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Alternative effective modality of Leipzig applicator with an electron beam for the treatment of superficial malignancies
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 508 (2003) 460–466
Alternative effective modality of Leipzig applicator with an electron beam for the treatment of superficial malignancies Ing-Ming Hwanga,b, Shen-Yeh Linc, Li-Ching Lind, Keh-Shih Chuangb, Hueisch-Jy Dinga,* a
Department of Medical Research, School of Technology for Radiological Medicine, Kaohsiung Medical University, 100 Shih-Chuan 1st Road, Kaohsiung, Taiwan, ROC b Institute of Nuclear Science, National Tsing-Hua University, Hsing-Chu, Taiwan, ROC c Fei-Long Radiation Oncology Clinic, Kaohsiung, Taiwan, ROC d Department of Radiation Oncology, Chi-Mei Foundation Hospital, Tainan, Taiwan, ROC Received 25 January 2003; received in revised form 23 April 2003; accepted 27 April 2003
Abstract Object: The project was designed to study the effect of the same aperture high-energy electron beams from a dual photon-energy linear accelerator with the Leipzig applicator being a successful method of treating superficial malignancy in achieving a maximum relative surface dose. Methods: Leipzig applicator with its protective cap beams and small field electron beams from a Varian Clinac 2100CD linear accelerator were used. Circular fields at 100 cm SSD were obtained from cadmium-free cerrobend shields attached to the bottom face of a 6 6 cm2 open wall electron cone. The central axis depth–dose curves, profiles and isodose curves measurement were carried out in a water phantom (NWP, Nucletron, The Netherlands) with a pair of silicon p-type diode electron detectors (Scanditronix, Sweden) with cross-section facing the source. The output factors, central-axis depth–dose curves, 90% isodose curve depth and area, flatness at depth of maximum dose were compared and the sagittal plane leakage of the Leipzig applicator were examined. Results: The data show that the maximum dose of Leipzig applicator and all 1 cm diameters electron beam are all located at the surface. The central-axis depth–dose curve of the Ir-192 beam is attenuated in an inverse square law manner. The surface to maximum dose ratio of the electron field decreases when the aperture size increases. The specified dose depths of Leipzig applicators are at least 3–6-fold less, and the area are at least 2–5-fold less than those of same aperture electron beams. The hot spot of a Leipzig applicator is located at the top and surrounding the source for both types of applicator with a leakage dose of up to 30% of Dmax : Conclusions: The Leipzig applicator with 192Ir high-dose-rate brachytherapy source can be used as an alternative method to electron beam irradiation in the treatment of superficial malignancies when the bulky electron applicator is difficult to set-up and approach. r 2003 Elsevier B.V. All rights reserved. Keywords: Dosimetry characteristics; Leipzig applicator; Small aperture electron; Beam; Surperficial malignancy treatment
*Corresponding author. Address for correspondence: Department of Medical Research, School of Technology for Radiological Medicine, Kaohsiung Medical University, 100, Shih-Chuan 1st Road, Kaohsiung, Taiwan, ROC. Tel.: +886-7-312-1101; fax: +886-7311-3449. E-mail address: [email protected] (H.J. Ding). 0168-9002/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-9002(03)01664-4
ARTICLE IN PRESS I.-M. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 460–466
1. Introduction Many radiation sources are available for the treatment of superficial malignancies [1–7]. Of these, orthovoltage, supervoltage X-rays, the 192Ir surface mold, and electron beams are the most often applied [8–11]. The main disadvantages of using superficial and orthovoltage machines for the treatment of superficial malignancies are those the output is quite low, requiring treatment times of up to 15 min, so they are being replaced by megavoltage electron beams [12,13]. Superficial malignancies require a dose distribution that is uniform from surface to the distal boundary of the target, followed by a rapid decline in the dose in the underlying normal tissue, an electron beam is ideal for radiation therapy for all skin and lip cancers since it has a characteristically sharp dropoff in dose beyond the depth of maximum dose. However it is also limited by its bulky size [14]. The Leipzig applicator has been used since 1987 as a standardized applicator for HDR brachytherapy of superficial malignancies. Including tumors of the skin, face, mouth, tongue, penis, perianal region and external genitalia, Kaposi-sarcomas, melanomas and skin manifestations of lymphomas, solid organ tumors, and keloids after excision, etc. The single doses were between 5 and 10 Gy once to twice per week with a total dose of 30–40 Gy and a range of standardized applicators demonstrates that this is a successful method of treating surface lesions [8,9]. The Leipzig applicators are optional accessories for the micro-Selectron HDR 192Ir system. However, the output of the Leipzig applicator has low output dose rate when the activity of the 192Ir source has decayed to a low level activity (o50 cGy/min at 1.7 cm SSD when the activity below 2 Ci). Then treatment time can take up to 20 min. To alternate the electron beam and Leipzig applicator treatment for surface lesions, dosimetry characteristics of both modalities with the same aperture field size were measured and compared. The dosimetry characteristics included the output factors at a depth of maximum dose (Dmax ), the central-axis depth–dose distribution, the surfaceto-maximum dose ratio (a-value), flatness of beam
461
profiles at depth of Dmax ; the isodose curve distributions and the area of 90% isodose curves were measured and compared. The leakage and hot spot of each type Leipzig applicators were also measured to help avoid radiation hazard to normal tissue.
2. Materials and methods Leipzig applicator with a protective cap (to eliminate electron contamination) beams from Nucletron micro-Selectron HDR system and small field electron beams from a Varian Clinac 2100CD linear accelerator were used. Energies of the electron beams studied were 6, 9, 12, 16, and 20 MeV. Circular fields of 1, 2, and 3 cm diameter at 100 cm SSD were obtained from cadmium-free cerrobend shields (thickness B1 cm) attached to the bottom face of a 6 6 cm2 open wall electron cone. The cerrobend inserts have transmission factors of 0.65% for the 6-MeV beam, 1.25% for the 9-MeV beam and 5.5% for the 20-MeV beam, respectively. The central axis depth–dose curves, profiles and isodose curves measurement were carried out in a 50 50 50 cm3 water phantom (NWP, Nucletron, The Netherlands) with a pair of silicon p-type diode electron detectors (Scanditronix, Sweden) with cross-section facing the source. The effective measuring depth and detection area were 0.5570.10 and 2.5 mm diameter, respectively [15,16]. The source-to-surface distance (SSD) for Leipzig applicator was 1.7 cm and for the electron beam was 100 cm. The flatness of the profile at depth of Dmax was analyzed using the software for the water phantom system. The output factors were measured in a polystyrene-sheet phantom (Nuclear Associate, New York, USA) at Dmax for electron beams according to the measurement results of central-axis depth– dose curves and at depth of 3 mm for Leipzig applicator [17] with an end-window 0.04 cm3 Markus parallel-plate ionization chamber (Nuclear Enterprise, New York, USA). The effective measurement point of the chamber was on the front surface electrode [18,19]. The ionization charges were measured with a Keithley 35040
ARTICLE IN PRESS I.-M. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 460–466 120
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digital electrometer (Keithley Instrument, Cleveland, OH, USA). The relative output for the electron beam was normalized at Dmax under 6 6 cm2 open cone, and for 192Ir source it was normalized at the same distance (2 cm) in full phantom. The a-value was defined as the ratio of surface dose, Ds ; to Dmax and was calculated relative to the observed relative surface dose variation when field size increased. Ideally, a should approach unity for the treatment of superficial malignancies [14]. The measurement of sagittal plane leakage of the Leipzig applicator was carried out in a bolus phantom using Kodak XV-2 (Kodak Rochester, New York, USA) ready pack films cut along the external contour of the applicator under darkroom conditions. The exposed film was developed and scanned using the water phantom densitometer software with optical density correction.
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The output factors of both type Leipzig applicators and various aperture electron beams are summarized in Table 1. The output factors of horizontal type Leipzig applicators (mean values=1.11) are greater than those of vertical ones (mean value=0.92) was found to be similar to
Table 1 Output factors of the two types of Leipzig applicator and various aperture electron beams Aperture diameter (cm)
Leipzig (Horizontal type) Leipzig (Vertical type) 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
Output factors 1
2
3
1.120 0.930 0.464 0.638 0.792 0.916 0.953
1.110 0.920 0.740 0.822 0.912 0.957 0.972
1.100 0.910 0.893 0.901 0.953 0.989 0.997
The norminal output of Leipzig applicator is the dose delivered by 192Ir source at the same distance of Leipzig applicator in solid phantom.
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Fig. 1. Central axis depth–dose distributions for 1 cm (a) and 3 cm (b) aperture Leipzig applicators and various energies electron beams with 6, 9, 12, 16, 20-MeV.
published data [9], showing that the vertical ones get more attenuation from the source itself. The central axis depth–dose curves of 1 and 3 cm aperture of Leipzig applicators and electron beams are shown in Figs. 1(a) and (b). The depth– dose curves of all the Leipzig applicators are nearly the same and attenuated in inverse square law in the relationship of {100% ½2=ð2 þ dÞ2 }. Field-size dependence of the central-axis percentage depth–dose distributions of small field electron beams was found to be similar to published data [20–25]. The Dmax of the Leipzig applicator and 1 cm diameter electron beams were found near located on the surface as shown in Fig. 1. For all electron
ARTICLE IN PRESS I.-M. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 460–466
energies studies, the surface dose, Ds ; increases as the field size becomes small, the depth of Dmax shifts toward the surface, and the dose fall-off region becomes more gradual. For all electron beams, the a-values become small when the beam diameter increased. The isodose curves of Leipzig applicator and 20and 6-MeV electron beams in 3 cm diameter are plotted in Figs. 2(a) and (b). In Fig. 2(a), the left-
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hand side was a vertical type and the right-hand side was a horizontal type of 3 cm aperture applicator. Concavity in all isodose curves along the central axis for the vertical type applicator was observed and found to be similar to measured data . from Kohler-Brock et al. [8] and Evans et al. [9]. The left-hand side of Fig. 2(b) shows the isodose curve distribution of 20-MeV, and the right-hand side shows that of 6-MeV distribution. The isodose curve distribution of the Leipzig applicator displayed a shell-shape and the divergence angle is greater than those of electron beams because of the short SSD and transmission at the applicator wall. The depth (in cm) and the area (in cm2) of the 90% isodose curve for the Leipzig applicator and all energies of electron beams were plotted and are compared in Figs. 3 and 4. The 80% depth of Leipzig applicator was located at 2 mm and was at least 3–6-fold less than those of electron beams, whereas the 90% isodose dose curve area was at least 2–5-fold less than those of electron beams. The flatness of each beam at Dmax was analyzed by the water phantom software and the values were summarized in Table 2. The high-energy electron beams with larger aperture were more uniform than those of low energy ones with small aperture field, demonstrating that high-energy electrons were mostly forward scattered at a depth of Dmax : The flatness of the Leipzig applicator beam was similar to that of a 9-MeV beam. Leakages of the two types of Leipzig applicator with 2 cm aperture are shown in Fig. 5. The main leakage was located around the source and at the top of both types of applicator, with 30% dose to Dmax :
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10% (b) Fig. 2. Isodose curve distributions for 3 cm (a) vertical type (left), horizontal type (right) and (b) 20-MeV (left), 6-MeV (right) electron beams at the 6 6 cm2 applicator.
The output factors of circular block shielded electron beams decreases monotonically with the field size from 3 to 1 cm diameter due to the loss of scatters. Correspondingly, the factors increased when the field diameter increased. The change in output factors with field size was small for the 20MeV beam due to the forward scattering nature of this beam.
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The electrons that are scattered at small angles from the cone were blocked by the cerrobend insert with a small opening minimizing the effect of cones on the output factors at small fields. The dose modification in the small field electron beam was caused by the absence of lateral electronic equilibrium inside the phantom and, to a lesser extent, by electron scatter and bremsstrah-
lung contamination introduced by the cerrobend insert. Practically all skin cancers can be irradiated successfully by electron beams ranging from 6 to 20 MeV, depending on the area and thickness of the lesion (depth). For most tumors, the electron beam energy selected puts the 80% or 90% isodose line at the desired treatment depth.
ARTICLE IN PRESS I.-M. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 460–466
Table 2 The flatness at depth of Dmax for various aperture of both type Leipzig applicators and electron beams Aperture diameter (cm)
Leipzig applicator 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
Flatness at Dmax 1 (%)
2 (%)
3 (%)
717.65 722.95 722.72 722.20 721.55 719.65
714.64 720.32 717.21 715.51 714.17 713.07
712.99 718.06 713.39 711.60 78.66 75.73
doses that destroy the basal layers of the epidermis but spare the underlying dermis [13]. Most skin cancers can be treated with either a Leipzig applicator or low energy electron beams, and optimal use of either modality requires knowledge of its specific beam characteristics. According to our measurements, Leipzig applicators deposit a maximum dose at the skin surface, and there is a decrease of dose according to depth 120
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Dosimetry characteristics of Leipzig applicator were observed similar to that of 6- and 9-MeV electron beams, but the electron beams penetrated 90% deeper and had a larger isodose curve area. Higher energy electron beams (>9-MeV) deposit higher doses at the deeper depth and in normal tissue. Epidermis, the outer layer of the skin varies in depth from 0.3 mm on the eyelids and flexural areas to 1.5 mm on the palms of the hands and soles of the feet. The most common malignant tumor of the skin is basal cell carcinoma, which arises from the basal layer of the epidermis. In general, treatment of most skin cancers requires
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Fig. 6. Comparison of central axis depth–dose curves of Leipzig applicator to 6-MeV, 1 cm aperture, and 6-MeV, 3 cm aperture plus 1 cm bolus.
Fig. 5. Radiation leakage measure of 2-cm aperture, both types of Leipzig applicator, left-horizontal type and right-vertical type.
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proportional to the inverse square, resulting in significant penetration and dosage to deeper structures relative to low energy electron beams (p9-MeV). Low energy electron beams offer the advantage of rapid reduction of depth–dose determined by beam energy, and therefore greater sparing of underlying normal structures. However, they have the disadvantage of skin sparing, with build-up to maximum dose beneath the skin surface at a larger aperture field. Although low energy electrons are suitable for superficial tumors because of their sharp dose cut-off, the use of electron beams that were not modified is limited and in some cases undesirable because of their pronounced dose build up and resultant low surface dose. Electron beam modifications (water-equivalent bolus, or beam spoiler) are necessary to increase the surface dose to 100% for the treatment of skin cancers [14,17], as shown in Fig. 6.
5. Conclusions The choice between these modalities should be determined by the size, depth, and anatomic location of the lesion, as well as the setup procedure, and the output. The quality of radiation should be chosen on the basis of the best ratio between surface dose and the ideal depth–dose (i.e. Dmax ; 90% or 80%). The use of Leipzig applicator is advocated as an alternative to electron beam irradiation in the treatment of superficial skin cancers. It is particularly useful in situations of lesions located around the eyelids, in the periorbital areas, in the medial triangle of the cheek, and in the ear and nose where electron applicator treatment is not suitable to setup and perform.
Acknowledgements The authors wish to express their sincere appreciation to the National Science Council of ROC for the financial support of this work. We would also like to thank Professor G.C. Liu for his valuable assistances.
References [1] K. Nemoto, S. Yamada, M. Hareyama, H. Nagakura, Y. Hirokawa, Int. J. Radiat. Oncol. Biol. Phys. 50 (3) (2001) 639. [2] J. Locke, S. Karimpour, G. Young, M.A. Lockett, C.A. Perez, Int. J. Radiat. Oncol. Biol. Phys. 51 (3) (2001) 748. [3] T. Kron, M. Haque, K. Foulke, R. Jeraj, Phys. Med. Biol. 47 (5) (2002) 713. [4] R. Jeraj, A. Sarvary, T. Kron, Phys. Med. Biol. 47 (5) (2002) 723. [5] A.S. Meigooni, S.A. Parker, J. Zheng, K.J. Kalbaugh, W.F. Regine, M. Mohiuddin, Med. Dosim. 27 (1) (2002) 37. [6] R. Yaparpalvi, D.P. Fontenla, J.J. Beitler, Br. J. Radiol. 75 (896) (2002) 670. [7] G. Malinverni, M. Stasi, B. Baiotto, C. Giordana, G. Scielzo, P. Gabriele, Tumori 88 (4) (2002) 331. . [8] A. Kohler-Brock, W. Prager, S. Pohlmann, et al., Strahlenther. Onkol. 175 (1999) 170. [9] M.D.C. Evans, M. Yassa, E.B. Podgorsak, T.N. Roman, J. Schreiner, L. Souhami, Int. J. Radia. Oncol. Biol. Phys. 39 (1997) 769. [10] S.N. Rustgi, R.L. Cumber, Med. Dosim. 18 (1993) 39. [11] V.H. Svoboda, J. Kovarik, F. Morvis, Int. J. Radiat. Oncol. Biol. Phys. 31 (1995) 967. [12] S. Shahabi, R. Stanton, D. Stinson, Medical Physics Publishing, Madison, Wisconsin, 1992. [13] M.J. Solan, L.W. Brady, S.A. Binnick, et al., Skin, in: Perez, C.A., Brady, L.W. (Eds.), Principles and Practices of Radiation Oncology, 3rd Edition, Lippincott-Raven, Philadelphia, New York, 1998. [14] I.J. Das, K.R. Kase, J.F. Copeland, et al., Int. J. Radiat. Oncol. Biol. Phys. 21 (1991) 1350. [15] AAPM (American Association of Physicists in Medicine) task group report 23, Total skin electron therapy, Technique and dosimetry; American Institute of Physics, New York, 1983. [16] F.M. Khan, K.P. Dophke, K.R. Hogstrom, et al., Med. Phys. 18 (1991) 73. [17] G.J. Kutcher, L. Coia, M. Gillin, et al., Med. Phys. 21 (1994) 581. [18] AAPM (American Association of Physicists in Medicine) Radiation Therapy Committee Task Group 21, A protocol for the determination of absorbed dose from high-energy photon and electron beam. Med. Phys. 10 (1983) 741–771. [19] P.R. Almond, P.J. Biggs, B.M. Coursey, et al., Med. Phys. 26 (1999) 1847. [20] S.C. Klevenhagen, Adam Hilger Ltd., Bristol and Boston, 1985. [21] S.Y. Lin, M.Y. Huang, J.Y. Tang, et al., Therapeu. Radio. Oncol. 5 (1998) 311 (In Chinese). [22] J.A. Mayer, J.R. Palta, K.R. Hogstrom, Med. Phys. 11 (1984) 670. [23] P.H. McGinley, J.R. McLaren, B.R. Barnett, Radiology 131 (1979) 231. [24] H. Rashid, M.K. Islam, H. Gaballa, et al., Med. Phys. 17 (1990) 710. [25] S.C. Sharma, D.L. Wilson, B. Jose, Med. Phys. 11 (1984) 697.
Nuclear Instruments and Methods in Physics Research A 508 (2003) 460–466
Alternative effective modality of Leipzig applicator with an electron beam for the treatment of superficial malignancies Ing-Ming Hwanga,b, Shen-Yeh Linc, Li-Ching Lind, Keh-Shih Chuangb, Hueisch-Jy Dinga,* a
Department of Medical Research, School of Technology for Radiological Medicine, Kaohsiung Medical University, 100 Shih-Chuan 1st Road, Kaohsiung, Taiwan, ROC b Institute of Nuclear Science, National Tsing-Hua University, Hsing-Chu, Taiwan, ROC c Fei-Long Radiation Oncology Clinic, Kaohsiung, Taiwan, ROC d Department of Radiation Oncology, Chi-Mei Foundation Hospital, Tainan, Taiwan, ROC Received 25 January 2003; received in revised form 23 April 2003; accepted 27 April 2003
Abstract Object: The project was designed to study the effect of the same aperture high-energy electron beams from a dual photon-energy linear accelerator with the Leipzig applicator being a successful method of treating superficial malignancy in achieving a maximum relative surface dose. Methods: Leipzig applicator with its protective cap beams and small field electron beams from a Varian Clinac 2100CD linear accelerator were used. Circular fields at 100 cm SSD were obtained from cadmium-free cerrobend shields attached to the bottom face of a 6 6 cm2 open wall electron cone. The central axis depth–dose curves, profiles and isodose curves measurement were carried out in a water phantom (NWP, Nucletron, The Netherlands) with a pair of silicon p-type diode electron detectors (Scanditronix, Sweden) with cross-section facing the source. The output factors, central-axis depth–dose curves, 90% isodose curve depth and area, flatness at depth of maximum dose were compared and the sagittal plane leakage of the Leipzig applicator were examined. Results: The data show that the maximum dose of Leipzig applicator and all 1 cm diameters electron beam are all located at the surface. The central-axis depth–dose curve of the Ir-192 beam is attenuated in an inverse square law manner. The surface to maximum dose ratio of the electron field decreases when the aperture size increases. The specified dose depths of Leipzig applicators are at least 3–6-fold less, and the area are at least 2–5-fold less than those of same aperture electron beams. The hot spot of a Leipzig applicator is located at the top and surrounding the source for both types of applicator with a leakage dose of up to 30% of Dmax : Conclusions: The Leipzig applicator with 192Ir high-dose-rate brachytherapy source can be used as an alternative method to electron beam irradiation in the treatment of superficial malignancies when the bulky electron applicator is difficult to set-up and approach. r 2003 Elsevier B.V. All rights reserved. Keywords: Dosimetry characteristics; Leipzig applicator; Small aperture electron; Beam; Surperficial malignancy treatment
*Corresponding author. Address for correspondence: Department of Medical Research, School of Technology for Radiological Medicine, Kaohsiung Medical University, 100, Shih-Chuan 1st Road, Kaohsiung, Taiwan, ROC. Tel.: +886-7-312-1101; fax: +886-7311-3449. E-mail address: [email protected] (H.J. Ding). 0168-9002/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-9002(03)01664-4
ARTICLE IN PRESS I.-M. Hwang et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 460–466
1. Introduction Many radiation sources are available for the treatment of superficial malignancies [1–7]. Of these, orthovoltage, supervoltage X-rays, the 192Ir surface mold, and electron beams are the most often applied [8–11]. The main disadvantages of using superficial and orthovoltage machines for the treatment of superficial malignancies are those the output is quite low, requiring treatment times of up to 15 min, so they are being replaced by megavoltage electron beams [12,13]. Superficial malignancies require a dose distribution that is uniform from surface to the distal boundary of the target, followed by a rapid decline in the dose in the underlying normal tissue, an electron beam is ideal for radiation therapy for all skin and lip cancers since it has a characteristically sharp dropoff in dose beyond the depth of maximum dose. However it is also limited by its bulky size [14]. The Leipzig applicator has been used since 1987 as a standardized applicator for HDR brachytherapy of superficial malignancies. Including tumors of the skin, face, mouth, tongue, penis, perianal region and external genitalia, Kaposi-sarcomas, melanomas and skin manifestations of lymphomas, solid organ tumors, and keloids after excision, etc. The single doses were between 5 and 10 Gy once to twice per week with a total dose of 30–40 Gy and a range of standardized applicators demonstrates that this is a successful method of treating surface lesions [8,9]. The Leipzig applicators are optional accessories for the micro-Selectron HDR 192Ir system. However, the output of the Leipzig applicator has low output dose rate when the activity of the 192Ir source has decayed to a low level activity (o50 cGy/min at 1.7 cm SSD when the activity below 2 Ci). Then treatment time can take up to 20 min. To alternate the electron beam and Leipzig applicator treatment for surface lesions, dosimetry characteristics of both modalities with the same aperture field size were measured and compared. The dosimetry characteristics included the output factors at a depth of maximum dose (Dmax ), the central-axis depth–dose distribution, the surfaceto-maximum dose ratio (a-value), flatness of beam
461
profiles at depth of Dmax ; the isodose curve distributions and the area of 90% isodose curves were measured and compared. The leakage and hot spot of each type Leipzig applicators were also measured to help avoid radiation hazard to normal tissue.
2. Materials and methods Leipzig applicator with a protective cap (to eliminate electron contamination) beams from Nucletron micro-Selectron HDR system and small field electron beams from a Varian Clinac 2100CD linear accelerator were used. Energies of the electron beams studied were 6, 9, 12, 16, and 20 MeV. Circular fields of 1, 2, and 3 cm diameter at 100 cm SSD were obtained from cadmium-free cerrobend shields (thickness B1 cm) attached to the bottom face of a 6 6 cm2 open wall electron cone. The cerrobend inserts have transmission factors of 0.65% for the 6-MeV beam, 1.25% for the 9-MeV beam and 5.5% for the 20-MeV beam, respectively. The central axis depth–dose curves, profiles and isodose curves measurement were carried out in a 50 50 50 cm3 water phantom (NWP, Nucletron, The Netherlands) with a pair of silicon p-type diode electron detectors (Scanditronix, Sweden) with cross-section facing the source. The effective measuring depth and detection area were 0.5570.10 and 2.5 mm diameter, respectively [15,16]. The source-to-surface distance (SSD) for Leipzig applicator was 1.7 cm and for the electron beam was 100 cm. The flatness of the profile at depth of Dmax was analyzed using the software for the water phantom system. The output factors were measured in a polystyrene-sheet phantom (Nuclear Associate, New York, USA) at Dmax for electron beams according to the measurement results of central-axis depth– dose curves and at depth of 3 mm for Leipzig applicator [17] with an end-window 0.04 cm3 Markus parallel-plate ionization chamber (Nuclear Enterprise, New York, USA). The effective measurement point of the chamber was on the front surface electrode [18,19]. The ionization charges were measured with a Keithley 35040
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digital electrometer (Keithley Instrument, Cleveland, OH, USA). The relative output for the electron beam was normalized at Dmax under 6 6 cm2 open cone, and for 192Ir source it was normalized at the same distance (2 cm) in full phantom. The a-value was defined as the ratio of surface dose, Ds ; to Dmax and was calculated relative to the observed relative surface dose variation when field size increased. Ideally, a should approach unity for the treatment of superficial malignancies [14]. The measurement of sagittal plane leakage of the Leipzig applicator was carried out in a bolus phantom using Kodak XV-2 (Kodak Rochester, New York, USA) ready pack films cut along the external contour of the applicator under darkroom conditions. The exposed film was developed and scanned using the water phantom densitometer software with optical density correction.
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The output factors of both type Leipzig applicators and various aperture electron beams are summarized in Table 1. The output factors of horizontal type Leipzig applicators (mean values=1.11) are greater than those of vertical ones (mean value=0.92) was found to be similar to
Table 1 Output factors of the two types of Leipzig applicator and various aperture electron beams Aperture diameter (cm)
Leipzig (Horizontal type) Leipzig (Vertical type) 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
Output factors 1
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1.120 0.930 0.464 0.638 0.792 0.916 0.953
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The norminal output of Leipzig applicator is the dose delivered by 192Ir source at the same distance of Leipzig applicator in solid phantom.
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1
2
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6 7 Depth (cm)
Fig. 1. Central axis depth–dose distributions for 1 cm (a) and 3 cm (b) aperture Leipzig applicators and various energies electron beams with 6, 9, 12, 16, 20-MeV.
published data [9], showing that the vertical ones get more attenuation from the source itself. The central axis depth–dose curves of 1 and 3 cm aperture of Leipzig applicators and electron beams are shown in Figs. 1(a) and (b). The depth– dose curves of all the Leipzig applicators are nearly the same and attenuated in inverse square law in the relationship of {100% ½2=ð2 þ dÞ2 }. Field-size dependence of the central-axis percentage depth–dose distributions of small field electron beams was found to be similar to published data [20–25]. The Dmax of the Leipzig applicator and 1 cm diameter electron beams were found near located on the surface as shown in Fig. 1. For all electron
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energies studies, the surface dose, Ds ; increases as the field size becomes small, the depth of Dmax shifts toward the surface, and the dose fall-off region becomes more gradual. For all electron beams, the a-values become small when the beam diameter increased. The isodose curves of Leipzig applicator and 20and 6-MeV electron beams in 3 cm diameter are plotted in Figs. 2(a) and (b). In Fig. 2(a), the left-
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hand side was a vertical type and the right-hand side was a horizontal type of 3 cm aperture applicator. Concavity in all isodose curves along the central axis for the vertical type applicator was observed and found to be similar to measured data . from Kohler-Brock et al. [8] and Evans et al. [9]. The left-hand side of Fig. 2(b) shows the isodose curve distribution of 20-MeV, and the right-hand side shows that of 6-MeV distribution. The isodose curve distribution of the Leipzig applicator displayed a shell-shape and the divergence angle is greater than those of electron beams because of the short SSD and transmission at the applicator wall. The depth (in cm) and the area (in cm2) of the 90% isodose curve for the Leipzig applicator and all energies of electron beams were plotted and are compared in Figs. 3 and 4. The 80% depth of Leipzig applicator was located at 2 mm and was at least 3–6-fold less than those of electron beams, whereas the 90% isodose dose curve area was at least 2–5-fold less than those of electron beams. The flatness of each beam at Dmax was analyzed by the water phantom software and the values were summarized in Table 2. The high-energy electron beams with larger aperture were more uniform than those of low energy ones with small aperture field, demonstrating that high-energy electrons were mostly forward scattered at a depth of Dmax : The flatness of the Leipzig applicator beam was similar to that of a 9-MeV beam. Leakages of the two types of Leipzig applicator with 2 cm aperture are shown in Fig. 5. The main leakage was located around the source and at the top of both types of applicator, with 30% dose to Dmax :
6cm
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10% (b) Fig. 2. Isodose curve distributions for 3 cm (a) vertical type (left), horizontal type (right) and (b) 20-MeV (left), 6-MeV (right) electron beams at the 6 6 cm2 applicator.
The output factors of circular block shielded electron beams decreases monotonically with the field size from 3 to 1 cm diameter due to the loss of scatters. Correspondingly, the factors increased when the field diameter increased. The change in output factors with field size was small for the 20MeV beam due to the forward scattering nature of this beam.
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80% depth 2mm
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7 6 5 4 3 2 1 0 Lz-H
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Fig. 4. 90% isodose curve area of both types of Leipzig applicator and electron beams with three different aperture sizes.
The electrons that are scattered at small angles from the cone were blocked by the cerrobend insert with a small opening minimizing the effect of cones on the output factors at small fields. The dose modification in the small field electron beam was caused by the absence of lateral electronic equilibrium inside the phantom and, to a lesser extent, by electron scatter and bremsstrah-
lung contamination introduced by the cerrobend insert. Practically all skin cancers can be irradiated successfully by electron beams ranging from 6 to 20 MeV, depending on the area and thickness of the lesion (depth). For most tumors, the electron beam energy selected puts the 80% or 90% isodose line at the desired treatment depth.
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Table 2 The flatness at depth of Dmax for various aperture of both type Leipzig applicators and electron beams Aperture diameter (cm)
Leipzig applicator 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
Flatness at Dmax 1 (%)
2 (%)
3 (%)
717.65 722.95 722.72 722.20 721.55 719.65
714.64 720.32 717.21 715.51 714.17 713.07
712.99 718.06 713.39 711.60 78.66 75.73
doses that destroy the basal layers of the epidermis but spare the underlying dermis [13]. Most skin cancers can be treated with either a Leipzig applicator or low energy electron beams, and optimal use of either modality requires knowledge of its specific beam characteristics. According to our measurements, Leipzig applicators deposit a maximum dose at the skin surface, and there is a decrease of dose according to depth 120
100 Percentage Depth Dose
Dosimetry characteristics of Leipzig applicator were observed similar to that of 6- and 9-MeV electron beams, but the electron beams penetrated 90% deeper and had a larger isodose curve area. Higher energy electron beams (>9-MeV) deposit higher doses at the deeper depth and in normal tissue. Epidermis, the outer layer of the skin varies in depth from 0.3 mm on the eyelids and flexural areas to 1.5 mm on the palms of the hands and soles of the feet. The most common malignant tumor of the skin is basal cell carcinoma, which arises from the basal layer of the epidermis. In general, treatment of most skin cancers requires
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20
0
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4 Depth (cm)
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6
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8
Fig. 6. Comparison of central axis depth–dose curves of Leipzig applicator to 6-MeV, 1 cm aperture, and 6-MeV, 3 cm aperture plus 1 cm bolus.
Fig. 5. Radiation leakage measure of 2-cm aperture, both types of Leipzig applicator, left-horizontal type and right-vertical type.
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proportional to the inverse square, resulting in significant penetration and dosage to deeper structures relative to low energy electron beams (p9-MeV). Low energy electron beams offer the advantage of rapid reduction of depth–dose determined by beam energy, and therefore greater sparing of underlying normal structures. However, they have the disadvantage of skin sparing, with build-up to maximum dose beneath the skin surface at a larger aperture field. Although low energy electrons are suitable for superficial tumors because of their sharp dose cut-off, the use of electron beams that were not modified is limited and in some cases undesirable because of their pronounced dose build up and resultant low surface dose. Electron beam modifications (water-equivalent bolus, or beam spoiler) are necessary to increase the surface dose to 100% for the treatment of skin cancers [14,17], as shown in Fig. 6.
5. Conclusions The choice between these modalities should be determined by the size, depth, and anatomic location of the lesion, as well as the setup procedure, and the output. The quality of radiation should be chosen on the basis of the best ratio between surface dose and the ideal depth–dose (i.e. Dmax ; 90% or 80%). The use of Leipzig applicator is advocated as an alternative to electron beam irradiation in the treatment of superficial skin cancers. It is particularly useful in situations of lesions located around the eyelids, in the periorbital areas, in the medial triangle of the cheek, and in the ear and nose where electron applicator treatment is not suitable to setup and perform.
Acknowledgements The authors wish to express their sincere appreciation to the National Science Council of ROC for the financial support of this work. We would also like to thank Professor G.C. Liu for his valuable assistances.
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