Influence of internal fixation systems on radiation therapy for spinal tumor

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1 JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 16, NUMBER 4, 2015 Influence of internl fixtion systems on rdition therpy for spinl tumor Jingfeng Li, Lei Yn, Jinping Wng, Lin Ci, Dongci Hu Deprtment of Orthopedics, Zhongnn Hospitl of Wuhn University, Wuhn, Hubei, Chin Received 29 November, 2014; ccepted 18 Februry, 2015 In this study, the influence of internl fixtion systems on rdition therpy for spinl tumor ws investigted in order to derive theoreticl bsis for djustment of rdition dose for ptients with spinl tumor nd internl fixtion. Bsed on common method of internl fixtion fter resection of spinl tumor, different models of spinl internl fixtion were constructed using the lumbr vertebr of fresh domestic pigs nd titnium lloy s the internl fixtion system. Vritions in rdition dose in the vertebrl body nd prtil spinl cord in different types of internl fixtion were studied under the sme rdition condition (6 MV nd 600 mgy) in different fixtion models nd compred with those irrdited bsed on the tretment plnning system (TPS). Our results showed tht spinl internl fixtion mterils hve gret impct on the rdition dose bsorbed by spinl tumors. Under the sme rdition condition, the influence of nterior internl fixtion mteril or combined nterior nd posterior pproch on rdition dose t the nterior border of the vertebrl body ws the gretest. Regrdless of the kinds of internl fixtion method employed, rdition dose t the nterior border of the vertebrl body ws significntly different from tht t other positions. Notbly, the influence of posterior internl fixtion mteril on the nterior wll of the vertebrl cnl ws the gretest. X-ry ttenution nd scttering should be tken into considertion for most ptients with bone metstsis tht receive fixtion of metl implnts. Further evlution should then be conducted with modified TPS in order to minimize the potentilly hrmful effects of inpproprite rdition dose. PACS number: D- Key words: internl fixtion system, spinl tumor, tretment plnning system, rdition dose I. INTRODUCTION Spinl tumors cn be divided into primry nd metsttic types, of which the former ccounts for 30% of spinl tumors nd 0.4% of ll tumors, nd of which the ltter ccounts for 10% to 30% of the new tumors dignoses nnully. (1) Metsttic spinl tumor is the most common of ll tumors nd cn be secondry to ny mlignnt tumor. Spinl metstsis is found in 90% of cncer ptients receiving pthologicl utopsy, nd 60% of these metstses re from lung cncer, brest cncer, nd prosttic cncer. (2-4) The min therpeutic objectives in both primry nd metsttic spinl tumors re to llevite pin, to mintin or improve neurl function, nd to mintin nd reconstruct spinl stbility. Tretment methods include surgicl removl of tumor, rdition tretment, nd chemotherpy. (5) Corresponding uthor: Jinping Wng, Deprtment of Orthopedics, Zhongnn Hospitl of Wuhn University, No.169 Donghu Rd., Wuchng District, Wuhn City, Hubei Province, Chin; phone: ; fx: ; emil: wngjp812@163.com

2 280 Li et l.: Influence of internl fixtion systems 280 Although surgicl tretment of spinl tumor cnnot significntly prolong the life of ptients, it cn considerbly improve their life qulity, including delying the loss of wlking bility, eliminting or lleviting pins, nd retrding or voiding prplegi. Routine methods for internl fixtion of spine include nterior internl fixtion, posterior internl fixtion, nd the combintion of the two. (6) A vriety of mterils re used for internl fixtion of spine such s stinless steel, titnium lloy, bone cement, nd utogenous bone. (7) Currently, the most commonly used internl fixtion system consists of screw-plte system, nil-stick system, nd titnium mesh. Although surgicl tretment is often sought to remove lesions of spinl tumors, they cnnot be completely resected in mny cses. Such lesions re treted by postopertive rdition therpy or chemotherpy. Rdition therpy, surgicl therpy, nd chemotherpy re the three mjor clinicl pproches currently in use for tumor tretment. (8) Rdition therpy cn directly kill tumor cells, llevite pin, prevent nd control pthologicl frcture, nd reduce the size of tumors, which cretes fvorble conditions for surgicl resection. (9) According to dt from the World Helth Orgniztion (WHO), bout 70% of tumor ptients need rdition therpy nd the rte of successful tretment is 45%, which includes 22% by surgicl therpy, 18% by rdition therpy, nd 5% by chemotherpy. (10,11) Therefore, rdition therpy is one of the mjor pproches for tretment of tumor. Use of n internl fixtion system in spines is indispensble for stbiliztion of spinl structure, (12) recovery of the bering cpcity of spine, nd protection of spinl function. However, the effective rdition dose cn be ltered by nonuniformity of the interfce between metl nd body tissue. (13) Specificlly, the metl used for internl fixtion cn increse the dose bsorbed by the interfce between the metl nd the tissue on the side where the rdition enters, nd reduce the dose bsorbed by the tissue behind the metl. (14,15) Alll et l. (13) used 60 Co s rdioctive source to study vrition in rdition bsorbed by the tissue t the interfce with metl implnt nd found tht rdition dose close to the surfce of titnium plte ws higher by 5% 7%. However, some studies hve reported results tht re inconsistent with these findings. (16,17) Moreover, due to the complexity of the ntomicl structure of the spine nd the interction with severl internl fixtion mterils, ccurte determintion of rdition dose in the vertebrl cnl nd on the vertebrl body is very difficult. (18) The rdition tolernce dose of spine is 45 Gy, which is 22 to 25 times tht of conventionl frctiontion. (19) The incidence of myelopthy is equl to 0.2% t the rdition level of 50 Gy, 6% t 60 Gy, nd 50% t ~ 69 Gy. (20) Excess rdition cn directly or indirectly dmge spinl neurons nd vsculr bed, resulting in rdioctive spinl cord injury s well s diffuse nd tiny dmges on the tissue being irrdited. The dmge is usully seen morphologiclly s demyelintion nd necrosis. For exmple, if the influence of titnium plte on rdition dose is neglected in ptients undergoing rdition therpy for cervicl spine tumor, the esophgus in front of the plte my be irrdited excessively, leding to rdioctive esophgitis. Moreover, lesions of vertebrl tumor behind re blocked by the titnium plte, so the dose of rdition bsorbed will be reduced nd the therpeutic gol my not be chieved. Therefore, rdition dose should be djusted in order to void unpredictble nd unrecoverble consequences for ptients with spinl tumors. Previous reserch on the influence of implnts on rdition therpy focused minly on the influence of metl mesh stents plced in the esophgus on the dose of rdition therpy, (21,22) but less on the spine itself. Following common method of internl fixtion fter resection of spinl tumor, different models of spinl internl fixtion were constructed using the lumbr vertebr of fresh domestic pigs nd titnium lloy s the internl fixtion system. Vritions in rdition dose in the vertebrl body nd the spinl cord in different types of internl fixtion systems were studied under the sme rdition condition (6 MV nd 600 mgy). Our results provide theoreticl bsis for djustment of rdition dose in ptients with spinl tumor fter internl fixtion.

3 281 Li et l.: Influence of internl fixtion systems 281 II. MATERIALS AND METHODS A. Ethics sttement This study ws crried out in strict ccordnce with the recommendtions in the Guide for the Cre nd Use of Lbortory Animls of the Ntionl Institutes of Helth. The protocol ws pproved by the Committee on the Ethics of Animl Experiments of Wuhn University. All nimls were scrificed under pentothl sodium nd pentobrbitl nesthesi, nd ll efforts were mde to minimize suffering. B. Smple processing Lumbr vertebre of fresh domestic pigs were used to construct different models of spinl internl fixtion. The surrounding muscles nd soft tissues were removed, nd the spinl dur mter ws bluntly dissected nd extrcted together with the spinl cord. Anterior longitudinl ligment, posterior longitudinl ligment, ligmentum flv, nd fcet joint cpsule were kept intct. The left pediculus rcus vertebre, trnsverse process, vertebrl lmin, nd the tissues between them were resected long the upper edge of L2 vertebrl body nd the lower edge of L3 vertebrl body, respectively, so s to expose the vertebrl cnls L2 nd L3. A fenestr ws creted on the seprted bone from the lterl side. C. Estblishment of models of spinl internl fixtion C.1 Model of nterior fixtion with titnium plte The position 1 cm bove the lower edge of L1 vertebrl body nd 1 cm in front of the posterior edge of vertebrl body ws the entry point of the posterolterl nil from the upper side. The position 1 cm below the upper edge of L3 vertebrl body nd 1 cm in front of the posterior edge of the vertebrl body ws the entry point of the posterolterl nil from the lower side. A four-bore titnium plte (Synthes AG, Bettlch, Switzerlnd) ws fixed on the vertebrl bodies L1 nd L3. Digonl drilling, tpping, screwing, nd nil fstening were routinely performed (see Fig. 1()). () (b) (c) (d) Fig. 1. The figures of model of nterior fixtion with titnium plte (), model of posterior fixtion with nil-stick system (b), model of fixtion by nterior bone grfting/ cement with titnium mesh + nterior screw-plte (c), nd model of fixtion by nterior bone grfting/cement with titnium mesh + posterior nil-stick (d). C.2 Model of posterior fixtion with nil-stick system The isthmus of vertebrl lmine L1 nd L3 tht is, the junction of the middle line of trnsverse process nd superior rticulr process ws chosen s the entry point. The bone rongeur ws first used to remove the corticl bone covering the entry point, nd then the cncellous bone ws exposed. A screw ws inserted into the nteromedil pediculus rcus vertebre from

4 282 Li et l.: Influence of internl fixtion systems 282 the posterior lterl side using drilling hmmer. The til of the nil ws lifted by 5 10 nd deflected outwrd by Pedicle screws (Synthes AG) nd connecting rods were inserted fter tpping (see Fig. 1(b)). C.3 Models of fixtion: nterior bone grfting with titnium mesh + nterior screw-plte; nterior bone grfting with titnium mesh + posterior nil-stick The L3 vertebrl body nd intervertebrl disc were resected using n osteotome nd n electric sw, but the posterior wll of the vertebrl body nd the posterior longitudinl ligment were left intct. The titnium mesh ws cut to height of 36 mm bsed on the distnce between vertebrl bodies nd volume vrition fter decompression. The resected bone ws cut into pieces, put in the titnium mesh, nd compcted. Then the titnium mesh ws plced between the vertebrl bodies prllel to the nterior border of the vertebrl body. Finlly, the nterior screw-plte or the posterior nil-stick (Synthes AG) ws used for fixtion, resulting in nterior bone grfting with titnium mesh + nterior screw-plte nd nterior bone grfting with titnium mesh + posterior nil-stick fixtion, respectively (see Figs. 1(c) nd (d)). C.4 Models of fixtion: nterior bone cement with titnium mesh + nterior screw-plte; nterior bone cement with titnium mesh + posterior nil-stick The specimens were dissected s described bove. A totl of 20 g of bone cement (crylic resin) nd 10 ml of wter were mixed well nd injected into the titnium mesh. After 15 min, the bone cement solidified nd the mesh ws plced in the position between the vertebrl bodies L2 nd L4 prllel to the nterior border of the vertebrl body. The nterior screw-plte or the posterior nil-stick ws used for fixtion to chieve nterior bone cement with titnium mesh + nterior screw-plte nd nterior bone cement with titnium mesh + posterior nil-stick fixtion, respectively (see Figs. 1(c) nd (d)). C.5 Fixtion of thermoluminescence dosimeter After prepring the models s described bove, five thermoluminescence dosimeters (TLD) were dhered to five reference points on the vertebrl cnls L2 nd L3 with 502 seccotine, nd were numbered s shown in Fig. 2. The dosimeters 1 nd 5 detect the influence of the internl fixtion system on the site of the lesion, while dosimeters 2, 3 nd 4 detect its influence on the spinl cord. Next, the bone resected during fenestrtion ws restored nd fixed with silk thred. Fig. 2. Cross section of the lumbr vertebre. Numbers show loctions of dosimeter plcement. 1 = nterior border of vertebrl body; 2 = nterior wll of vertebrl cnl; 3 = center of vertebrl cnl; 4 = posterior wll of vertebrl cnl; 5 = middle prt of spinous process.

5 283 Li et l.: Influence of internl fixtion systems 283 C.6 Estblishment of the control group The control model without ny internl fixtion mteril ws plced in the wter phntom nd irrdited using 6 MV therpy unit. The verge rdition dose bsorbed by ech point ws clculted fter ten repeted mesurements. The correction method is the ETAR method. (23) C.7 Group of tretment plnning system (TPS) In order to reduce the influence of metl implnts on the dose of rdition therpy s much s possible, tretment plnning system (TPS) (Computerized Medicl Systems (CMS)-XIO, St. Louis, MO), with its pencil bem lgorithm, ws used for clcultion of the isodose curves. TPS ws generlly employed for simultion clcultion so s to correct the rdition dose during rdition tretment plnning, which ws fixed in energy 6 MV, irrdition dose 600 mgy, rdition field cm, source-to-tumor distnce 108 cm, source-to-skin distnce 100 cm. In this study, the model of nterior fixtion with titnium lloy screw-plte system, the model of nterior bone cement with titnium mesh + nterior titnium lloy screw-plte system, the model of nterior bone cement with titnium mesh + posterior nil-stick system, nd the blnk control group were compred with the TPS group in order to determine the influence of different internl fixtion models on rdition therpy. D. Irrdition The prepred model of spinl internl fixtion ws plced in wter with the spinous process fcing upwrd. Then it ws plced on the stndrd position of X-ry extrction window of the tretment bed (6 MV tretment unit). The wter level ws 5 mm bove the surfce of the spinous process. The center of the L3 vertebrl body ws 8 cm wy from the wter surfce, nd the source-to-skin distnce ws 80 cm. Bsed on the source-to-tumor distnce of 88 cm, 60 cgy of rdition dose ws selected with field of 20 cm 20 cm. Mesurements were performed ten times on the specimens of ech group. The thermoluminescence elements were tken out nd kept still for 48 hr, fter which the mesurements were red (TLD Reder Model 3000, Ksei Optonix, Ltd., Odwr, Jpn) nd the verges were clculted. E. Sttisticl nlysis SPSS 20.0 (SPSS Inc., Chicgo, IL) ws employed to nlyze the dt nd the significnce level ws set t p < Comprison between the two models ws conducted by the Wilcoxon signed-rnk test. Dt for the vrious reference points in the different models were nlyzed by ANOVA, nd t-test ws conducted to evlute sttisticl significnce of differences between the two groups. III. RESULTS A. Fixtion models with different internl fixtion systems A.1 Model of nterior titnium lloy screw-plte system, model of nterior bone cement with titnium mesh + nterior titnium lloy screw-plte system, model of nterior bone cement with titnium mesh + posterior nil-stick system, nd blnk control group No difference ws found in rdition doses t the nterior wll, center, nd posterior wll of vertebrl cnl (the inside of vertebrl cnl). However, doses t the nterior border nd the middle prt of the spinous process were different from those t other positions.

6 284 Li et l.: Influence of internl fixtion systems 284 A.2 Model of posterior titnium lloy screw-plte system No difference ws found mong rdition doses of the nterior wll, center, nd posterior wll of the vertebrl cnl. Rdition doses t the nterior wll nd the center of the vertebrl cnl were not different from tht in the middle prt of the spinous process. However, rdition dose in the posterior prt of the vertebrl wll ws different from tht in the middle prt of the spinous process (p = 0.049). Moreover, the nterior border of the vertebrl body hd significntly different rdition dose compred with ll other positions (Tbles 1 nd 2). Tble 1. Comprison of rdition doses t ech position in different models of internl fixtion systems under the sme rdition condition. Rdition Dose Vlues (mgy) Anterior Bone Cement with Anterior Bone Titnium Cement with Anterior Mesh+Anterior Titnium Posterior Titnium Alloy Titnium Alloy Mesh+Posterior Titnium Alloy Blnk Screw-Plte Screw-Plte Nil-Stick Screw-Plte Control Position of TLD System System System System Group ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±38.2 Ech system model is fixed in energy 6 MV, irrdition dose 600 mgy, rdition field cm, source-to-tumor distnce 108 cm, source-to-skin distnce 100 cm. 1 = nterior border of vertebrl body; 2 = nterior wll of vertebrl cnl; 3 = center of vertebrl cnl; 4 = posterior wll of vertebrl cnl; 5 = middle prt of spinous process. Tble 2. Pirwise comprison of rdition doses t different positions in ech model of internl fixtion system under the sme rdition condition. P-vlues Anterior Bone Cement with Anterior Bone Titnium Cement with Anterior Mesh+Anterior Titnium Posterior Titnium Alloy Titnium Alloy Mesh+Posterior Titnium Alloy Blnk Screw-Plte Screw-Plte Nil-Stick Screw-Plte Control Position of TLD System System System System Group 1 VS VS VS VS VS VS VS VS VS VS Ech system model is fixed in energy 6 MV, irrdition dose 600 mgy, rdition field 15 15cm, source-to-tumor distnce 108 cm, source-to-skin distnce 100 cm. 1 = nterior border of vertebrl body; 2 = nterior wll of vertebrl cnl; 3 = center of vertebrl cnl; 4 = posterior wll of vertebrl cnl; 5 = middle prt of spinous process.

7 285 Li et l.: Influence of internl fixtion systems 285 B. Comprison of tretment groups nd TPS group in different internl fixtion models 1. Under the sme rdition condition, rdition doses t the nterior border of the vertebrl body, nterior wll of the vertebrl cnl, center of the vertebrl cnl, nd middle prt of the spinous process of the tretment group incresed by 12.26%, 8.08%, 3.60%, nd 2.29%, respectively, nd the dose on the posterior wll decresed by 1.96% compred with those in the TPS group using the nterior titnium lloy screw-plte system (Tble 3). 2. Under the sme rdition condition, rdition doses in the nterior border of the vertebrl body, nterior wll of the vertebrl cnl, center of the vertebrl cnl, posterior wll of the vertebrl cnl, nd middle prt of the spinous process of the tretment group incresed by 6.94%, 5.41%, 1.34%, 0.05%, nd 5.00%, respectively, compred with those in the TPS group using the nterior bone cement with titnium mesh + nterior titnium lloy screwplte system (Tble 4). 3. Under the sme rdition condition, rdition doses in the nterior border of the vertebrl body, nterior wll of the vertebrl cnl, center of the vertebrl cnl, posterior wll of the vertebrl cnl, nd middle prt of the spinous process of tretment group incresed by 8.27%, 7.21%, 5.45%, 7.71%, nd 6.51%, respectively, compred with those in the TPS group using the nterior bone cement with titnium mesh + posterior titnium lloy screwplte system (Tble 5). 4. Under the sme rdition condition, rdition doses in the nterior border of the vertebrl body, nterior wll of the vertebrl cnl, center of the vertebrl cnl, posterior wll of the vertebrl cnl, nd middle prt of the spinous process of the tretment group incresed by Tble 3. Comprison of tretment group with the nterior titnium lloy screw-plte system (tretment group) nd the TPS group. Rdition Doses Incrementl (mgy) Percentge Position of TLD Tretment Group TPS Group Difference (%) Ech system model is fixed in energy 6 MV, irrdition dose 600 mgy, rdition field 15 15cm, source-to-tumor distnce 108 cm, source-to-skin distnce 100 cm. 1 = nterior border of vertebrl body; 2 = nterior wll of vertebrl cnl; 3 = center of vertebrl cnl; 4 = posterior wll of vertebrl cnl; 5 = middle prt of spinous process. Tble 4. Comprison of tretment group receiving nterior bone cement with titnium mesh + nterior titnium lloy screw-plte system nd the TPS group. Rdition Doses Incrementl (mgy) Percentge Position of TLD Tretment Group TPS Group Difference (%) Ech system model is fixed in energy 6 MV, irrdition dose 600 mgy, rdition field 15 15cm, source-to-tumor distnce 108 cm, source-to-skin distnce 100 cm. 1 = nterior border of vertebrl body; 2 = nterior wll of vertebrl cnl; 3 = center of vertebrl cnl; 4 = posterior wll of vertebrl cnl; 5 = middle prt of spinous process.

8 286 Li et l.: Influence of internl fixtion systems %, 14.24%, 11.42%, 7.60%, nd 10.03%, respectively, compred with those in the TPS group using the posterior titnium lloy screw-plte system (Tble 6). 5. Under the sme rdition condition, rdition doses in the nterior border of the vertebrl body, nterior wll of the vertebrl cnl, nd middle prt of the spinous process of the tretment group incresed by 1.68%, 4.32%, nd 2.55%, respectively, nd the doses in the center nd posterior wll of the vertebrl cnl decresed by 0.80% nd 4.54%, respectively, compred with those in the TPS group of the blnk controls (Tble 7). Tble 5. Comprison of tretment group receiving nterior bone cement with titnium mesh + posterior titnium lloy screw-plte system nd the TPS group. Rdition Doses Incrementl (mgy) Percentge Position of TLD Tretment Group TPS Group Difference (%) Ech system model is fixed in energy 6 MV, irrdition dose 600 mgy, rdition field 15 15cm, source-to-tumor distnce 108 cm, source-to-skin distnce 100 cm. 1 = nterior border of vertebrl body; 2 = nterior wll of vertebrl cnl; 3 = center of vertebrl cnl; 4 = posterior wll of vertebrl cnl; 5 = middle prt of spinous process. Tble 6. Comprison of tretment group with the posterior screw-plte system nd the TPS group. Rdition Doses Incrementl (mgy) Percentge Position of TLD Tretment Group TPS Group Difference (%) Ech system model is fixed in energy 6 MV, irrdition dose 600 mgy, rdition field 15 15cm, source-to-tumor distnce 108 cm, source-to-skin distnce 100 cm. 1 = nterior border of vertebrl body; 2 = nterior wll of vertebrl cnl; 3 = center of vertebrl cnl; 4 = posterior wll of vertebrl cnl; 5 = middle prt of spinous process. Tble 7. Comprison of the blnk control group with the TPS group. Rdition Doses (mgy) Incrementl Blnk Control Percentge Position of TLD Group TPS Group Difference (%) Ech system model is fixed in energy 6 MV, irrdition dose 600 mgy, rdition field 15 15cm, source-to-tumor distnce 108 cm, source-to-skin distnce 100 cm. 1 = nterior border of vertebrl body; 2 = nterior wll of vertebrl cnl; 3 = center of vertebrl cnl; 4 = posterior wll of vertebrl cnl; 5 = middle prt of spinous process.

9 287 Li et l.: Influence of internl fixtion systems 287 IV. DISCUSSION Our experimentl results show tht spinl internl fixtion mterils hve gret impct on the rdition dose bsorbed by spinl tumors. Under the sme rdition condition, the influence of nterior internl fixtion mteril or combined nterior nd posterior pproch on rdition dose ws the gretest t the nterior border of the vertebrl body. Irrespective of the internl fixtion method employed, rdition dose t the nterior border of the vertebrl body ws different from tht t other positions. Notbly, the influence of posterior internl fixtion mteril on the nterior wll of the vertebrl cnl ws the gretest. Tken together, our results show tht rdition therpy is influenced by spinl internl fixtion mterils s follows: 1) X-ry is ttenuted gretly fter pssing through the spinl internl fixtion mteril, which will ffect the rdition dose ctully bsorbed by the tumor; 2) bckscttering of X-ry by spinl internl fixtion mterils cn result in incresed effective rdition dose on the surfce of the incident plne; 3) scttered photons nd secondry electrons in front of the incident plne of the spinl internl fixtion mteril cn enter only prtilly or cnnot enter the emergent plne due to ttenution. (24,25) Previous studies hve shown tht spinl internl fixtion mterils hve some effect on rdition therpy of ptients with spinl tumors but no obvious impct on chemotherpy. (26) The volume nd thickness of spinl internl fixtion mterils re lrge in reltion to spinl tumors. In cses where the mterils re locted on the bem pth, ttenution of X-rys is obvious, nd increment of rdition cused by scttering will decrese quickly fter leving the interfce. Therefore, in ddition to ttenution of X-rys, the influence of scttered rys should lso be considered. Moreover, rdition dose for ptients with implnttion of spinl internl fixtion mterils should be corrected so s to reduce the probbility of filure of rdition therpy nd the incidence of side effects. The determintion of rdition dose depends on the specifics of n individul s clinicl condition. Although some scholrs dvocte djustment of the rdition dose, (27) most dvocte irrdition from two fields or irrdition with the sme center from severl fields insted. The ltter methods llow vrition of dose to be reduced to 4%, which meets the requirement (Report ICRU 24) tht the totl uncertinty of dose in the trget region must be less thn 5%. (21,28,29) In order to reduce the influence of metl implnts on the dose of rdition therpy, the tretment plnning system (TPS) is usully employed for simultion clcultion. (30) Thus, the rdition dose cn be corrected during rdition tretment plnning. The clcultion of dose with TPS minly depends on reltive electron density, which is derived from CT vlue. A lrge rtifct my occur when scnning metl implnt with high density, which will result in error in CT clcultion. Therefore, the TPS must be modified to tke such scenrios into ccount. Currently, the common correction methods for TPS re the EPL, Btho, nd ETAR methods. (23,31,32) The EPL method is not used widely due to its excessive simplifiction of the model. (31) The Btho method, or the TAR (tissue-to-ir rtio) method, consists of 1D nonuniformity correction of microstructures, nd is minly used to correct the influence of nonuniformity informtion on the mjor photon trnsmission pth nd the distribution of rdition dose. (23) On the other hnd, the ETAR method consists of 3D nonuniformity correction of microstructures. In this method, the influence of nonuniform microstructure surrounding the site being irrdited on the distribution of rdition dose is considered on 3D scle. Since correction by the ETAR method is more effective, it is widely pplied in TPS. However, the FDS tem thinks tht the ETAR method is not ccurte enough for clcultion of rdition dose, especilly for inverse optimiztion. (33) And the three more sophisticted dose clcultion lgorithms for TPS in order of incresing ccurcy/decresing performnce re pencil bem, superposition/ convolution (S/C), nd Monte Crlo (MC). (34) In clinicl, Monte Crlo method is the unique method ble to clculte the dose ccurtely ner high-z inhomogeneity. (35) In this rticle, models of the nterior titnium lloy screw-plte system, the nterior bone cement with titnium mesh + nterior titnium lloy screw-plte system, the nterior bone cement with titnium

10 288 Li et l.: Influence of internl fixtion systems 288 mesh + posterior nil-stick system, nd the blnk control group were compred with the TPS group. Our results showed tht spinl internl fixtion mterils hve significnt impct on rdition dose, nd which could be more ccurte corrected by the ETAR method. The specific method used in clinicl reserch is s follows: (32) if there re no importnt orgns nd fst-recting tissues ner the metl implnts (2 3 cm) nd the implnt is mde up of lowdensity metl, modified TPS is used to evlute the influence of implnt on dose distribution. In ddition, the influence of X-ry scttered by the metl in CT imge is lso considered in this system. X-ry ttenution nd scttering is considered for most ptients with bone metstsis fter fixtion of metl implnts. Further evlution is then conducted with modified TPS so s to void or reduce the effect of incorrect rdition dose. The results of our study indicte tht metl implnts hve negtive impct on rdition therpy conducted fter spinl tumor surgery. However, there is still controversy regrding the best method to determine correct rdition dose. Future studies should focus on wys to void the side effects of metl implnts, s well s the kinds of metl implnts to be used in rdition therpy for different types of bone metstsis. V. CONCLUSIONS X-ry ttenution nd scttering by metl implnts should be tken into considertion for most ptients with bone metstsis receiving fixtion of metl implnts. Further evlution should be conducted with modified TPS so s to void or reduce the potentilly dmging effects of inpproprite rdition dose. ACKNOWLEDGMENTS This work ws finncilly supported by the Nturl Science Foundtion of Hubei Province (2010), the Ntionl Nturl Sciences Foundtion of Chin (No: ), nd the Youth Science nd Technology Morning Progrm of Wuhn (Grnt No: ). REFERENCES 1. Dunning EC, Butler JS, Morris S. Complictions in the mngement of metsttic spinl disese. World J Orthop. 2012;3(8): Cheng P, Su X, Go H, Zhng T. All vertebrl body metstses of brest cncer: cse report nd literture review. Eur J Gynecol Oncol. 2013;34(5): Ktsenos S nd Nikolopoulou M. Intrmedullry thorcic spinl metstsis from smll-cell lung cncer. Monldi Arch Chest Dis. 2013;79(3-4): Crnlic S, Hildingsson C, Bergh A, Widmrk A, Svensson O, Löfvenberg R. Erly dignosis nd tretment is crucil for neurologicl recovery fter surgery for metsttic spinl cord compression in prostte cncer. Act Oncol. 2013;52(4): Chn NK, Abdullh KG, Lubelski D, et l. Stereotctic rdiosurgery for metsttic spine tumors. J Neurosurg Sci. 2014;58(1): Jnkowski R, Nowk S, Zukiel R, Blok T, Pprzycki W, Szymś J. Appliction of internl stbilistion in the surgicl tretment of spinl metstses. Neurol Neurochir Pol. 2008;42(4): Bellut D, Mutter UM, Sutter M, Eggspuehler A, Mnnion AF, Porchet F. Bck pin in ptients with degenertive spine disese nd intrdurl spinl tumor: wht to tret? when to tret? Eur Spine J. 2014;23(4): Glimelius B. Optiml time intervls between pre-opertive rdiotherpy or chemordiotherpy nd surgery in rectl cncer? Front Oncol. 2014;4: Jnsen DR, Krijger GC, Kolr ZI, Zonnenberg BA, Zeevrt JR. Trgeted rdiotherpy of bone mlignncies. Curr Drug Discov Technol. 2010;7(4): Lufer I, Rubin DG, Lis E, et l. The NOMS frmework: pproch to the tretment of spinl metsttic tumors. Oncologist. 2013;18(6): Chwl S, Schell MC, Milno MT. Stereotctic body rdition for the spine: review. Am J Clin Oncol. 2013;36(6):

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