- Email: [email protected]
A conformal technique for a ring shaped conjunctive lymphoma treatment
Radiotherapy and Oncology 49 (1999) 315±318
Technical note
A conformal technique for a ring shaped conjunctive lymphoma treatment Rafael ArraÂns a,*, Sara Alonso a, Francisco SaÂnchez-Doblado a, b, Jose Antonio SaÂnchez-Calzado c, Antonio Leal b, Maria Perucha b b
a Department of Medical Physics, University Hospital Virgen Macarena, Av. Dr. Fedriani, 3. E-41009 Sevilla, Spain Department of Medical Physiology and Biophysics, University of Sevilla, Av. Sanchez Pijuan, 9. E-41009 Sevilla, Spain c Department of Radiotherapy, University Hospital Virgen Macarena, Av. Dr. Fedriani, 3. E-41009 Sevilla, Spain
Received 7 May 1998; received in revised form 13 January 1999; accepted 3 February 1999
Abstract Radiotherapy is commonly utilised as standard treatment in the so called mucosa-associated lymphoid tissues (MALT), due to the low probability of distant relapse.The particularities of the lesion, make necessary both energy degradation and beam conformation. To keep homogeneity within acceptable limits, a lengthener attached to the electron applicator has been devised to closely ®t the anatomy of the patient. Considering the small area of the outcoming ®eld, ®lm dosimetry is preferred, since the dimensions of an ionisation chamber and even of a semiconductor probe might be comparable to the ®eld size. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Electron therapy; Energy degradation; Small ®eld dosimetry; Mucosa-associated lymphoid tissues
1. Introduction Lymphoid elements associated with epithelium in the respiratory and gastro-instinal tracts, salivary glands, conjunctiva and others are referred to as the mucosa-associated lymphoid tissues (MALT) [2±5,8,10,11,14]. MALTtype lymphomas of the conjunctiva are rare, and some patients are affected bilaterally [6,13]. Radiotherapy is usually considered to be the choice of treatment because the risk of distant relapse is low in such situations [1,13]. A case of MALT-type conjunctival malignant lymphoma is reported. The patient, a 28 year old female embroiderer, was referred to our radiotherapy department with the diagnosis of conjunctival lymphoma of the marginal zone. Physical examination revealed a reddish circumferential perilimbal elevated lesion. The prescription was 36 Gy in 20 sessions (1.8 Gy/session) at the 90% level. The duration of the treatment was four weeks (®ve sessions/week). Due to the particular geometry and depth of the target region, a partial shielding of the smallest electron applicator available was needed as well as a customised energy degradation. Both beam variations may in¯uence the dosimetry as the ®eld size becomes comparable to the detector dimension. Furthermore, the relative output factor (ROF) needs to be carefully considered since the depth of dose maximum dmax moves towards the surface as the ®eld gets smaller. * Corresponding author.
The irradiation technique and the devices developed to deliver adequately the treatment will be described in this work.
2. Materials and methods The 6 MeV nominal electron energy provided by a Siemens Mevatron 74 linac, was degraded until a practical range of some 15 mm was reached, to match the clinical requirements. This was achieved with a PMMA plate 12.7 mm thick. The outer diameter of the electron cone did not allow to place the centre of the ®eld in contact with the patient's skin, due to the contour irregularities introduced by the nasal septum and orbit of the patient which left in an air gap of 2 cm, which would widen the penumbra to unacceptable levels. To minimise the beam spread associated to these low energies, a lengthener crown was designed. It was attached to the electron applicator, which permitted the end of the cone to be in close contact with the patient's skin. It was made by means of a tube of brass, which allowed for the divergence of the beam, having a very narrow circular ¯ange at its distal end to prevent the PMMA degrader from dropping. The inner diameter of this tube was chosen according to the lesion size (external diameter of approximately 35 mm including safety margins).
0167-8140/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0167-814 0(99)00039-0
316
R. ArraÂns et al. / Radiotherapy and Oncology 50 (1999) 315±318
reliable detector we possess in our centre to measure low energies, is the parallel plate ionisation chamber. However, the NACP chamber housing dimensions (diameter of 30 mm), were approximately the same size of the beam de®ned with the lengthener (diameter of 35 mm). Direct evaluation of the dose by means of such an ionisation chamber should therefore be avoided. The method which was followed instead to perform the absolute dosimetry used the correlation of the depth dose distributions of both a large ®eld and the customised one, should these be comparable, one could then correlate them by ROFs, de®ned as ROF A Dose A; dmax =Dose Aref ; dmaxref Mu A; dmax =Mu Aref ;dmax
ref
£ sw;air A; dmax =sw;air Aref ; dmaxref £ Pu dmax =Pu dmaxref Fig. 1. Pro®le of a beam with lengthener, PMMA degrader and lead shielding, measured both with semiconductor probe and with ®lm dosimetry.
To prevent any damage to either cornea or eye lens, a 2mm thick circular shielding made of lead was located in the centre of the ®eld over the patient's eyelid. The protection was given the curvature of the eyeball to achieve the best ®t to the patient's anatomy. To ensure the position of this shielding, the patient was instructed to stare at a ®xed point on the linac head with the unaffected eye. Pro®les and PDD curves were measured with a radiation ®eld analyser RFA-300 manufactured by Scanditronix. With the degraded beam, a PDD curve was measured, both with a NACP parallel plate chamber and with a diode, to be sure of its response with electron energy. The agreement between both curves was within ^0.5%, even at the X ray tail where the diode could overestimate the dose due to its increased sensitivity below 400 keV. Nevertheless, because of the small size of the effective beam surface, the dimension of the detector might be comparable to that of the ®eld, even using semiconductor probes. An alternative method, such as ®lm dosimetry, should then be used to get the proper shape of the pro®les. We used Kodak X-Omat V ®lms which were developed at the same time to avoid variations due to the developing process. The optical density to dose calibration was done by exposing individual ®lms of the same batch to known values of dose, from zero to 0.5 Gy, in steps of 0.05 Gy. Although stopping power and linear energy transfer of electrons increase faster in ®lm than in phantom, it has been found that ®lm sensitivity remains independent of electron energy [7]. This fact allows reliability of measurements both in PDD curves and near the edges of a pro®le. In dealing with small ®elds an additional problem arises when one faces up to the absolute dosimetry. The most
where [9,12] Mu are the meter readings, corrected for temperature, pressure, recombination, etc., at the depths of maximum absorbed dose for the ®eld sizes A and Aref, and Pu are the perturbation factors. For large ®elds, this expression reduces to ROF A Mu A; dmax =Mu Aref ; dmaxref and, moreover, the depths of maximum absorbed dose are co-incident for both ®elds. For small ®eld sizes, however, there is a shift of dmax towards shallower depths and consequently, the quotient of stopping powers, sw,air, is no longer unity. The method followed was the comparison between a ®rst depth dose curve coming from a ®eld large enough to encompass largely the detector, and a second one corresponding to the customised treatment ®eld without the circular shielding, provided we can assume the energy to be the same for both and the displacement of dmax to be negligible. The ROF was then measured by relating the readings of a very small semiconductor probe (Scanditronix EDD-5) located at the surface of a solid water phantom. It was ®rst irradiated by a 10 £ 10 cm 2 and subsequently by the conformal ®eld without circular shielding, both with the same degrading thickness. In addition we checked that the height of the trace for the shielded ®eld did not change when the lead was removed, thus ensuring that the same ROF was valid for both the leaded and unleaded ®elds. 3. Results and discussion The effect of the beam energy degradation lowers R85 from 18.5 to 6.8 mm, while the mean energy at the surface drops from 5.4 to 2.4 MeV as can be derived from the R50 value [9,12]. At this point, it should be taken into account that the mean energy at the surface E0 is de®ned for large
R. ArraÂns et al. / Radiotherapy and Oncology 50 (1999) 315±318
317
of view, and accordingly relate the dose delivered by these two ®elds just by means of ROF. Regarding absolute dosimetry, the calibration factor for the 10 £ 10 cm 2 degraded beam was 0:910 ^ 0:002 £ 102 Gy/M.U. This was modi®ed consequently by the ROF of the customised ®eld which was 0:708 ^ 0:001. This yielded a calibration factor for the customised ®eld of 0:644 ^ 0:002 £ 102 Gy/M.U. 4. Conclusions
Fig. 2. Comparison of two PDD curves for the use of ROF. The reference (10 £ 10 cm 2) and the outcoming conformal ®elds are correlated, both with the same degrading thickness.
®elds at standard distances. This is not strictly so in our case, but nevertheless, it may be an easy parameter to give an approximate description of the energy. The lengthener designed to overcome the beam spread introduced by the air gap slightly varies the depth dose distribution, lowering R85 to 5.2 mm while R50 is now 9.3 mm, which yields a mean energy at the surface of 2.2 MeV. Inserting the circular lead shielding implies an effective beam area so small that it might be comparable to the detector size when measuring a beam pro®le. This fact could distort the true shape of the scan. To take this into account, the same beam set-ups were measured with two different dosimetric methods, semiconductor probes and ®lm. The smaller the ®eld the larger the differences that may be found. Fig. 1 plots the crossplane pro®le (symmetry was found satisfactory) with a particular arrangement (12 mm diameter protection), showing that the semiconductor probe underestimates the dose both at the open part of the beam and under the shielding. Fig. 2 exhibits the depth dose distribution behaviour of a 10 £ 10 cm 2, compared to the 3.5 cm diameter unshielded circular ®eld, both with a beam degrader of the same thickness as the one designed for the treatment. We may see that the dose gradient G de®ned as Rp = Rp 2 Rq is practically the same for both curves, which means that there is no noticeable hardening of the beam and therefore, sw,air can be considered to be the same for both ®eld sizes. We may see also, that there is a negligible (less than l mm) shift of dmax and consequently, the variation of sw,air will be insigni®cant [15] from a practical approach. We may consider, therefore, that the results do not differ from the energy point
The shallow depth of this lymphoma forced us to degrade the lowest available nominal energy, until a penetration compatible with the clinical requirements was reached. It was also necessary to conform the ®eld shape to avoid late effects to either the cornea or the eye lens, yielding an effective beam area comparable with the detector size. This fact has to be taken into account in relative measurements. Regarding absolute dosimetry, additional considerations have to be made concerning constancy of sw,air and dmax. Should these quantities remain approximately constant, reference and customised ®elds can be related by means of ROF. However, if these constrains are not matched, ROFs are overestimated for small ®elds since dmax shifts to shallower depths and sw,air values decrease consequently. In this case, the stopping power ratio should be obtained by means of Monte Carlo calculations or else, another alternative method would be advisable for absolute dosimetry, such us TLD or ®lm. The results obtained with this particular treatment can be easily extrapolated to other cases where either beam conformation or electron energy degradation are needed. Acknowledgements The authors wish to thank Dr. Ben Mijnheer (The Netherlands Cancer Institute) for his valuable comments and criticism. Furthermore, the authors gratefully acknowledge the work of Felipe Sanchez, Segundo Delgado and Jose Rodriguez, from our Hospital workshop, in the construction of the lengthener and the beam degrader. References [1] Gustafsson A. Non-hodgkin's lymphoma (NHL). Acta Oncol. 1990;35(Suppl. 7):102±116. [2] Isaacson PG, Spencer J. Malignant Lymphoma of mucosa-associated lymphoid tissue. Histopathology 1987;11:445±462. [3] Isaacson PG, Spencer J. Malignant lymphoma of mucosa-associated lymphoid tissue (MALT). In: Jones DB, Wright DH, editors. Lymphoproliferative diseases. Immunology in medicine series, 15. Norwell: KIuwer, 1990: 123±146. [4] Isaacson PG. Extranodal lymphomas, the MALT concept. Verh. Dtsch. Ges. Pathol. 1992;76:14±23. [5] Isaacson PG. Pathogenesis and early lesions in extranodal lymphoma. Toxicol. Lett. 1993;67(1±3):237±247.
318
R. ArraÂns et al. / Radiotherapy and Oncology 50 (1999) 315±318
[6] Kawamoto K, Miyanaga Y, Tonyama S. A case of bilateral MALT (mucosa-associated lymphoid tissue) type conjunctival malignant lymphoma. Nippon Ganka Gakkai Zasshi 1996;100(3):246±252. [7] Khan FM, Doppke KP, Hogstrom KR, et al. Clinical electron beam dosimetry, report of AAPM radiation therapy committee task group no. 25. Med. Phys. 1991;18(1):73±109. [8] Luppi M, Longo C, Ferrari MG, et al. Additional neoplasm and HCV infection in low-grade lymphoma of MALT type. Br. J. Haematol. 1996;94(2):373±375. [9] Nordic Association of Clinical Physicists (NACP), Procedures in external radiation therapy dosimetry with electron and photon beams with maximum energies between 1 and 50 MeV, Acta Radiol. Oncol., 19: 55-78, 1980. [10] Thieblemont C, Berger F, Coif®er B. Mucosa-associated lymphoid tissue lymphomas. Curr. Opin. Oncol. 1995;7(5):415±420. [11] Schmitt-Graff A, Raff T, Rahn W, et al. Primare Pulmonale
[12]
[13] [14] [15]
B-Zell-Lymphome vom MALT-typ. Pathologe 1995;16(5):328± 335. Sociedad Espa®ola de Fisica Medica (SEFM), Procedimientos recomendados para la dosimetrõÂa de fotones y electrones de energõÂas comprendidas entre 1 y 50 MeV en radioterapia de haces externos, (SEFM Publ. No. 1/1984), Comite de DosimetrõÂa en Radioterapia, Madrid, 1984. Vitu L, Cosset JM, Briot E, et al. Les lymphomes malins non hodgkiniens de la conjonctive. A propos de 14 cas traites a l'institut Gustave-Roussy. Bull, Cancer Radiother. 1990;77(1):61±68. Wotherspoon AC, Hardman-Lea S, Isaacson PC. Mucosa-associated lymphoid tissue (MALT) in the human conjunctiva. J. Pathol. 1994;174(1):33±37. Zhang GG, Rogers DWO, Cygler JE, Mackie TR. Corrections for electron beam relative output factors due to changes in dmax and ®eld size. Med. Phys. 1998;25:1711±1716.
Technical note
A conformal technique for a ring shaped conjunctive lymphoma treatment Rafael ArraÂns a,*, Sara Alonso a, Francisco SaÂnchez-Doblado a, b, Jose Antonio SaÂnchez-Calzado c, Antonio Leal b, Maria Perucha b b
a Department of Medical Physics, University Hospital Virgen Macarena, Av. Dr. Fedriani, 3. E-41009 Sevilla, Spain Department of Medical Physiology and Biophysics, University of Sevilla, Av. Sanchez Pijuan, 9. E-41009 Sevilla, Spain c Department of Radiotherapy, University Hospital Virgen Macarena, Av. Dr. Fedriani, 3. E-41009 Sevilla, Spain
Received 7 May 1998; received in revised form 13 January 1999; accepted 3 February 1999
Abstract Radiotherapy is commonly utilised as standard treatment in the so called mucosa-associated lymphoid tissues (MALT), due to the low probability of distant relapse.The particularities of the lesion, make necessary both energy degradation and beam conformation. To keep homogeneity within acceptable limits, a lengthener attached to the electron applicator has been devised to closely ®t the anatomy of the patient. Considering the small area of the outcoming ®eld, ®lm dosimetry is preferred, since the dimensions of an ionisation chamber and even of a semiconductor probe might be comparable to the ®eld size. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Electron therapy; Energy degradation; Small ®eld dosimetry; Mucosa-associated lymphoid tissues
1. Introduction Lymphoid elements associated with epithelium in the respiratory and gastro-instinal tracts, salivary glands, conjunctiva and others are referred to as the mucosa-associated lymphoid tissues (MALT) [2±5,8,10,11,14]. MALTtype lymphomas of the conjunctiva are rare, and some patients are affected bilaterally [6,13]. Radiotherapy is usually considered to be the choice of treatment because the risk of distant relapse is low in such situations [1,13]. A case of MALT-type conjunctival malignant lymphoma is reported. The patient, a 28 year old female embroiderer, was referred to our radiotherapy department with the diagnosis of conjunctival lymphoma of the marginal zone. Physical examination revealed a reddish circumferential perilimbal elevated lesion. The prescription was 36 Gy in 20 sessions (1.8 Gy/session) at the 90% level. The duration of the treatment was four weeks (®ve sessions/week). Due to the particular geometry and depth of the target region, a partial shielding of the smallest electron applicator available was needed as well as a customised energy degradation. Both beam variations may in¯uence the dosimetry as the ®eld size becomes comparable to the detector dimension. Furthermore, the relative output factor (ROF) needs to be carefully considered since the depth of dose maximum dmax moves towards the surface as the ®eld gets smaller. * Corresponding author.
The irradiation technique and the devices developed to deliver adequately the treatment will be described in this work.
2. Materials and methods The 6 MeV nominal electron energy provided by a Siemens Mevatron 74 linac, was degraded until a practical range of some 15 mm was reached, to match the clinical requirements. This was achieved with a PMMA plate 12.7 mm thick. The outer diameter of the electron cone did not allow to place the centre of the ®eld in contact with the patient's skin, due to the contour irregularities introduced by the nasal septum and orbit of the patient which left in an air gap of 2 cm, which would widen the penumbra to unacceptable levels. To minimise the beam spread associated to these low energies, a lengthener crown was designed. It was attached to the electron applicator, which permitted the end of the cone to be in close contact with the patient's skin. It was made by means of a tube of brass, which allowed for the divergence of the beam, having a very narrow circular ¯ange at its distal end to prevent the PMMA degrader from dropping. The inner diameter of this tube was chosen according to the lesion size (external diameter of approximately 35 mm including safety margins).
0167-8140/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0167-814 0(99)00039-0
316
R. ArraÂns et al. / Radiotherapy and Oncology 50 (1999) 315±318
reliable detector we possess in our centre to measure low energies, is the parallel plate ionisation chamber. However, the NACP chamber housing dimensions (diameter of 30 mm), were approximately the same size of the beam de®ned with the lengthener (diameter of 35 mm). Direct evaluation of the dose by means of such an ionisation chamber should therefore be avoided. The method which was followed instead to perform the absolute dosimetry used the correlation of the depth dose distributions of both a large ®eld and the customised one, should these be comparable, one could then correlate them by ROFs, de®ned as ROF A Dose A; dmax =Dose Aref ; dmaxref Mu A; dmax =Mu Aref ;dmax
ref
£ sw;air A; dmax =sw;air Aref ; dmaxref £ Pu dmax =Pu dmaxref Fig. 1. Pro®le of a beam with lengthener, PMMA degrader and lead shielding, measured both with semiconductor probe and with ®lm dosimetry.
To prevent any damage to either cornea or eye lens, a 2mm thick circular shielding made of lead was located in the centre of the ®eld over the patient's eyelid. The protection was given the curvature of the eyeball to achieve the best ®t to the patient's anatomy. To ensure the position of this shielding, the patient was instructed to stare at a ®xed point on the linac head with the unaffected eye. Pro®les and PDD curves were measured with a radiation ®eld analyser RFA-300 manufactured by Scanditronix. With the degraded beam, a PDD curve was measured, both with a NACP parallel plate chamber and with a diode, to be sure of its response with electron energy. The agreement between both curves was within ^0.5%, even at the X ray tail where the diode could overestimate the dose due to its increased sensitivity below 400 keV. Nevertheless, because of the small size of the effective beam surface, the dimension of the detector might be comparable to that of the ®eld, even using semiconductor probes. An alternative method, such as ®lm dosimetry, should then be used to get the proper shape of the pro®les. We used Kodak X-Omat V ®lms which were developed at the same time to avoid variations due to the developing process. The optical density to dose calibration was done by exposing individual ®lms of the same batch to known values of dose, from zero to 0.5 Gy, in steps of 0.05 Gy. Although stopping power and linear energy transfer of electrons increase faster in ®lm than in phantom, it has been found that ®lm sensitivity remains independent of electron energy [7]. This fact allows reliability of measurements both in PDD curves and near the edges of a pro®le. In dealing with small ®elds an additional problem arises when one faces up to the absolute dosimetry. The most
where [9,12] Mu are the meter readings, corrected for temperature, pressure, recombination, etc., at the depths of maximum absorbed dose for the ®eld sizes A and Aref, and Pu are the perturbation factors. For large ®elds, this expression reduces to ROF A Mu A; dmax =Mu Aref ; dmaxref and, moreover, the depths of maximum absorbed dose are co-incident for both ®elds. For small ®eld sizes, however, there is a shift of dmax towards shallower depths and consequently, the quotient of stopping powers, sw,air, is no longer unity. The method followed was the comparison between a ®rst depth dose curve coming from a ®eld large enough to encompass largely the detector, and a second one corresponding to the customised treatment ®eld without the circular shielding, provided we can assume the energy to be the same for both and the displacement of dmax to be negligible. The ROF was then measured by relating the readings of a very small semiconductor probe (Scanditronix EDD-5) located at the surface of a solid water phantom. It was ®rst irradiated by a 10 £ 10 cm 2 and subsequently by the conformal ®eld without circular shielding, both with the same degrading thickness. In addition we checked that the height of the trace for the shielded ®eld did not change when the lead was removed, thus ensuring that the same ROF was valid for both the leaded and unleaded ®elds. 3. Results and discussion The effect of the beam energy degradation lowers R85 from 18.5 to 6.8 mm, while the mean energy at the surface drops from 5.4 to 2.4 MeV as can be derived from the R50 value [9,12]. At this point, it should be taken into account that the mean energy at the surface E0 is de®ned for large
R. ArraÂns et al. / Radiotherapy and Oncology 50 (1999) 315±318
317
of view, and accordingly relate the dose delivered by these two ®elds just by means of ROF. Regarding absolute dosimetry, the calibration factor for the 10 £ 10 cm 2 degraded beam was 0:910 ^ 0:002 £ 102 Gy/M.U. This was modi®ed consequently by the ROF of the customised ®eld which was 0:708 ^ 0:001. This yielded a calibration factor for the customised ®eld of 0:644 ^ 0:002 £ 102 Gy/M.U. 4. Conclusions
Fig. 2. Comparison of two PDD curves for the use of ROF. The reference (10 £ 10 cm 2) and the outcoming conformal ®elds are correlated, both with the same degrading thickness.
®elds at standard distances. This is not strictly so in our case, but nevertheless, it may be an easy parameter to give an approximate description of the energy. The lengthener designed to overcome the beam spread introduced by the air gap slightly varies the depth dose distribution, lowering R85 to 5.2 mm while R50 is now 9.3 mm, which yields a mean energy at the surface of 2.2 MeV. Inserting the circular lead shielding implies an effective beam area so small that it might be comparable to the detector size when measuring a beam pro®le. This fact could distort the true shape of the scan. To take this into account, the same beam set-ups were measured with two different dosimetric methods, semiconductor probes and ®lm. The smaller the ®eld the larger the differences that may be found. Fig. 1 plots the crossplane pro®le (symmetry was found satisfactory) with a particular arrangement (12 mm diameter protection), showing that the semiconductor probe underestimates the dose both at the open part of the beam and under the shielding. Fig. 2 exhibits the depth dose distribution behaviour of a 10 £ 10 cm 2, compared to the 3.5 cm diameter unshielded circular ®eld, both with a beam degrader of the same thickness as the one designed for the treatment. We may see that the dose gradient G de®ned as Rp = Rp 2 Rq is practically the same for both curves, which means that there is no noticeable hardening of the beam and therefore, sw,air can be considered to be the same for both ®eld sizes. We may see also, that there is a negligible (less than l mm) shift of dmax and consequently, the variation of sw,air will be insigni®cant [15] from a practical approach. We may consider, therefore, that the results do not differ from the energy point
The shallow depth of this lymphoma forced us to degrade the lowest available nominal energy, until a penetration compatible with the clinical requirements was reached. It was also necessary to conform the ®eld shape to avoid late effects to either the cornea or the eye lens, yielding an effective beam area comparable with the detector size. This fact has to be taken into account in relative measurements. Regarding absolute dosimetry, additional considerations have to be made concerning constancy of sw,air and dmax. Should these quantities remain approximately constant, reference and customised ®elds can be related by means of ROF. However, if these constrains are not matched, ROFs are overestimated for small ®elds since dmax shifts to shallower depths and sw,air values decrease consequently. In this case, the stopping power ratio should be obtained by means of Monte Carlo calculations or else, another alternative method would be advisable for absolute dosimetry, such us TLD or ®lm. The results obtained with this particular treatment can be easily extrapolated to other cases where either beam conformation or electron energy degradation are needed. Acknowledgements The authors wish to thank Dr. Ben Mijnheer (The Netherlands Cancer Institute) for his valuable comments and criticism. Furthermore, the authors gratefully acknowledge the work of Felipe Sanchez, Segundo Delgado and Jose Rodriguez, from our Hospital workshop, in the construction of the lengthener and the beam degrader. References [1] Gustafsson A. Non-hodgkin's lymphoma (NHL). Acta Oncol. 1990;35(Suppl. 7):102±116. [2] Isaacson PG, Spencer J. Malignant Lymphoma of mucosa-associated lymphoid tissue. Histopathology 1987;11:445±462. [3] Isaacson PG, Spencer J. Malignant lymphoma of mucosa-associated lymphoid tissue (MALT). In: Jones DB, Wright DH, editors. Lymphoproliferative diseases. Immunology in medicine series, 15. Norwell: KIuwer, 1990: 123±146. [4] Isaacson PG. Extranodal lymphomas, the MALT concept. Verh. Dtsch. Ges. Pathol. 1992;76:14±23. [5] Isaacson PG. Pathogenesis and early lesions in extranodal lymphoma. Toxicol. Lett. 1993;67(1±3):237±247.
318
R. ArraÂns et al. / Radiotherapy and Oncology 50 (1999) 315±318
[6] Kawamoto K, Miyanaga Y, Tonyama S. A case of bilateral MALT (mucosa-associated lymphoid tissue) type conjunctival malignant lymphoma. Nippon Ganka Gakkai Zasshi 1996;100(3):246±252. [7] Khan FM, Doppke KP, Hogstrom KR, et al. Clinical electron beam dosimetry, report of AAPM radiation therapy committee task group no. 25. Med. Phys. 1991;18(1):73±109. [8] Luppi M, Longo C, Ferrari MG, et al. Additional neoplasm and HCV infection in low-grade lymphoma of MALT type. Br. J. Haematol. 1996;94(2):373±375. [9] Nordic Association of Clinical Physicists (NACP), Procedures in external radiation therapy dosimetry with electron and photon beams with maximum energies between 1 and 50 MeV, Acta Radiol. Oncol., 19: 55-78, 1980. [10] Thieblemont C, Berger F, Coif®er B. Mucosa-associated lymphoid tissue lymphomas. Curr. Opin. Oncol. 1995;7(5):415±420. [11] Schmitt-Graff A, Raff T, Rahn W, et al. Primare Pulmonale
[12]
[13] [14] [15]
B-Zell-Lymphome vom MALT-typ. Pathologe 1995;16(5):328± 335. Sociedad Espa®ola de Fisica Medica (SEFM), Procedimientos recomendados para la dosimetrõÂa de fotones y electrones de energõÂas comprendidas entre 1 y 50 MeV en radioterapia de haces externos, (SEFM Publ. No. 1/1984), Comite de DosimetrõÂa en Radioterapia, Madrid, 1984. Vitu L, Cosset JM, Briot E, et al. Les lymphomes malins non hodgkiniens de la conjonctive. A propos de 14 cas traites a l'institut Gustave-Roussy. Bull, Cancer Radiother. 1990;77(1):61±68. Wotherspoon AC, Hardman-Lea S, Isaacson PC. Mucosa-associated lymphoid tissue (MALT) in the human conjunctiva. J. Pathol. 1994;174(1):33±37. Zhang GG, Rogers DWO, Cygler JE, Mackie TR. Corrections for electron beam relative output factors due to changes in dmax and ®eld size. Med. Phys. 1998;25:1711±1716.