Optimization of three dimensional planning dosimetric in breast phantom for match region of supraclavicular and tangential fields

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1 Original Article Optimization of three dimensional planning dosimetric in breast phantom for match region of supraclavicular and tangential fields ABSTRACT Aim: Complex geometry of breast tissue causes perturbation in dose distribution. This problem can beget overdose or under-dose points in match region of three fields. The aim of this study is to create dose homogeneity distribution in match region between tangential and supraclavicular fields (SCF) with Gafchromic external beam therapy (EBT) film. Materials and Methods: In this study, an anatomical slab phantom was designed with cork lung inhomogeneity and plexiy colored heart part. Conventional and three dimensional (3D) methods were utilized along with Gafchromic EBT film. Results: In asymmetric fields (3D method) much better results in match region were observed (i.e., maximum amount overlap area of 0.43 cm 2, overlap depth of 3.55 cm and an average overlap width of 0.75 cm). Conclusion: This study revealed that EBT film is a proper tool for two dimensional (2D) relative-dose measurements. The study showed difficulties in achieving homogenous dose distribution in match region of tangential and supraclavicular. KEY WORDS: Breast radiotherapy, dose inhomogeneity, EBT film dosimetry, match region Allahverdi Mahmoud, Nourollahi Somayeh, Esfahani Mahbod 1, Aghili Mehdi 1, Changizi Vahid, Geraily Ghazale Departments of Medical Physics and 1 Radiotherapy, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran INTRODUCTION Breast cancer is the most common malignancy in women and accounts for one third of all female cancers. [1] Radiotherapy is given for primary carcinoma of the breast to reduce the risk of locoregional recurrence. The entire breast is included in the target volume and if affection, lymph nodes regional should be considered too. The tangential fields are used to treat the target volume or planning treatment volume (PTV). Irradiation of the supraclavicular fossa with separate field supraclavicular fields (SCF) is used as adjuvant therapy to improve local control and to reduce the incidence of symptomatic disease in this region. [1-4] In many centers, this treatment is performed using a conventional two dimensional (2D) method, with a single plane hand-generated contour, in which just central plane is considered. In fact, no information about dose distribution in the off-axis plane is acquired and variation contour of the body in other plans and also lung correction are neglected. [5-8] In three dimensional (3D) method, a full of computer tomogeraphy (CT) cuts are used to obtain the whole coverage of breast tissue and reduce an irradiated volume of heart and lung tissue with taking into consideration of the international commission on radiation units and measurements (ICRU )50 recommendation. [9] Irregular geometry of breast shape, slope of chest wall, inhomogeneity such as lung and divergence of tangential beams make it technically difficult to deliver a homogeneous dose to treatment volume and cause dose inhomogeneity, pneumonia, cardiac and cosmetic results. [2] Such a complex treatment requires a proper matching between tangential and anterior fields both on the body surface, and depth because match region between supraclavicular and tangential fields suffers from the problem of overdose and under dose. [10] Number of techniques has been documented to overcome this dose inhomogeneity, but all of them could be made over dose and under dose at match line. [3,11-16] The aim of this study is to decrease overdose and under dose points in match line between supraclavicular and tangential fields as much as possible, in order to using the results commonly in clinic. For correspondence: Somayeh Nourollahi, Department of Medical Physics, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran. y.nourollahi@ yahoo.com Access this article online Website: DOI: / PMID: *** Quick Response Code: 64 Journal of Cancer Research and Therapeutics - January-March Volume 9 - Issue 1

2 MATERIALS AND METHODS A slab anthropomorphic phantom, including lung inhomogeneity and heart part was constructed from 61 transverse slices, each with 5 mm thick. Materials used in the phantom were chosen to confirm with the requirement of ICRU report No. 44. [17] Breast intact was made of transparent plexiy with density of 1.01 g/cm 3, lung part of cork with density of 0.23 g/cm 3 and heart part of colored plexiy, which is well differentiated in breast intact [Figure 1]. border of supraclav field was considered as a reference point and measurements were made in both regions of supraclav and tangential fields up to 2 cm with the step of 5 mm from the reference point. Width, depth and area of overlap region appeared in each film can show dose distribution in match region of three fields [Figure 3]. CT Scans of 14 patients had been used to get averaging the size of typical breast and determine required dimensions such as chest wall separation and height breast. [18] From this estimation, CT scans of one patient (who had dimensions nearest to this typical size) were selected and contour of body, heart, lung were drawn on these slices. Auto Cad software was used to get real size of slabs of designed phantom. EBT Gafchromic film (International specialty products manufacturer) with advantages of being applicable in water and solid phantom, easy to handle, relatively insensitive to environmental light, no needing to processor facilities and having high resolution was chosen as a 2D radiation dosimeter in this research. [19] So in design of phantom grooves for placement of these films were considered. Four grooves in order to insert dosimeters, with width of 7 cm and variable length (because of slope of chest wall) were created vertically in match region between supraclav and tangential beams 2 cm apart each others [Figure 2]. Figure 1: The anthropomorphic breast phantom with cork lung, plexiy heart and four sagittal cuts for film measurement Phantom underwent CT scan with 5 mm thickness slice on the treatment position then sent to the radiation therapy (RT) dose planning system and PTV was drawn by the radiotherapist. To measure dose delivery with film first a calibration process was performed. A slab cubic phantom with dimensions of (30 cm 30 cm 1 cm) was used and EBT films were placed at depth of 5 cm within it. To get a calibration curve radiation was repeated 3 times for every dose range and obtained optical densities were depicted versus known dose values. All of calibration processes were done according to TG 55 and ISP Company reports. [20] Figure 2: Placement of dosimeters (films) in phantom In this study, radiation was performed with 6 mega voltage (MV) photon energy generated by Varian 2100C/D linear accelerator. 48 h after each radiation (to complete density growth post exposure), films read by scanner Microtek 9800 XL (Microtek International Inc. Husinchu, Taiwan). This digitizer is a flatbed scanner with a scan area of 406 mm mm 2, in color mode (red green blue (RGB) 48 bit) with 127 resolutions. To analysis the obtained results, matrix laboratory (MATLAB) software (version 2007) and Image J software were used. In this study, axillaries field was withdrawal because of no significant effect on match region between three fields. Inferior Figure 3: Width, depth and area of overlap region in match region of three fields Journal of Cancer Research and Therapeutics - January-March Volume 9 - Issue 1 65

3 Techniques with varying degree of complexity such as combination of gantry, collimator, couch rotation and gap in adjacent fields for both conventional and 3D methods were employed to match the border of tangential and supraclav fields in order to get dose homogeneity in this region. Table 1 summarizes the adopted matching techniques. Conventional methods Conventional methods accomplished with hand-generated contour on central axis and border of fields draw on surface of phantom. The size of SCF was specified as a width of 12 cm and a length of 9 cm. The gantry angle was set to 58 and 238 for medial and lateral tangential fields respectively. Length and width of tangential fields were determined to cover inferior portion of treatment volume and the breast anterior respectively. Table 1: Different parameters used in conventional and three dimensional methods Number of Techniques Modification to techniques superior border of tangent fields 1 SSD match Symmetric, without gap collimator angle 2 SSD match Symmetric, with gap collimator angle 3 3D Usual Symmetric, collimator angle 4 3D Asymmetric, Asymmetric collimator angle 5 3D Noncoplanar Asymmetric, collimator angle Modification to inferior border of SC field Symmetric, collimator angle Symmetric, collimator angle, gap Symmetric Asymmetric SSD = 100 cm, SC = Supraclavicular, 3D = Three dimensional Symmetric, couch angle, collimator angle, gantry angle Conventional methods were classified into two techniques both with source skin distance (SSD) match. In the first technique called SSD match without gap, caudal border of supraclav field was fixed to cranial border of tangential fields while in second one called SSD match with gap there was 5 mm gap between supraclav and tangential fields, in which the placement of supraclav was changed and tangential fields was leaved unchanged. Three dimensional methods In this method, the effects of different parameters were investigated and a full set CT slices was used. In 3D methods with implying different options like photon energy and different weight of dose the best dose distribution in the match region between three fields and good coverage of PTV can be got. These methods were classified into three techniques (called 3D usual, 3D asymmetric and 3D non-coplanar) all are considered as source axis distance (SAD). In 3D, usual technique tangential and supraclav fields are symmetric while in 3D asymmetric technique inferior border of SCF field and superior border of tangential fields set asymmetric in order to overcome beam divergency at match region of three fields. In 3D non-coplanar technique combination of couch, collimator, gantry angle, symmetric field were applied to supraclavicular beams and asymmetric fields, collimator rotationss were applied to tangential beams to get the best dose homogeneity in match region of three fields. Summary of different parameters used in five techniques (two for conventional methods and three for 3D methods) are presented in Table 1. RESULTS Calibration curve for EBT film shown in Figure 4 were used to get dose distribution from obtained optical density from film measurements in five techniques. Dose profiles across the match region, from reference point toward supraclav and tangential fields, which are shown in Figure 5 along with Figure 4: Calibration curve for external beam therapy (EBT) Gafchromic film with 6MV photon area [Figure 6], depth [Figure 7] and width of overlap region [Figures 8 and 9] in each film show dose perturbation in conjugation of three fields. Furthermore, overdose and under dose values in the match region of three fields relative to prescribed dose are presented in Table 2 for each technique. SSD match without gap According to Figure 5, increasing dose was seen in film number 2, 3 and 4, which are placed under greater beams divergency. 75% of the maximum dose occurred at 5 mm from the reference point toward SCF. Range of overlap depth in this technique was between 23 mm to 63 mm [Figure 7]. It can be seen from Figures 8 and 9 that the most overlap width happened in film number 4 and 3 at 2 cm depth (13 mm width). The magnitude of overlap area increased dose value up to 55% over prescribed does, which is largest among other techniques [Table 2]. SSD match with gap From beam profiles depicted in Figure 5, it can be deducted that 60% of the maximum dose occurred at 5 mm from reference line toward SCF. Presented results in Figures 7 and 8 shows that depth and width of overlap are not appeared in film number 1 and 2 but there was under dose especially in film number 1. Range of overlap depth in film number 3 and 4 was between 53 mm to 62 mm. As shown in Figure 9, the most overlap width was seen in film number 4 with 4 mm wide at 2 cm 66 Journal of Cancer Research and Therapeutics - January-March Volume 9 - Issue 1

4 [Downloaded free from on Thursday, April 18, 2013, IP: ] Click here to download free Android application for this journal a b c d e f g h Figure 5: Dose profile related to different techniques. (a) Film 1 in 1 cm depth. (b) Film 1 in 2 cm depth. (c) Film 2 in 1 cm depth. (d) Film 2 in 2 cm depths. (e) Film 3 in 1 cm depth. (f) Film 3 in 2 cm depths. (g) Film 4 in 1 cm depth. (h) Film 4 in 2 cm depths Journal of Cancer Research and Therapeutics - January-March Volume 9 - Issue 1 67

5 Table 2: Overdose and underdose from prescribed dose in match region Number of techniques Techniques Over prescribed dose % Under prescribed dose % 1 SSD match without gap SSD match with gap D Usual D Asymmetric D non-coplanar 14 0 SSD = 100 cm, 3D = Three dimensional Figure 6: Overlap area of three fields depth. Also the averaging of overdose and under dose at match region of these fields were 11% and 31% respectively [Table 2]. Three dimensional usual Beam profiles in Figure 5 show there is overdose for most of conjunction points of fields. Figure 6 shows that the maximum area of overlap happened in this method which is 6.4 cm 2 in film number 4. Also magnitude of overlap depth was between 3.95 cm to 7.15 cm that is more than other techniques [Figure 7]. The most width of overlap was seen in this technique which is 14.5 mm and appeared in film number 4 at 2 cm depth [Figure 9]. According to Table 2 the overdose at conjunction of three fields was 47% of prescribed dose. Figure 7: Overlap depth of three fields Figure 8: Overlap width of three fields at 1 cm depth Three dimensional Asymmetric Shape of beam profiles shown in Figure 5 confirmed the nondivergent characteristic of beams for this technique. It can be seen that there is a good matching in all films. As shown in Figure 6 the joint common area of three fields is little such that minimum value of overlap area is related to this technique (0.06 cm 2 ). The maximum overlap width in this technique occurred at the surface of film number 4 with 2 mm wide [Figures 8 and 9] also the maximum of overlap depth occurred at 36 mm deep [Figure 7]. Measured dose in match line was 9% over prescribed dose that was less than other techniques while 4% under dose in the match region was found in this technique [Table 2]. Three dimensional non-coplanar Beam profiles in Figure 5, show relatively good dose homogeneity in match line with 14% overdose relative to prescribed dose [Table 2]. Maximum overlap depth was found 7.3 cm [Figure 7]. The maximum overlap width (15 mm) occurred in surface and 1 cm deep at film number 1 and 2 [Figure 8]. DISCUSSION Figure 9: Overlap width of three fields at 2 cm depth This study attempts to create a good dose distribution in the match region between tangential and supraclav fields with different techniques and use the result at clinical application with the aim of increasing dose homogeneity to improve the cosmetic results. The uncertainties in measurements could be attributed to all processes of dosimetry and design 68 Journal of Cancer Research and Therapeutics - January-March Volume 9 - Issue 1

6 of phantom such as film scan process, phantom setup and film placement. The location of the film in phantom should not be such as width that film moves freely in it and such as narrow that scratches the film because impacting between film and phantom wall affects on pixel value readings. In this study, measurements were carried out three times to reduce statistical errors. Beam profiles depicted in Figure 5 demonstrate the main effect occurred across the match line, which is proper or leads to potential over-under dose followed by complication of dose inhomogeneity. Usually, in most techniques, maximum dose occurred in part of supraclav field included in overlap region. It means that dose in this region was more than match line, that could be related to slope of the chest wall in match line and increasing of beams divergency toward supraclav field. Results show maximum overdose at SSD match without gap technique because fields are exactly beside each other in surface, so beam divergency produces overlap in depth. The minimum value of under dose in the match region was happened in this technique, especially in film number 1, but this region is not included in PTV. The maximum width of overlap at match region was observed in 3D usual technique, because at this technique, the inferior border of SCF and superior border of tangential fields maintain with more margins to refuse missing PTV. In addition, more beam divergency is related to symmetric fields, which results in more overlap in 3D usual technique. Conventional methods are simpler than 3D methods, but need a high precision alignment because of using separated set up for each field and absent of isocentre point. Beam profiles showed worse dose distribution in match line at both SSD matches without gap and 3D usual techniques. Therefore, there is no benefit for 3D usual technique over conventional methods. 3D asymmetric technique produces the best results in terms of depth, area, width of overlap and prescribed dose. The advantages of this technique are a good match between the tangential and supraclav fields and flat dose profiles. Asymmetric fields have less beam divergency than symmetric fields, which result in high steep dose gradient in edges of field. This affects dramatically on dose distribution in the match line. The accuracy of set up in asymmetric fields is very important and any misalignment of setup may increase the overlap area and mismatch between three fields because of the sharpness penumbra in the edge of fields. The similar studies carried out by Miles et al., in 2009 confirms our results. [21] In 3D non-coplanar technique, although the depth of overlap was more than others, relatively good distribution in match region has obtained. However, in this technique, it takes more times to set up than asymmetric beams and is not suggested in a busy center. In this study lung is not involved in overlap region. Only 3D usual and 3D non-coplanar techniques show the coverage between lung and depth of overlap however is not remarkable. Although the lung included at separately supraclav and tangential fields can be remarkable. Get a perfect matching between these fields is difficult even on a phantom. Dose distribution in the match region strongly depends on the set up accuracy of the fields. Idzes et al., in 1998 have shown that dose distribution in the match region of supraclav and tangential fields for conventional methods is more dependent on set up uncertainties. [16] It s noteworthy that dosimetry measurements were carried out just with 4 films in 4 places, but not along the entire supraclav and tangential match line. If more films can be used at entire of match region, dose homogeneity can be investigated more accurately. CONCLUSION EBT film is a proper tool for 2D dosimetry and relative dose measurement and shows the dose distribution in match region very well. Techniques without geometric alignment (conventional methods and 3D usual technique) lead to improper dose distribution in match line. SSD match with gap technique decreases overdose in match region, but in some points made trouble (under dose) in match region and besides poor coverage on PTV is resulted. This article recommends using appropriate geometry alignment (as asymmetric fields) in match region of three fields. 3D methods show better dose distribution in match region. 3D asymmetric technique could result good dose homogeneity in match region than other techniques. For centers with limitation facility that should use conventional methods, SSD match with the gap technique is a good suggestion to minimize the risk of overdose. For gap techniques, considering anatomical position of gap relative to the gross region or region of potential lymph node is important. It is important to notice that in conventional methods there was not good coverage of PTV and also in 3D methods volume of involved contralateral breast was more than conventional methods. It should be noted that the good results depend mainly on the good cooperation between treatment team (i.e., radiotherapist, physicist and technologist). REFERENCES 1. Sonnik D, Selvaraj RN, Faul C, Gerszten K, Heron DE, King GC. Treatment techniques for 3D conformal radiation to breast and chest wall including the internal mammary chain. Med Dosim 2007;32: Siddon RL, Buck BA, Harris JR, Svensson GK. Three-field technique for breast irradiation using tangential field corner blocks. Int J Radiat Oncol Biol Phys 1983;9: Lichter AS, Fraass BA, van de Geijn J, Padikal TN. A technique for field matching in primary breast irradiation. 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