generated by the TPS using the AAPM TG-43 dose formalism [1] is usually compared with the calculations using the Sievert summation [2], Monte Carlo simulation [3], or dose distributions measured with Gafchromic film [4]. Gafchromic film is an excellent tool for dosimetry but involves a consumable cost, requires characterization of both the filmand the scanner, and presents several technical challenges in yielding accurate dosimetric results [5]. Recently, 2D ionization chamber arrays have become increasingly popular for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plan verification. The detector was found to be linear with dose, independent of dose rate,and a suitable device for quality assurance and 2D dose verifications of megavoltage beams.

MATERIALS AND METHOD
In this study, Nucletron (Nucletron, Veenendaal, The Netherlands) MicroSelectron remote-afterloading HDR brachytherapy 30 channels unit (for treatment applicator connection) fed with Nucletron Ir-192 HDR-v2r (new design) radiation source is used to deliver the prescribed radiation dose planned by Oncentra-Master-Plan-Brachy treatment planning system whose calculation algorithm is based on AAPM TG-43 dose formalism. The dosimetric verification is performed using the two-dimensional (2D) ionization chamber array MatriXX Evolution developed by IBA Dosimetry (IBA Dosimetry, Germany). The MatriXX Evolution consists of 1020 vented, plane-parallel cylindrical ionization chambers arranged in a 32 x 32 matrix with a maximum field-of-view of 24.4 x 24.4 cm 2 . The chamber size is 4.5-mm diameter and 5-mm height, center-to-centerdistance is 0.76 cm, active volume is 0.08 cm 3 [6,7]. The detector area is covered with the Nucletron Freiburg Flap Applicator Set (Nucletron BV,Veenendaal, the Netherlands)with catheters such that the detector plane was set to 0.86 cm from the catheter plane [8]. The Freiburg Flap Applicator Set shown in figurer 1 is a flexible mesh style surface mold made of silicone rubber with 36 channels for implant tubeswith a separation of 10 mm and a channel length of 24 cm.

Fig 1: The Freiburg Flap Applicator Set
The Freiburg Flap Applicator is used in conjunction with the OncoSmart flexible implant tubes which are used to guide the source to the points of planned implant.Fixed slabs of RW3 (Perspex) -Slabs made of Water-equivalent material (Goettingen White Water) -were added below and above the 2D-array to provide full scattering conditions.The phantom was scanned on computed tomography (CT) for the treatment planning -figure 2-with 2.5-mm slice thickness and the scanned images are exported to the Oncentra Master Brachy treatment planning system for performing different test plans. Based on the CT data of the phantom, three different plans imitating three different intracavitary brachytherapy treatment applications were calculated by the planning system.The first plan consists of two active channels to imitate the Fletcher GYN applicator set for vault application -figure 3-and planned to deliver 2 Gy at 1 cm (point A) lateral to the central axis mid the two transfer tubes.The second plan consists of three active channels to imitate the Fletcher GYN applicator set for full application -figure 4-and planned to deliver 2 Gy at 2 cm (point A) lateral to the tandem-like transfer tube central axis and at 4 cm lateral to that axis at the top level of the two other transfer tubes. The third plan consists of only one active channel to imitate the Fletcher GYN applicator set for cylinder application -figure 5-and planned to deliver 2 Gy at 2 cm (point A) lateral to the central axis of the cylinder-like transfer tube. The treatment plans were then exported to theconsole of the MicroSelectron HDR afterloader (version 3, Nucletron BV) which contains the Ir-192 sourcefor practical application on the measurement phantom. The ion chamber array was irradiated, setting the position of the detector plane of measurement to correspond with the computed plan. The TPS generated plans were also exported to the VeriSoft software for comparison with measured data. For comparison of dose distributions, both dose planes were normalized to the global maximum dose of the reference matrix (measured data set). The calculated and measured dose distributions were then compared using the Gamma index method [9] where both plans were normalized to the maximum dose ofthe reference matrix (measured data set), this normalization decreases the possibility of miss matching between measured and calculated values hence gamma passing rate results increase. Gamma indexes were evaluated using a dose-difference criterion of 3% and a distance criterion of 3 mm (γ≤1).

Figures 6-11 show comparisons of measured and TPS-calculated dose distributions for the three test patterns.
Comparisons of dose distributions were done using Gamma 2D global method [9] where the passing criterion is γ≤1 (with 3% delta dose and 3-mm distance criteria).Tables 1-3 illustrate the comparison points' statistics for the three test applications. Such that the total number of evaluated dose points for the vault test case (table 1) is 9755, 98.6 % of them (9623 point) passed the criteria of acceptability (3% delta dose and 3-mm distance criteria) and 1.4% of them (132 point) failed it.The total number of evaluated dose points for the full test case (table 2) is 19964,93.6 % of them (18683 point) passed the criteria of acceptability and 6.4% of them (1281point) failed it.And the total number of evaluated dose points for the cylinder test case (table 3)    ; it can be found that as the number of these pixels increases the possibility of having more failing points increases.
More likely to be illustrated in the full test case whose the maximum number of dose points (19964) and the highest failing percentage (6.4%) of its total evaluated dose points, and in the vault test case whose the minimum number of dose points (9755) and the lowest failing percentage (1.4%) of its total evaluated dose points.The relation between the passing J a n u a r y 2 3 , 2 0 1 6 percentage of the comparison and the number of the total evaluated dose points can be interpreted to the comparison matrix area which when is increased (increased number of pixels evaluated) the possibility to have more failing points is increased and vies versa for smaller areas.

CONCLUSION
The array measurement technique has been successfully validated for three different test cases (vault, full and cylinderlike applications) against TPS dose distribution calculations. The 2D chamber array can be applied as a routine quality control toolproviding reliableverification toolof the actual dose distribution delivered by HDRtreatment equipment for the different test cases. The comparisons made led to make a relation between the passing percentage values to the number of the evaluated dose points for each test case and it was found that as the number of these pixels increases the possibility of having more failing points increases.