an alternative approach is active breath - hold , which is a temporary suspension of breathing at a desired respiratory phase .
however , it is argued that the suspension mechanism facilitates the protection of the heart better in breast cancer patients than in lung cancer patients who tend to have a smaller breathing capacity .
gating was initially proposed in proton therapy for liver tumors using a pressure sensor on the abdominal wall .
gated radiotherapy using an external respiratory signal has been clinically implemented using commercially available systems .
however , it has been reported that respiratory - gated beam delivery can not result in margin reduction without respiratory - correlated image guidance . for more accurate
gated radiotherapy , a gold fiducial marker can be implanted near a lung tumor for real - time diagnostic x - ray fluoroscopy . however , a major problem of any gating technique is longer delivery time leading to lower patient throughput as well as the possibility of reduced tumor control .
another approach is dynamic tumor tracking with a gimbaled treatment head that allows continuous beam delivery , where the moving target may be located by an implanted fiducial marker and diagnostic x - ray fluoroscopy in real time .
although it is assumed that the spatial relationship between tumor volume and a point marker remains unchanged , this may not always be true due to the mobility of the marker .
in addition , accuracy of treatment may ultimately be defined as a volumetric relationship between the tumor and the dose distribution during the entire beam delivery .
still another method for managing respiratory motion is the use of an internal target volume ( itv ) determined by contouring clinical target volumes ( ctvs ) on 10-phase 4d planning ct images . in this case , the problem of respiratory motion management can be reduced to a tumor registration problem with a larger , but virtually still , target volume on the treatment couch . by constraining the breathing amplitude with the aid of abdominal compression , the itv approach may decrease the target volume and thus the dose to normal lung . using 4d cone - beam ct ( cbct ) , a respiratory - correlated lung tumor volume can be visualized immediately before treatment [ 79 ] .
an advantage of the 4d - cbct - based itv approach is that the above - mentioned volumetric accuracy can be directly confirmed on a cbct workstation prior to each fractionated treatment . to the authors ' knowledge
, a clinical workflow using 4d cbct for lung treatment has not previously been established .
the purpose of this study was to propose a clinical workflow of stereotactic volumetric modulated arc therapy ( vmat ) for lung tumors from planning to tumor position verification .
a large bore 16-multislice ct scanner , aquillion lb ( toshiba , japan ) , an anzai belt ( anzai medical , japan ) and a body fixation device , bodyfix ( elekta , germany ) , were employed to obtain 10-phase respiratory - correlated ct data for a lung patient under constrained breathing conditions induced by an abdominal compression plate .
the patient was diagnosed with stage iv metastatic lung tumor ( squamous cell carcinoma ) from an anal cancer .
the lung tumor had a major diameter of 15 mm and a minor diameter of 8 mm .
the amplitude of the tumor motion was 11.7 mm in the craniocaudal direction , 1.8 mm in the lateral direction and 0.8 mm in the anteroposterior direction . a planning target volume ( ptv )
was defined by adding a 5-mm margin to an itv created by 10 clinical target volumes , each of which was delineated on each of the 10-phase 4d planning ct images .
a single - arc vmat plan was created with a d95 prescription dose of 50 gy in four fractions on the maximum exhalation phase ct images   where the photon energy was 6 mv with a maximum dose rate of 580 mu / min .
the ptv contours were exported to a kilovoltage cbct x - ray volume imaging ( xvi ) version 4.5 , equipped with a synergy linear accelerator ( linac ) ( elekta , uk ) .
immediately before treatment , 10-phase 4d cbct data were reconstructed with an acquisition time of 4 min , generating animated lung tumor imaging .
initial bone matching was performed between frame - averaged 4d planning ct and frame - averaged 4d cbct datasets , where the frame - averaged 4d planning ct data were calculated by in - house software and the frame - averaged 4d cbct data were provided by xvi built - in functionality .
subsequently , the imported ptv contours and the animated moving tumor were simultaneously demonstrated on the xvi display monitor , and a manual 4d registration was interactively performed by moving the imported itv and ptv contours using a mouse on the axial , coronal and sagittal views until the moving tumor was symmetrically positioned inside the ptv .
after confirmation of the registration results on the display monitor , the patient couch was repositioned and stereotactic vmat beams were delivered . during the delivery , another set of 4d cbct projection data was acquired to verify the tumor position , and phase - sorted volumetric reconstruction was performed using in - house software .
this in - house program was needed because the current xvi version does not provide cbct imaging during treatment .
the current study is in compliance with ethical guidelines of the hospital and written informed consent was obtained before the treatment was initiated .
manual 4d registration using the ptv contours and the animated lung tumor required only a few seconds .
figure 1a  e show 4d cbct images on the first day overlaid with the ptv ( in sky blue ) and itv ( in yellow ) contours after the 4d lung tumor registration .
registration required a couch translation of 3.2 mm toward the patient 's right , 1.5 mm toward the caudal direction , and 4.4 mm toward the anterior direction , respectively .
the figures show images for five consecutive respiratory phases that cover half a breathing cycle , in which the tumor moves from cranial to caudal direction during the half cycle . 

1.4d cbct images on the first day overlaid with ptv ( in sky blue ) and itv ( in yellow ) contours after lung tumor registration for five consecutive respiratory phases covering half a breathing cycle , where the tumor moves from cranial to caudal direction during the half cycle . after confirmation of the registration results on the display monitor , stereotactic vmat is ready to run .
4d cbct images on the first day overlaid with ptv ( in sky blue ) and itv ( in yellow ) contours after lung tumor registration for five consecutive respiratory phases covering half a breathing cycle , where the tumor moves from cranial to caudal direction during the half cycle . after confirmation of the registration results on the display monitor , stereotactic vmat is ready to run .
figure 2a  d shows 4d cbct images acquired during vmat delivery on the first day and reconstructed afterwards to verify the in - treatment moving tumor position in reference to the ptv and itv contours .
the vmat delivery time was 210 s. for the in - treatment cbct , the number of phase bins was four , which covered an entire breathing cycle .
it was confirmed that during the vmat delivery on the first day the moving tumor was also located inside the ptv .
2.4d cbct images acquired during vmat delivery on the first day to verify the in - treatment moving tumor position in reference to the ptv and itv contours . in this case , the number of phase bins was four , which covered an entire breathing cycle .
4d cbct images acquired during vmat delivery on the first day to verify the in - treatment moving tumor position in reference to the ptv and itv contours . in this case , the number of phase bins was four , which covered an entire breathing cycle .
it is known that vmat delivery is faster than any other intensity - modulated treatment .
in addition , in - treatment cbct can be acquired during vmat delivery for 4d tumor position verification .
2 a  d show some blurring , possibly due to the smaller number of phase bins , which may be improved by providing 10-phase in - treatment 4d cbct datasets
. a limitation of this technique would be that the amplitude of tumor motion may not be sufficiently reduced by abdominal compression in the case of some tumors having a large breathing motion , possibly leading to a prohibitively large itv volume .
the measured respiratory movement in this study was relatively large and could have been reduced by increasing the compression of the plate to the abdominal surface . whereas the dose calculation in this study was based on maximum exhalation phase ct images , the authors are currently updating their procedure using mid - ventilation ct data .
we have proposed a clinical workflow of stereotactic lung vmat capable of 4d registration and 4d verification of the tumor position , and initial promising results have been obtained .
the authors believe that the proposed 4d workflow for stereotactic lung vmat is valid for respiratory motion management because the vmat delivery is sufficiently fast to maintain respiratory periodicity , but much longer than the patient breathing cycle for obtaining 4d cbct .
our lung vmat delivery employs a sequence with a maximum leaf speed of 1 mm / degree , and therefore it is anticipated that doubling the dose rate using flattening filter free techniques , combined with doubled leaf speeds , may reduce the treatment time down to an order of 100 seconds for delivering a prescribed dose of 50 gy in four fractions .
 we propose a clinical workflow of stereotactic volumetric modulated arc therapy ( vmat ) for a lung tumor from planning to tumor position verification using 4d planning computed tomography ( ct ) and 4d cone - beam ct ( cbct ) . 
 a 4d ct scanner , an anzai belt and a bodyfix were employed to obtain 10-phase respiratory - correlated ct data for a lung patient under constrained breathing conditions . 
 a planning target volume ( ptv ) was defined by adding a 5-mm margin to an internal target volume created from 10 clinical target volumes , each of which was delineated on each of the 10-phase planning ct data . 
 a single - arc vmat plan was created with a d95 prescription dose of 50 gy in four fractions on the maximum exhalation phase ct images . 
 the ptv contours were exported to a kilovoltage cbct x - ray volume imaging ( xvi ) equipped with a linear accelerator ( linac ) . immediately before treatment , 10-phase 4d cbct images were reconstructed leading to animated lung tumor imaging . 
 initial bone matching was performed between frame - averaged 4d planning ct and frame - averaged 4d cbct datasets . 
 subsequently , the imported ptv contours and the animated moving tumor were simultaneously displayed on the xvi monitor , and a manual 4d registration was interactively performed on the monitor until the moving tumor was symmetrically positioned inside the ptv . 
 a vmat beam was delivered to the patient and during the delivery further 4d cbct projection data were acquired to verify the tumor position . 
 the entire process was repeated for each fraction . 
 it was confirmed that the moving tumor was positioned inside the ptv during the vmat delivery . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION
FUNDING