Spatially fractionated radiotherapy (GRID) using helical tomotherapy

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1 JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 17, NUMBER 1, 2016 Sptilly frctionted rdiotherpy (GRID) using helicl tomotherpy Xin Zhng, 1 Jose Pengricno, 1 Yulong Yn, 2 Xioying Ling, 1 Steven Morrill, 1 Robert J. Griffin, 1 Peter Corry, 1 Vneert Rtnthrthorn 1 Rdition Oncology Deprtment, 1 University of Arknss for Medicl Science, Little Rock, AR; Deprtment of Rdition Oncology, 2 University of Texs Southwestern Medicl Center, Dlls, TX, USA xinzhng2003@gmil.com Received 1 July, 2015; ccepted 6 October, 2015 Sptilly frctionted rdiotherpy (GRID) ws designed to tret lrge tumors while spring skin, nd it is usully delivered with liner ccelertor using commercilly vilble block or multilef collimtor (LINAC-GRID). For deep-seted (skin to tumor distnce (> 8 cm)) tumors, it is lwys chllenge to chieve dequte tumor dose coverge. A novel method to perform GRID tretment using helicl tomotherpy (HT-GRID) ws developed t our institution. Our pproch llows treting ptients by generting ptient-specific virtul GRID block (softwregenerted) nd using IMRT technique to optimize the tretment pln. Here, we report our initil clinicl experience using HT-GRID, nd dosimetric comprison results between HT-GRID nd LINAC-GRID. This study evlutes 10 previously treted ptients who hd deep-seted bulky tumors with complex geometries. Five of these ptients were treted with HT-GRID nd replnned with LINAC-GRID for comprison. Similrly, five other ptients were treted with LINAC-GRID nd replnned with HT-GRID for comprison. The prescription ws set such tht the mximum dose to the GTV is 20 Gy in single frction. Dosimetric prmeters compred included: men GTV dose (D GTV ), GTV dose inhomogeneity (vlleyto-pek dose rtio (VPR)), norml tissue doses (D N men men ), nd other orgns-t-risk (OARs) doses. In ddition, equivlent uniform doses (EUD) for both GTV nd norml tissue were evluted. In summry, HT-GRID technique is ptient-specific, nd llows djustment of the GRID pttern to mtch different tumor sizes nd shpes when they re deep-seted nd cnnot be dequtely treted with LINAC-GRID. HT-GRID delivers higher D GTV, EUD, nd VPR compred to LINAC-GRID. men HT-GRID delivers higher D N men nd lower EUD for norml tissue compred to LINAC-GRID. HT-GRID plns lso hve more options for tumors with complex ntomicl reltionships between the GTV nd the voidnce OARs (butment or close proximity). PACS numbers: D, de, ne, tg Key words: GRID, helicl tomotherpy, tretment plnning system, dosimetric comprison I. INTRODUCTION Sptilly frctionted rdiotherpy (GRID) is generlly used to tret ptients using conventionl liner ccelertor with either commercilly vilble GRID block or MLC-bsed GRID pttern (LINAC-GRID). (1-3) It is used to tret lrge bulky mlignnt tumors nd it hs been Corresponding uthor: Xin Zhng, Deprtment of Rdition Oncology, University of Arknss for Medicl Science, 4301 W. Mrkhm St., #771, Little Rock, AR 72205, USA; phone: (501) ; fx: (501) ; emil: xinzhng2003@gmil.com

2 397 Zhng et l.: GRID therpy using tomotherpy 397 shown to be n effective technique to enhnce tumor locl control. (4,5) However, LINAC-GRID techniques fce mjor chllenge to effectively tret bulky tumors with lrge skin to tumor distnces. Tht is, there is indequte dose to the trget with most of the dose lost into norml tissue. Frequently, in such cses, the mximum dose (D mx ) flls outside of the trget. For helicl tomotherpy, 51 projections or bems round the trget re used to perform IMRT tretment plnning nd, becuse of the helicl nture of the tretment delivery (the couch nd gntry re moving during the tretment), it is possible for the system to deliver dose to deep-seted trgets. The University of Arknss for Medicl Sciences (UAMS) developed customizble ptientspecific virtul GRID block to tret ptients with the new TomoHDA V2.0 system (Accurcy Inc., Sunnyvle, CA) (HT-GRID) to enble dequte GRID tretment of deep-seted tumors. This is done by using softwre-generted customized virtul GRID block nd n IMRT delivery technique. We hve previously investigted the bsic design of the virtul GRID block, the bem chrcteristics of HT-GRID, nd the possibility of pplying HT-GRID in the clinic. (6) In the current study, we focused on the clinicl ppliction of HT-GRID for deep-seted bulky tumors (medin distnce from skin to the proximl edge of the tumor of 12.3 cm (rnge cm)) nd the dosimetric comprison between the HT-GRID nd LINAC-GRID in group of nonconsecutive ptients treted t UAMS. II. MATERIALS AND METHODS Ten previously treted ptients who hd deep-seted bulky tumors with complex geometries were selected for this study. Among these ptients, five ptients were previously treted with HT-GRID techniques nd were replnned with LINAC-GRID technique for comprison purposes. Another five ptients were previously treted with LINAC-GRID technique nd were replnned with HT-GRID technique for the comprison purposes s well. The prescription ws set such tht the mximum dose to the GTV is 20 Gy in single frction. Ptient chrcteristics re presented in Tble 1. Tble 1. Ptient chrcteristics. Trget Mx. Trget Trget Volume Dimension Pd mx Dd b mx Pt. # Dignosis Loction (cc) (cm) (cm) (cm) 1 Renl cell crcinom Axill Squmous crcinom of the hed nd neck of unknown primry Neck Renl cell crcinom Shoulder Srcom Lung Squmous crcinom of the cervicl esophgus Neck Squmous crcinom of the lrynx Neck Melnom Shoulder Nonsmll cell lung cncer Lung Squmous crcinom Anus Crcinosrcom Uterus Pd mx : mximum distnce from skin to proximl edge of the trget. b Dd mx : mximum distnce from skin to deepest prt of trget.

3 398 Zhng et l.: GRID therpy using tomotherpy 398 A. LINAC-GRID technique tretment pln LINAC-GRID tretment plns were generted using the Pinncle v. 9.0 tretment plnning sttion (TPS) (Philips Helthcre, Andover, MA) with commercilly vilble GRID block mnufctured by Rdition Products Design, Inc. (Albertive, MN). Ptient CT dtsets were cquired on CT-simultor (Brillince CT Big Bore, Philips, Clevelnd, OH) nd trnsferred to the Pinncle TPS for contouring nd plnning. Ptient GTV ws contoured by the sme physicin. The norml tissue ws defined s the volume of the plnning CT dtset tht received t lest 5% of the prescription dose (100 cgy) s region of very low dose minus the volume of GTV. The commercilly vilble GRID block ws imported to the Pinncle TPS s GRID block using the in-house dedicted GRID script generted in Pinncle TPS. LINAC-GRID usully uses one bem ngle or two bems in prllel opposed fshion. (7) The detils of LINAC-GRID plns nd their bem profiles were studied in our previous publictions. (3,6) B. HT-GRID technique tretment pln B.1 Virtul tomo GRID block The virtul GRID block used in helicl tomotherpy (HT) system were generted by in-house softwre DICOMn. (8) Two new structures (GRID Trget nd GRID Avoidnce) were generted inside the GTV structure using DICOMn softwre. In summry, the softwre-generted virtul GRID block consists of three elements: GTV (ptient gross tumor volume), the GRID Trget (the cylindricl structures or holes mimicking openings of commercilly vilble GRID blocks inside the GTV), nd the GRID Avoidnce volume creted by extrcting the GRID Trget from the GTV (mimicking the shielded re of the commercilly vilble GRID block nd thus used s n orgn t risk, but still locted inside the GTV). By chnging the dimeter of the opening holes nd the center-to-center distnce between the holes, DICOMn cn configure the ptient-specific virtul GRID block in vrious settings of GRID Trget nd GRID voidnce structures (Figs. 1() nd 1(b)). () (b) Fig. 1. Illustrtion of HT-GRID virtul GRID block () from Ptient #4 (GTV volume = 4646 cc); (b) virtul GRID block from Ptient #1 (GTV volume = 646 cc). B.2 HT-GRID technique tretment pln The sme ptient CT imge dtsets nd sme contours generted from Pinncle TPS were trnsferred to the in-house softwre DICOMn to generte the virtul GRID block. Ptient CT nd contours including the two new contours of GRID Trget nd GRID Avoidnce were then trnsferred to the HT TPS for tretment plnning. Figs. 1() nd 1(b) show exmples of the virtul GRID blocks. The Virtul GRID block is ptient-specific nd cn be optimized by mnipulting the dimeter of opening holes nd the center-to-center distnce between nerby holes in order to suit the prticulr ptient GTV nd OAR ntomicl reltionship. The virtul

4 399 Zhng et l.: GRID therpy using tomotherpy 399 GRID block could be further modified fter trnsferring to the HT TPS, to void hot or cold spots in the unintended res. Prescribed dose ws set to 20 Gy to the mximum point of the GTV in single frction. This is the sme prescription s set in LINAC-GRID. HT-GRID used 6 MV X-ry bems nd IMRT tretment technique with dynmic jw to tret the ptients. The plnning prmeters such s field size, pitch number, nd modultion fctor need to be optimized to produce tretment pln tht is both dosimetriclly cceptble nd clinicl deliverble. In HT, long bem-on time (BOT) is of concern becuse of potentil ptient motion. As such, optimiztion prmeters were chosen tht would reduce BOT. The 5 cm dynmic jw, pitch of 0.43, nd modultion fctor of 1.5 were selected in this study to strike fine blnce between obtining dosimetriclly cceptble nd cliniclly deliverble pln while mximlly reduce the BOT. For the HT optimiztion process, the GRID Trget structure generted from DICOMn ws used s the HT trget constrint to gin high dose inside the cylindricl structures of the virtul GRID block, while GRID Avoidnce structure ws set s the regions-t-risk constrints to limit dose s much s possible. This mimics the GRID bem delivery pttern. Figure 2 shows the exmple of the isodose distribution of the HT-GRID. From our previous experience of treting LINAC-GRID bsed ptients, the gol men GTV dose ws usully round 8 to10 Gy, (3,6) wheres the gol for HT-GRID tretment pln ws to optimize the pln to rech men GTV dose of pproximtely 8 12 Gy. The difference in men GTV dose between LINAC-GRID nd HT-GRID tkes into considertion the potentil extension of the high-dose region outside the GRID trget. The dose to criticl OAR ws optimized by setting optimiztion gols to lower their doses. The systemtic chnge of importnce, dose objectives, dose-volume objectives, nd penlties were similr to our clinicl experience in treting non-grid HT ptients. (9,10) Fig. 2. Illustrtion of HT-GRID isodose distribution (trnsverse, coronl, sgittl plne) from Ptient #6. B.3 Dosimetric comprison between HT-GRID nd LINAC-GRID tretment plns The tretment pln evlution prmeters include equivlent uniform dose (EUD) nd the men dose (D GTV ) for the GTV. The heterogeneity of the dose distribution inside the GTV for men the ptient ws evluted by the vlley-to-pek dose rtio (VPR) nd is defined s the rtio of minimum to mximum dose inside the GTV. Pln evlution prmeters for OARs include norml tissue men dose (D N men ) nd EUD, nd the men doses (D men ) or mximum doses (D mx ) for certin OARs. The EUD is clculted s the following eqution: (11) N EUD = ( v i D i ) Σ i = 1 1 (1) In the eqution bove, is unitless model prmeter describing the dose response of ech structure nd v i is the weight of the ith prtil volume receiving dose D i in Gy. This model cn

5 400 Zhng et l.: GRID therpy using tomotherpy 400 be used for both tumors nd norml tissue. In this study, we use = -5 for tumor nd = 5 for norml tissue. These vlues were selected bsed on our prior experience with IMRT tretment plnning using the EUD s constrint/objective. However, the vlue of EUD with lrge negtive vlue of is close to the miniml dose which is chrcteristic consistent with the response of highly inhomogeneously irrdited tumors. On the other hnd, for lrge positive vlue of the prmeter, the EUD is ner the mximum dose. This is consistent with nonuniformly irrdited seril norml structures. (12) Tomotherpy tretment pln cn export differentil DVH (including corresponding prtil volume v i or ith prtil volume) to n excel file. Differentil EUD ws clculted in the excel file. The totl EUD ws then clculted by dding ech differentil EUD. B.4 HT-GRID tretment pln dose verifiction Ptient-specific qulity ssurnce (QA) for HT-GRID tretment pln ws performed using PTW OCTAVIUS phntom nd 729 2D ion chmber rry (PTW, Freiburg, Germny). The dose mesurement results were nlyzed using the PTW-VeriSoft softwre. III. RESULTS Deep-seted bulky tumors were selected in this study. The medin tumor volume ws 634 cc ( cc). The medin distnce from skin to the proximl edge of the tumors ws 12.7 cm (rnge cm) nd the medin distnce from skin to the deepest prt of tumor ws 22 cm (rnge cm). The results for GTV EUD, (D GTV) nd VPR nd the results for norml tissue men DN men nd EUD, nd results of OAR doses from both HT-GRID nd LINAC-GRID tretment plns for the selected 10 ptients re listed in Tbles 2, 3 nd 4. The medin men GTV dose ws 6.73 Gy ( Gy) for LINAC-GRID, nd ws Gy ( Gy) for HT-GRID. The medin GTV EUD for LINAC-GRID tretment pln ws 3.95 Gy ( Gy), nd ws 7.62 Gy ( Gy) for HT-GRID tretment pln. The medin norml tissue men dose ws 1.24 Gy ( Gy) for HT-GRID nd 0.61 Gy ( Gy) for LINAC-GRID; the medin norml tissue EUD for HT-GRID ws 5.45 Gy ( Gy) nd 6 Gy ( Gy) for LINAC-GRID plns. Tble 2. GTV EUD, D GTV, nd VPR for HT-GRID nd LINAC-GRID. men EUD (cgy) (cgy) VPR Pt.# HT LINAC HT LINAC HT LINAC D GTV men

6 401 Zhng et l.: GRID therpy using tomotherpy 401 Tble 3. Norml tissue D N men nd EUD for HT-GRID nd LINAC-GRID. Dose (cgy) Dmen N EUD DN men EUD DN men EUD DN men EUD DN men EUD Pt. # HT LINAC Diff Pt. # HT LINAC Diff Diff = the dose difference between HT-GRID nd LINAC-GRID (cgy). Tble 4. OAR doses for 10 selected ptients from HT-GRID nd LINAC-GRID. Dose (cgy) HT LINAC HT LINAC HT LINAC HT LINAC HT LINAC Pt. # Cord Lt. Humerus b Rt. Protid Orl cvity Lt. Plexus Rt. Plexus Lrynx Pt. # Dose (cgy) HT Linc HT Linc HT Linc HT Linc HT Linc Cord Orl cvity Rt. Lung b Esophgus b Bowel D mx. b D men. - = OAR not of interest for this ptient. HT-GRID ptient QAs were performed nd the results showed tht the gmm greement index score ws 97.8% nd higher (3%, 3 mm). Figure 3 shows the mesurement dose mp result for coronl plne from Ptient #6. The QA result shows tht rdition high dose ws Fig. 3. Ptient QA dose mp (coronl plne) mesurement result for HT-GRID technique using PTW OCTAVIUS phntom nd 729 2D ion chmber rry from Ptient #6.

7 402 Zhng et l.: GRID therpy using tomotherpy 402 only delivered inside the cylindricl structure of the GRID Trget within GTV nd it hd good greement compred with the clculted dose mp (coronl plne) from sme ptient showed in Fig. 2. IV. DISCUSSION HT-GRID technique with the novel virtul GRID block ws successfully pplied in our clinic. In this pper, deep-seted bulky tumors especilly with lrge distnces from skin to tumor were selected. Tretment plns nd dosimetric prmeters were compred between HT-GRID nd LINAC-GRID tretment plns nd severl chrcters for HT-GRID tretment pln re discussed. The men GTV doses were compred between HT-GRID nd LINAC-GRID plns nd the comprison results showed tht the men GTV doses from HT-GRID were generlly higher thn the LINAC-GRID tretment pln. This is minly becuse the HT-GRID pln hd lrger VPR compred to the LINAC-GRID pln (medin VPR ws with rnge of 0.05 to 0.29 for HT-GRID nd ws with rnge of to 0.14 for LINAC-GRID). Compred to the commercilly vilble LINAC block, the virtul GRID block is not rel physicl block. The chrcteristics of the virtul GRID block nd the nture of HT bems cn result in the extension of the high-dose region from the open re (GRID Trget) to the block re (GRID voidnce) within the GTV. We did not find ny GTV EUD studies in the literture. The comprison results showed tht the GTV EUDs were substntilly higher for HT-GRID tretment plns compred to the EUDs from LINAC-GRID tretment plns. This study lso found tht, for some ptients with LINAC- GRID tretment pln, the GTV EUDs were very low even though the men doses were within the 7 12 Gy rnge. This ws due to portions of very low dose inside the GTV. For exmple, Ptient #5, the EUD ws only 0.15 Gy, even though the men GTV dose ws 8.5 Gy. For the sme ptient with n HT-GRID tretment pln, the EUD ws 7.53 Gy nd the men GTV dose ws Gy. For this specific ptient, the shpe of the GTV ws very irregulr (mximum dimension ws 14.2 cm in the superior nd inferior dimension, nd the tumor ws 2.4 times wider in the sgittl plne s compred to the coronl plne) (Fig. 4). Becuse of the elongted GTV shpe long the superior nd inferior direction, one single GRID field could not cover the entire GTV volume when using LINAC-GRID; in ddition, this ptient hd hed nd neck cncer where mny criticl structures were in the vicinity of the GTV. As result, there were reltively lrger portions of low-dose volumes ner the superior nd inferior end of the GTV (Fig. 5()). By definition, the EUD is clculted by finding the sum of ll prtil volumes receiving dose in the GTV structure. Hence, ny prtil volumes receiving dose close or ner zero would yield very low GTV EUD. Fig. 4. Illustrtion of irregulr deep-seted bulky tumor from Ptient #5.

8 403 Zhng et l.: GRID therpy using tomotherpy 403 () (b) Fig. 5. Isodose distribution () from LINAC-GRID Ptient # 5 (illustrtion of low-dose re in superior nd inferior portions of the tumor volume in coronl plne); isodose distribution (b) from HT-GRID Ptient #5 (coronl pln from sme CT slice s compred to LINAC-GRID nd trnsverse plne). For HT-GRID, the virtul GRID block cn be modified by dding more open holes in the low dose re t the superior nd inferior direction of the GTV to improve on the low-dose re. In ddition by using IMRT technique, GTV EUD cn be improved upon, while mintining the GRID bem chrcteristics nd spring the criticl OARs. It is importnt to note tht vrying the dimeter of the hole nd the center-to-center spcing cn ffect the therpeutic rtio. (13) However, it is beyond the scope of this pper to determine the mgnitude of this chnge. Figure 5(b) shows the dose distribution t coronl plne for the sme CT slice cut s compred to LINAC-GRID pln. The 600 cgy dose distribution (blue color) covered more of the GTV compred to LINAC-GRID pln nd no lrge cold spot re t the superior nd inferior direction of the GTV ws seen. Another exmple is Ptient #10 which hd very lrge volume GTV (4511 cc) nd criticl orgn of smll bowel butting the GTV (Fig. 6). The GTV EUD ws 2.95 Gy for LINAC-GRID pln nd 9.25 Gy for the HT-GRID pln for the sme resons. The literture provides dt regrding the use of prllel opposed GRID therpy to improve upon the dose distribution of the GRID GTV when treting deep-seted tumors. Meigooni et l. (7) reported on GRID therpy using prllel opposed bems. It ws shown tht prllel opposed GRID is vible option for tretment of deep-seted tumors s the sptilly frctionted dose distribution is preserved. However, this pproch cn expose the volume of norml tissue proximl nd distl to the trget to potentilly high doses of rdition (Fig. 7). This is minly due to the deep-seted nture of the lrge tumors. Tble 1 shows the rnge of the mximum distnce from the skin to the proximl edge of the trget ws from 3.4 cm to 22.6 cm for the ptients

9 404 Zhng et l.: GRID therpy using tomotherpy 404 Fig. 6. Illustrtion of lrge volume of GTV with criticl orgn (smll bowel) round it from Ptient #10. Fig. 7. Isodose distribution for LINAC-GRID pln using prllel opposed bems from Ptient #9. selected in our study. So one or two rdition bems (djcent but nonopposing fields) were used in this study, depending on the tumor volume nd loction, to reduce norml tissue exposure. Norml tissue men doses were generlly higher for HT-GRID pln compred to LINAC- GRID pln. This result cn be explined by the following resons. HT-GRID plns used n IMRT technique, wheres only one or two sttic field bems were delivered for LINAC-GRID pln. In ddition, the HT bem is delivered in helicl fshion, so the integrl dose is usully higher compred to LINAC rdiotherpy using sttic bems. Our study results showed the norml tissue men dose ws higher for HT-GRID pln compred to LINAC-GRID plns nd tht result ws in greement with the reported results from other published ppers. (14) However, HT-GRID generlly hs lower norml tissue EUD compred to LINAC-GRID plns. This could be explined by severl resons. The LINAC-GRID pln only used one or two sttic bems without ny bem modultion, for the deep-seted tumors selected in this study, nd the mximum doses ll fell outside the GTV trget (Figs. 8() nd 8(b)). However, by using IMRT technique in HT-GRID plns, there is more control of the dose deposition within the GTV nd the mximum dose could be confined within the GTV. In this wy, the pln could be optimized by dding different constrints to GTV trget nd OARs. In ddition, plnning structures, such s rings, cn be used to further void the high dose outside the GTV trget. In this study, the mximum doses ll fell inside the GTV trget. In results, HT-GRID plns hd lower norml tissue dose ner the edge of the GTV nd higher norml tissue dose beyond the GTV boundry compred to LINAC-GRID, so the EUD over the entire norml tissue ws lower compred to LINAC-GRID plns. However, there ws n exception for Ptient #10, where the HT-GRID plns hd higher thn norml tissue EUD, compred to the LINAC-GRID pln (689 cgy vs. 513 cgy). This ptient hd very lrge, deeply seted GTV (4511 cc) (Fig. 7), nd portion of the GTV tht received ner zero doses in the LINAC-GRID pln minimized the EUD results of the LINAC- GRID pln compred to the HT-GRID pln. For HT-GRID, in order to constrin the bowel dose, the HT bems were highly intensified nd, s result, incresed the norml tissue EUD.

10 405 Zhng et l.: GRID therpy using tomotherpy 405 () (b) Fig. 8. LINAC-GRID pln () in which mximum dose flls outside the GTV, from Ptient #1; HT-GRID pln (b) where mximum dose (cross line) flls inside the GTV, from Ptient #1. OARs doses were lso compred in this study. Generlly the OARs doses were comprble between the HT-GRID pln nd the LINAC-GRID pln. However, occsionlly it is very difficult to void the bem pssing through specific OAR in LINAC-GRID plns. For exmple, in the cse of Ptient #8, the spinl cord could be blocked by closing hlf jw (Fig. 9()) to reduce the spinl cord dose; however, this would underdose substntil portion of the GTV nd, s result, reduce its EUD. For the sme ptient, if the spinl cord is not excluded from the bem s pth, the mximum spinl cord dose would be Gy for single frction. This would crry n uncceptble risk of spinl cord myelitis. HT-GRID could solve this problem by modifying the virtul GRID block to both mximize exclusion of the spinl cord nd minimize the portion of the GTV not receiving ny dose. Additionlly, the use of inverse plnning in HT-GRID llows for optimiztion of dose deposition within the GTV (Fig. 9(b)). This study showed tht HT-GRID pln could substntilly reduce the spinl cord dose while keeping the comprble or better GTV dose distribution, compred to the LINAC-GRID pln (Tble 4). Another exmple is Ptient #10 (Fig. 7) who hd lrge GTV (4511 cc) nd smll bowel structure butting it. For LINAC-GRID, single, lrge, sttic field would not be enough to cover the entire GTV nd multiple bems would be needed to receive the cceptble GTV dose. However, this would significntly increse the smll bowel dose to n uncceptble level. An

11 406 Zhng et l.: GRID therpy using tomotherpy 406 () (b) Fig. 9. LINAC-GRID () with hlf jw block to spre the cord dose (Ptient #8); HT-GRID (b) with spring cord dose by modifying the HT virtul GRID block nd using IMRT technique (Ptient #8). HT-GRID pln could reduce the bowel dose by closing some open res of the GTV ner the smll bowel structure nd, together with IMRT optimiztion, rech the cceptble GTV dose nd spre the smll bowel dose to n cceptble level. V. CONCLUSIONS Ptients who re eligible to receive GRID therpy were successfully treted using HT-GRID tretment technique t UAMS. By generting the virtul GRID block, HT-GRID cn mimic the dose delivery pttern defined by LINAC-GRID tretments with commercilly vilble GRID block. The virtul GRID block needs to be optimized for ech individul ptient in order to get best GTV dose coverge while spring the OAR doses to the cceptble level. In this study, both HT-GRID nd LINAC-GRID tretment techniques hve their pros nd cons. The LINAC-GRID hd smller VPR nd ws esy to pln, wheres HT-GRID plnning is better suited for GTVs showing lrge skin-to-tumor distnce or complex ntomicl reltionship between the GTV nd the OAR (butment or close proximity). LINAC-GRID hs limittions in treting this type of deep-seted tumor s seen in Ptients #5 nd #8 who were not suited for LINAC-GRID tretment. This stresses the importnce of EUD in these cses, s the verge doses to the GRID GTV seemed pproprite but the EUD ws very low thus indicting tht the tretment my be filure. In ddition, HT-GRID uses IMRT optimiztion to enhnce the dose distribution on the GTV. EUD, in conjunction with the men GTV dose, ws found to be good indictor to evlute the GRID dose coverge.

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