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Orbital lymphoma: a simple treatment using electrons
Medical Dosimetry, Vol. 28, No. 2, pp. 95–98, 2003 Copyright © 2003 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/03/$–see front matter
doi:10.1016/S0958-3947(02)00243-1
ORBITAL LYMPHOMA: A SIMPLE TREATMENT USING ELECTRONS KYM RYKERS, PH.D., GENINE UDEN, B. APP. SC., and VICKI THOMPSON, DIP. APP. SC. Radiation Oncology Victoria, Melbourne, Victoria, Australia (Accepted 10 May 2002)
Abstract—Previous papers have discussed the successful treatment of orbital lymphoid tumors using a lowvacuum contact lens placed directly on the eye and the use of a lead bar suspended over the eye with a retort stand. In this paper, a novel approach using a cerrobend bar attached to an electron shield for the treatment of a conjunctival lymphoma is presented. With this approach, the entire eye, excluding the lens, may be treated with a single field. Isodose distributions measured in a water phantom are compared with those planned on a 3D radiotherapy treatment planning system, where the effects of an external eye bar can be shown. The clinical outcome of the treatment is also shown. © 2003 American Association of Medical Dosimetrists. Key Words:
Orbital lymphoma, Electrons, Radiation treatment planning.
a single field. The approach presented here avoids blocking any area of the field and avoids the hot spots associated with the low-vac lens.1
INTRODUCTION Lymphomas of the orbit are rare.1 Less than 1% of patients with lymphoma present with an orbital primary site. They can occur in the orbit, globe, or surrounding adnexal structures, with involvement in 1 or both eyes. Presentation is generally with a slowly increasing mass. Excessive tearing and eye discomfort are common symptoms. The conjunctiva is the most commonly involved site. These lesions appear as a diffuse salmon-colored fleshy mass, with relatively few symptoms. The majority are low-grade neoplasms, often localized and carrying a good prognosis. Diagnosis is commonly made by biopsy, and a full range of diagnostic studies may be performed to stage the disease.1,2 Radiotherapy is often the primary form of treatment, as the lesions are radiosensitive and radiocurable.2 The conjunctival lesions are superficial and, therefore, preferably treated with electron beam therapy.2 The borders of the lesions are often indistinct; consequently, it is often necessary to treat the entire conjunctiva. The lens is the most sensitive structure and should be shielded from irradiation to prevent the formation of cataracts.1,2 In the dose range required for control of small non-Hodgkin lymphomas, the tolerance of other adjoining structures is of little concern. Previous papers1–3 employing electron beam radiation therapy have achieved lens shielding through the use of blocks attached to the electron cone, lead cutouts fashioned from plaster casts, a lead block mounted on a “low-vac lens,” and external eye shields held in a retort stand suspended over the lens. The use of any of these approaches does not allow for the entire conjunctiva to be treated with a single field. Each method results in blocking or excluding some part of the conjunctiva from
MATERIALS AND METHODS Patient history A 30-year-old female recently presented with a 9-month history of a unilateral lesion involving the conjunctiva of the right eye. This was a classic salmoncolored, fleshy lesion extending to within 3 to 4 mm of the cornea. The lesion had been biopsied and diagnosis confirmed as a low-grade B-cell lymphoma. Physical examination and a clear chest x-ray confirmed that this was a localized mass. A CT scan of the abdomen was deemed unnecessary, given that these lesions are virtually totally restricted to the eye. Initially, the alternative of chemotherapy was discussed with the patient. This was rejected due to her age, and the possibility of depressed ovarian function caused by the alkylating agent, Chlorambucil.4 Treatment method In keeping with current practice,2,3,5 a course of radiotherapy was the treatment of choice. The treatment goal was to irradiate the entire involved area, while keeping the dose to the lens as low as possible to avoid formation of cataracts.1–3 To immobilize the patient, a personalized cast of the head and neck was made, and the treatment area cut out to expose the eye. A spiral CT scan with 2-mm-thick slices, 2-mm-slice index, and 1.5-mm pitch was acquired with a Marconi PQ2000S CT scanner (Marconi Medical Systems, Highland Heights, OH). The x-ray tube was operated at 130 kVp and 200 mA. The planning treatment volume was delineated clinically by the Radiation Oncologist at simulation. A 4-cmround electron insert provided adequate coverage of the
Reprint requests to: Dr. K. Rykers, Radiation Oncology Centre, Austin and Repatriation Medical Centre, 330 Waterdale Rd., Heidelberg Heights 3081, Australia. 95
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Medical Dosimetry
Fig. 1. Lens shielding attached to downstream side of plastic film.
lesion. The isocenter was permanently marked on the cast at this stage. A rigid, thin (approximately 0.2 mm) plastic sheet was glued to the upstream side of the electron insert. The position of the center of the lens requiring shielding was marked onto the plastic film. A tapered, approximately 1.2 ⫻ 1.0 cm cross-section oval bar was manufactured from cerrobend (Cerrobend Alloy [bismuth 50%, tin 13%, lead 27%, cadmium 10% (158 – 163F) fusible alloy, mass density 9.75 g cm⫺3]; Consolidated Alloy, Thomastown, Australia) to provide said shielding. The height of the block from the patient was calculated so that it would sit 1.0 cm from the eye during treatment when attached to the downstream side of the plastic sheet. This distance is in keeping with previous papers discussing shield of the lens.2,3 The patient was instructed to look straight ahead during treatment. A standard treatment distance of 100 cm focus to surface distance was used. The lens shield was screwed and glued into place on the downstream side of the plastic film (Fig. 1). Bar positioning with patient setup was checked prior to the first treatment (Fig. 2). Dosimetry When selecting the appropriate energy for the electron beam, surface dose, depth dose, and flatness char-
Fig. 2. Bar positioning check prior to first treatment.
Volume 28, Number 2, 2003
acteristics of the available energies with field size were considered, as was the degree of lens protection.2 Minimizing dose to the lens was a concern in energy selection.1,2 With all of these considerations, 12 MeV was chosen for this treatment. The small size of the treatment area, combined with the central shielding, meant that the delivered beam deviated significantly from a “typical” 12-MeV electron field in terms of percentage depth dose (PDD) and field flatness. The influence of field size on these parameters is well known.2,6 Measurements were made in a water phantom to determine the PDD and field flatness achievable with the personalized shielding. These measurements were also made to compare the measured dose with the dose predicted by the planning system (ADAC Pinnacle 3D; ADAC Laboratories, Milpitas, CA). The relative dose delivered per nominal monitor unit setting for the small field was determined by comparison with the dose delivered with a 10 ⫻ 10-cm2 field. Treatment planning The CT data set was transferred from the AcQSim (Marconi Medical Systems) to the ADAC Pinnacle planning system. The cerrobend lens shield was added manually as a region of interest, with a density of 9.75 g cm⫺3. Dose calculation with the planning system was then performed and a 3D dose distribution was produced. RESULTS AND DISCUSSION Dosimetry All measurements were performed for 12-MeV electrons at 100-cm FSD. The dose distributions were measured with IC-10 ion chambers in a 48 ⫻ 48 ⫻ 48 Wellhofer water tank (Wellhofer Dosimetrie, Shwarzenbruck, Germany). Dose distributions were measured for the personalized, 4-cm-round centrally-blocked electron shield and for a reference 10 ⫻ 10-cm2 field. Maximum dose was delivered for the personalized shield at approximately 1-cm depth and 1 cm away from central axis. Maximum dose for the reference field was delivered at 2.9-cm depth. All scanning for the personalized shield was done with the field chamber readings normalized to the reading at Dmax for the reference 10 ⫻ 10-cm2 field. Crossplane/inplane scans for the personalized shield at 1.0 cm and 2.9 cm are given in Fig. 3a and b. The personalized shield had a maximum output equal to approximately 88% of the maximum output of the reference field. This relative output was also confirmed through charge measurements made with a Markus chamber (PTW Frieburg) and Farmer electrometer (9NE Technology Limited, Berkshire, England) in RW3 solid water (PTW Frieburg). The uniformity of the field was also assessed through inspection of the crossplane/inplane scans. Additionally, inplane/beam and crossplane/beam measure-
Orbital lymphoma ● K. RYKERS et al.
97
Fig. 3. Personalized shield: isodose distribution at (a) 1.0-cm depth with 12 MeV (b) 2.9-cm depth with 12 MeV.
ments were taken. The inplane/beam isodose values are given in Fig. 4; a similar distribution occurred in the crossplane/beam direction. If the 30% isodose line is considered, then a volume approximately delineated by a cylinder of 1-cm diameter and 1-cm length is shielded by the electron field central blocking. The lens of the eye lies at a depth of 6 to 9 mm beneath the surface of the cornea, while the lens diameter is approximately 0.8 cm. Thus, the shielded region corresponds to the position of the lens of the eye, which was desired. The remaining open portion of the field covers the conjunctiva and allows for a therapeutic dose to be delivered. Treatment planning and delivery The personalized field was entered and an isodose distribution was produced normalized to the dose at the Dmax of 2.9 cm for 12 MeV. The resultant isodose distribution gave a maximum dose of 107% at a point approximately 1.8 cm below the skin surface with an off-axis distance of approximately 1.0 cm. The dose distribution produced by the planning system in the axial plane at central axis using the patient-specific anatomy is shown in Fig. 5. The distribution was calculated using 0.28-cm3 voxels and a collapsed cone convolution algo-
Fig. 4. Isodose distribution for personalized shield in the inplane/beam plane.
rithm with corrections made for heterogeneities. The lens was also outlined. This distribution is comparable with that measured in the water tank when heterogeneities and non-flat patient anatomy is considered. The presence of the plastic sheet produced no significant variation to the electron isodose distribution. The oncologist inspected the isodose distribution and a dose of 30 Gy total dose in 15 fractions with 5 fractions per week was prescribed. Dose to the entire lens volume was calculated using a dose volume histogram and found to be less than 300 cGy to 100% of the volume. This is well below the 600-cGy level associated with cataract formation.1,2 The treatment area and block position was checked once again prior to commencement of the first treatment. The shielding was inspected on each day of treatment for mechanical damage. The integrity of the plastic sheet was not compromised and the setup was found to be reliable and consistent for the 3 weeks of treatment.
Fig. 5. Central axis dose distribution produced by 3D RTP; isodose lines of 500, 2000, 2700, and 3100 cGy are shown. The lens is also outlined.
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Volume 28, Number 2, 2003
CONCLUSIONS The use of island shielding with an electron shield allowed for the entire conjunctiva to be treated with a single field. The manufacture of the shielding was relatively simple and the use of thin plastic, glue, and screws to attach the central blocking was robust enough for daily treatments to be delivered. Patient setup times were on par with other electron treatments. The clinical result was excellent and the technique has since been used for further patients. The shielding is personalized for each patient and a new insert with new plastic is used for each patient. REFERENCES
Fig. 6. (a) Before and after treatment of the upper conjunctiva. (b) Before and after treatment of the lower conjunctiva.
Patient outcome After approximately 5 increments, the lesion was visibly smaller. At the end of treatment, the lymphoma was no longer visible on the eye (Fig. 6).
1. Jereb, B.; Hyun, L.; Jakobiec, F.A.; Kutcher, J. Radiation therapy of conjunctival and orbital lymphoid tumours. Int. J. Radiat. Oncol. Biol. Phys. 10:1013–9; 1984. 2. Donaldson, S.S.; Findley, D.O. Treatment of orbital lymphoid tumours with electron beams. Front. Radiat. Ther. Oncol. 25:187– 200; 1991. 3. Smitt, M.C.; Donaldson, S.S. Radiotherapy is successful treatment for orbital lymphoma. Int. J. Radiat. Oncol. Biol. Phys. 26:59 –66; 1993. 4. MIMS Australia, MediMedia Aust. Pty. Ltd.; 2001. 5. Perez, C.A.; Brady, L.W. Principles and Practice of Radiation Oncology. Philadelphia: Lippincott Company; 1992. 6. Klevenhagen, S.C. Physics and Dosimetry of Therapy Electron Beams. Medical Physics Publishing; 1993.
doi:10.1016/S0958-3947(02)00243-1
ORBITAL LYMPHOMA: A SIMPLE TREATMENT USING ELECTRONS KYM RYKERS, PH.D., GENINE UDEN, B. APP. SC., and VICKI THOMPSON, DIP. APP. SC. Radiation Oncology Victoria, Melbourne, Victoria, Australia (Accepted 10 May 2002)
Abstract—Previous papers have discussed the successful treatment of orbital lymphoid tumors using a lowvacuum contact lens placed directly on the eye and the use of a lead bar suspended over the eye with a retort stand. In this paper, a novel approach using a cerrobend bar attached to an electron shield for the treatment of a conjunctival lymphoma is presented. With this approach, the entire eye, excluding the lens, may be treated with a single field. Isodose distributions measured in a water phantom are compared with those planned on a 3D radiotherapy treatment planning system, where the effects of an external eye bar can be shown. The clinical outcome of the treatment is also shown. © 2003 American Association of Medical Dosimetrists. Key Words:
Orbital lymphoma, Electrons, Radiation treatment planning.
a single field. The approach presented here avoids blocking any area of the field and avoids the hot spots associated with the low-vac lens.1
INTRODUCTION Lymphomas of the orbit are rare.1 Less than 1% of patients with lymphoma present with an orbital primary site. They can occur in the orbit, globe, or surrounding adnexal structures, with involvement in 1 or both eyes. Presentation is generally with a slowly increasing mass. Excessive tearing and eye discomfort are common symptoms. The conjunctiva is the most commonly involved site. These lesions appear as a diffuse salmon-colored fleshy mass, with relatively few symptoms. The majority are low-grade neoplasms, often localized and carrying a good prognosis. Diagnosis is commonly made by biopsy, and a full range of diagnostic studies may be performed to stage the disease.1,2 Radiotherapy is often the primary form of treatment, as the lesions are radiosensitive and radiocurable.2 The conjunctival lesions are superficial and, therefore, preferably treated with electron beam therapy.2 The borders of the lesions are often indistinct; consequently, it is often necessary to treat the entire conjunctiva. The lens is the most sensitive structure and should be shielded from irradiation to prevent the formation of cataracts.1,2 In the dose range required for control of small non-Hodgkin lymphomas, the tolerance of other adjoining structures is of little concern. Previous papers1–3 employing electron beam radiation therapy have achieved lens shielding through the use of blocks attached to the electron cone, lead cutouts fashioned from plaster casts, a lead block mounted on a “low-vac lens,” and external eye shields held in a retort stand suspended over the lens. The use of any of these approaches does not allow for the entire conjunctiva to be treated with a single field. Each method results in blocking or excluding some part of the conjunctiva from
MATERIALS AND METHODS Patient history A 30-year-old female recently presented with a 9-month history of a unilateral lesion involving the conjunctiva of the right eye. This was a classic salmoncolored, fleshy lesion extending to within 3 to 4 mm of the cornea. The lesion had been biopsied and diagnosis confirmed as a low-grade B-cell lymphoma. Physical examination and a clear chest x-ray confirmed that this was a localized mass. A CT scan of the abdomen was deemed unnecessary, given that these lesions are virtually totally restricted to the eye. Initially, the alternative of chemotherapy was discussed with the patient. This was rejected due to her age, and the possibility of depressed ovarian function caused by the alkylating agent, Chlorambucil.4 Treatment method In keeping with current practice,2,3,5 a course of radiotherapy was the treatment of choice. The treatment goal was to irradiate the entire involved area, while keeping the dose to the lens as low as possible to avoid formation of cataracts.1–3 To immobilize the patient, a personalized cast of the head and neck was made, and the treatment area cut out to expose the eye. A spiral CT scan with 2-mm-thick slices, 2-mm-slice index, and 1.5-mm pitch was acquired with a Marconi PQ2000S CT scanner (Marconi Medical Systems, Highland Heights, OH). The x-ray tube was operated at 130 kVp and 200 mA. The planning treatment volume was delineated clinically by the Radiation Oncologist at simulation. A 4-cmround electron insert provided adequate coverage of the
Reprint requests to: Dr. K. Rykers, Radiation Oncology Centre, Austin and Repatriation Medical Centre, 330 Waterdale Rd., Heidelberg Heights 3081, Australia. 95
96
Medical Dosimetry
Fig. 1. Lens shielding attached to downstream side of plastic film.
lesion. The isocenter was permanently marked on the cast at this stage. A rigid, thin (approximately 0.2 mm) plastic sheet was glued to the upstream side of the electron insert. The position of the center of the lens requiring shielding was marked onto the plastic film. A tapered, approximately 1.2 ⫻ 1.0 cm cross-section oval bar was manufactured from cerrobend (Cerrobend Alloy [bismuth 50%, tin 13%, lead 27%, cadmium 10% (158 – 163F) fusible alloy, mass density 9.75 g cm⫺3]; Consolidated Alloy, Thomastown, Australia) to provide said shielding. The height of the block from the patient was calculated so that it would sit 1.0 cm from the eye during treatment when attached to the downstream side of the plastic sheet. This distance is in keeping with previous papers discussing shield of the lens.2,3 The patient was instructed to look straight ahead during treatment. A standard treatment distance of 100 cm focus to surface distance was used. The lens shield was screwed and glued into place on the downstream side of the plastic film (Fig. 1). Bar positioning with patient setup was checked prior to the first treatment (Fig. 2). Dosimetry When selecting the appropriate energy for the electron beam, surface dose, depth dose, and flatness char-
Fig. 2. Bar positioning check prior to first treatment.
Volume 28, Number 2, 2003
acteristics of the available energies with field size were considered, as was the degree of lens protection.2 Minimizing dose to the lens was a concern in energy selection.1,2 With all of these considerations, 12 MeV was chosen for this treatment. The small size of the treatment area, combined with the central shielding, meant that the delivered beam deviated significantly from a “typical” 12-MeV electron field in terms of percentage depth dose (PDD) and field flatness. The influence of field size on these parameters is well known.2,6 Measurements were made in a water phantom to determine the PDD and field flatness achievable with the personalized shielding. These measurements were also made to compare the measured dose with the dose predicted by the planning system (ADAC Pinnacle 3D; ADAC Laboratories, Milpitas, CA). The relative dose delivered per nominal monitor unit setting for the small field was determined by comparison with the dose delivered with a 10 ⫻ 10-cm2 field. Treatment planning The CT data set was transferred from the AcQSim (Marconi Medical Systems) to the ADAC Pinnacle planning system. The cerrobend lens shield was added manually as a region of interest, with a density of 9.75 g cm⫺3. Dose calculation with the planning system was then performed and a 3D dose distribution was produced. RESULTS AND DISCUSSION Dosimetry All measurements were performed for 12-MeV electrons at 100-cm FSD. The dose distributions were measured with IC-10 ion chambers in a 48 ⫻ 48 ⫻ 48 Wellhofer water tank (Wellhofer Dosimetrie, Shwarzenbruck, Germany). Dose distributions were measured for the personalized, 4-cm-round centrally-blocked electron shield and for a reference 10 ⫻ 10-cm2 field. Maximum dose was delivered for the personalized shield at approximately 1-cm depth and 1 cm away from central axis. Maximum dose for the reference field was delivered at 2.9-cm depth. All scanning for the personalized shield was done with the field chamber readings normalized to the reading at Dmax for the reference 10 ⫻ 10-cm2 field. Crossplane/inplane scans for the personalized shield at 1.0 cm and 2.9 cm are given in Fig. 3a and b. The personalized shield had a maximum output equal to approximately 88% of the maximum output of the reference field. This relative output was also confirmed through charge measurements made with a Markus chamber (PTW Frieburg) and Farmer electrometer (9NE Technology Limited, Berkshire, England) in RW3 solid water (PTW Frieburg). The uniformity of the field was also assessed through inspection of the crossplane/inplane scans. Additionally, inplane/beam and crossplane/beam measure-
Orbital lymphoma ● K. RYKERS et al.
97
Fig. 3. Personalized shield: isodose distribution at (a) 1.0-cm depth with 12 MeV (b) 2.9-cm depth with 12 MeV.
ments were taken. The inplane/beam isodose values are given in Fig. 4; a similar distribution occurred in the crossplane/beam direction. If the 30% isodose line is considered, then a volume approximately delineated by a cylinder of 1-cm diameter and 1-cm length is shielded by the electron field central blocking. The lens of the eye lies at a depth of 6 to 9 mm beneath the surface of the cornea, while the lens diameter is approximately 0.8 cm. Thus, the shielded region corresponds to the position of the lens of the eye, which was desired. The remaining open portion of the field covers the conjunctiva and allows for a therapeutic dose to be delivered. Treatment planning and delivery The personalized field was entered and an isodose distribution was produced normalized to the dose at the Dmax of 2.9 cm for 12 MeV. The resultant isodose distribution gave a maximum dose of 107% at a point approximately 1.8 cm below the skin surface with an off-axis distance of approximately 1.0 cm. The dose distribution produced by the planning system in the axial plane at central axis using the patient-specific anatomy is shown in Fig. 5. The distribution was calculated using 0.28-cm3 voxels and a collapsed cone convolution algo-
Fig. 4. Isodose distribution for personalized shield in the inplane/beam plane.
rithm with corrections made for heterogeneities. The lens was also outlined. This distribution is comparable with that measured in the water tank when heterogeneities and non-flat patient anatomy is considered. The presence of the plastic sheet produced no significant variation to the electron isodose distribution. The oncologist inspected the isodose distribution and a dose of 30 Gy total dose in 15 fractions with 5 fractions per week was prescribed. Dose to the entire lens volume was calculated using a dose volume histogram and found to be less than 300 cGy to 100% of the volume. This is well below the 600-cGy level associated with cataract formation.1,2 The treatment area and block position was checked once again prior to commencement of the first treatment. The shielding was inspected on each day of treatment for mechanical damage. The integrity of the plastic sheet was not compromised and the setup was found to be reliable and consistent for the 3 weeks of treatment.
Fig. 5. Central axis dose distribution produced by 3D RTP; isodose lines of 500, 2000, 2700, and 3100 cGy are shown. The lens is also outlined.
98
Medical Dosimetry
Volume 28, Number 2, 2003
CONCLUSIONS The use of island shielding with an electron shield allowed for the entire conjunctiva to be treated with a single field. The manufacture of the shielding was relatively simple and the use of thin plastic, glue, and screws to attach the central blocking was robust enough for daily treatments to be delivered. Patient setup times were on par with other electron treatments. The clinical result was excellent and the technique has since been used for further patients. The shielding is personalized for each patient and a new insert with new plastic is used for each patient. REFERENCES
Fig. 6. (a) Before and after treatment of the upper conjunctiva. (b) Before and after treatment of the lower conjunctiva.
Patient outcome After approximately 5 increments, the lesion was visibly smaller. At the end of treatment, the lymphoma was no longer visible on the eye (Fig. 6).
1. Jereb, B.; Hyun, L.; Jakobiec, F.A.; Kutcher, J. Radiation therapy of conjunctival and orbital lymphoid tumours. Int. J. Radiat. Oncol. Biol. Phys. 10:1013–9; 1984. 2. Donaldson, S.S.; Findley, D.O. Treatment of orbital lymphoid tumours with electron beams. Front. Radiat. Ther. Oncol. 25:187– 200; 1991. 3. Smitt, M.C.; Donaldson, S.S. Radiotherapy is successful treatment for orbital lymphoma. Int. J. Radiat. Oncol. Biol. Phys. 26:59 –66; 1993. 4. MIMS Australia, MediMedia Aust. Pty. Ltd.; 2001. 5. Perez, C.A.; Brady, L.W. Principles and Practice of Radiation Oncology. Philadelphia: Lippincott Company; 1992. 6. Klevenhagen, S.C. Physics and Dosimetry of Therapy Electron Beams. Medical Physics Publishing; 1993.