3d conformal radiotherapy is the present standard treatment modality for radiotherapy treatment of thoracic and abdominal - pelvic malignancies . respiratory motion leads to inaccuracy in target volume definition on the planning computed tomography ( ct ) scan . to ensure sufficient dose coverage throughout the treatment course , internal margin ( i m ) and setup margin ( sm ) are added to the clinical target volume ( ctv ) to make up the planning target volume ( ptv ) . simple linear addition of the two margins generally leads to an excessively large ptv that would go beyond the patient 's tolerance , and does not reflect the actual clinical consequences . thus , the risk of missing part of the ctv must be balanced against the risk of complications due to large ptv . geometric margins to account the respiratory motion are usually derived from fluoroscopy , clinical experience or values reported in the literature . patient - nonspecific margins either lead to the possibility of geometric miss or treat excess normal tissue . a geometric miss is still possible if tumor motion is greater than the assumed average motion . in addition , there can be unnecessary irradiation of normal tissue if tumor motion is smaller than expected . it is very difficult to decide margins unless we know the exact motion of tumor . patient - specific margins for ctv have been evaluated using fluoroscopy motion study , repeated volumetric ct scan , breath hold ct scan , slow ct scan and time - resolved ct scan ( 4dct scan ) . according to the guideline of aapm tg 76 , if the target motion is > 5 mm , a method of respiratory motion management is available and if the patient can tolerate the procedure , respiratory motion management technology is appropriate . a study shows that respiratory gating permits a reduction in field sizes because irradiation can be limited to phases in which the mobile target volume is in a predetermined position . in this article , we describe the clinical implementation of patient - specific ptv design using 4dct scanning . we report the initial results on 4d target volume designs and comparison with conventional ptv volume and we also studied dose to surrounding critical structure for different ptvs . ten patients were immobilized with a vacuum bag in the supine position , with arms raised during simulation . the 4dct scanning was performed during free breathing on a 16-slice pet / ct ( gemedical systems , waukesha , wi , usa ) . a plastic box with a pair of reflective markers was placed on the patient 's anterior abdominal surface , approximately midway between the xiphoid and the umbilicus . markers motion was captured by an infrared camera and the respiratory signal was recorded in synchronization with the x - ray  on  signal from the ct scanner . real - time positioning management ( rpm , varian ; palo alto , ca , usa ) was used for recording the breathing curve . the scanner was operated in an axial cine mode and continuous scans were performed at each couch position , with an interval equal to the patient 's average respiratory period so that complete data were acquired over an average respiratory cycle . data acquisition was repeated at each couch position until full longitudinal coverage of the region of interest was obtained . parameters for ct acquisition were 120 kv , 300 - 400 ma and slice thickness of 2.5 mm . the whole scanning time was 90 - 120 s. typically , a total of 1200 - 1500 slices were acquired for each patient . after 4dct scanning , the 4d software ( advantage 4d , gemedical systems , waukesha , wi , usa ) used all reconstructed images along with the respiratory phases that were calculated by the rpm system . images were sorted into 10 phases based on the temporal correlation between surface motion and data acquisition with an advantage workstation . the 10 respiratory phases were evenly distributed over a breathing cycle labeled as ct0% , ct10% , ct20% . . . ct0% corresponds to end - inhalation , ct20% to mid - exhalation and ct50% to end - exhalation , as shown in figure 1 . respiratory cycle if the respiratory cycle is viewed as a waveform , then a phase is a position within the breathing cycle where 0% = peak inspiration , 50% = peak expiration and 100% = peak inspiration gtvs were manually contoured on all 10 phases of the 4dct scan . target volumes were defined as gtv , which represented the primary lesion visualized on the ct images . the combined volume of the gtvs in the multiple ct phases was called as gtv4d . gtv4d plus an isotropic margin of 1.0 cm was called as ctv4d . two sets of ptv4d were derived from the ctv4d , i.e. ptv4d2 cm with a conventional margin of 2 cm ( ctv4d plus 1 cm sm and 1 cm i m ) and ptv4d1.5 cm ( ctv4d plus 1 cm sm and 0.5 cm i m ) , with reduction in the i m margin . for ptvgated , only three respiratory phases were selected in which organs at risk are at maximum distance from the tumor . ptv3d was derived from a ctv3d of the helical fast ct scan with conventional margins of 2 cm . all volumes of ptv , i.e. ptv4d2 cm , ptv4d1.5 cm , ptvgated and ptv3d were compared . to quantify the extent of the motion , we selected the two phases where the tumor exhibited the greatest range of motion , i.e. full inspiration ( 0% ) and full expiration ( 50% ) phases . we studied overlapping of ptv3d and ptv4d1.5 cm using the boolean operator or . to study the effect of ptv volumes on dose to critical structure , a set of three plans were generated for every patient with three different ptv volumes , i.e. ptv3d , ptv4d1.5 cm and ptvgated . for each set of plans , an identical number of beams , gantry angle and dose prescription were used . for ca lung patients , lung mean dose and v20 dose were calculated and for abdominal tumors , mean dose of the kidney and liver were calculated . the average of ratio of ctv3d volume and ctv4d volume of all patients was found to be 0.72 , with an sd of 0.07 [ table 1 ] . ptv4d1.5 cm shows reduction by an average value of 14% . similarly , ptvgating shows reduction by an average value of 21% of ptv3d . comparison of ctv3d and ctv4d volume comparison of ptv3d , ptvgated and ptv4d with different ctv expansion table 3 shows the observed tumor motion . the motion of tumor in peak to peak amplitude was found with a variation of 0.2 cm ( min ) and 1.2 cm ( max ) . in ca lung patients , the tumor motion during breathing was found to be varying widely in all directions ( first five patients in table 3 ) , but for abdominal cancer patients , tumor motion was dominant in the superior to inferior ( si ) direction only ( last five patients in table 3 ) . to determine the impact of motion on position of ptvs , overlapping of ptv4d1.5 cm and ptv3d was studied . it was noticed that in only one patient , ptv4d1.5 cm was completely overlapped by ptv3d , as shown in table 2 . because a 2 cm margin was used for ctv3d to get ptv3d and a 1.5 cm margin was used for ctv4d to get ptv4d1.5 cm , ptv4d1.5 cm should be completely overlapped by ptv3d . tumor motion and ptv overlapping as shown in figure 2 , abdomen tumor ptvs were not completely overlapped in the si direction because tumor motion was in the same direction , i.e. in the si direction . as tumor motion was very complex in lung patients , nonoverlapping of ptvs was also in a different direction . this clearly indicates that the nonpatient 's specific margin of 2 cm to ctv is also not sufficient if we do not know the tumor motion . nonoverlapping region is in the superior to inferior direction table 4 shows dose to critical structures . among the three ptvs plan ( ptv3d , ptv4d , ptvgated ) , the ptvgated plan has the least lung mean dose and lung v20dose . for the ptvgated plan , v20 lung dose and lung mean dose was lesser than ptv3d by 24% and 25% , respectively . similarly , for the ptv4d1.5 cm plan , the lung v20 dose and lung mean dose were smaller than the ptv3d plan by 9.87% and 13.2% . the averages doses in terms of percentages of prescribed dose for normal tissues for three different ptvs the same pattern of dose reduction was also observed for abdominal tumors . mean kidney dose reduction of 12% for the ptvgated plan and 16% for ptv4d1.5 cm as compared with the ptv3d plan were observed . ptvgated has the smallest volume among the three ptvs and it has less overlapping with organ at risk . there are no general patterns of respiratory behavior that can be assumed for a particular patient prior to observation and treatment . the individual characteristics of breathing , like quiet versus deep , chest versus abdominal , healthy versus compromised , etc . , and many motion variations associated with tumor location and pathology lead to distinct individual patterns in displacement , direction and phase of tumor motion . therefore , it is important to know the exact tumor motion to account for i m . generalized i m increases the probability of not missing target and treating extra normal tissue . the ptv must account for all interfractional as well as intrafractional variations throughout a treatment course . several sources of errors potentially remain , including ( 1 ) residual imaging artifacts , ( 2 ) setup variations , ( 3 ) delineation variations , ( 4 ) interfractional means tumor position variations and ( 5 ) treatment - induced changes in target size and motion patterns . the 4d target design eliminates systematic errors in ct imaging introduced by imaging a nonrepresentative target position during a standard helical scan . patient dose from 4dct is a major concern because the total radiation dose for a 4dct scan is eight - times the dose from a single conventional scan . the average of ratio of ctv3d volume and ctv4d volume of all patients was found to be 0.72 , with an sd of 0.07 [ table 1 ] . ptv4d1.5 cm shows reduction by an average value of 14% . similarly , ptvgating shows reduction by an average value of 21% of ptv3d . comparison of ctv3d and ctv4d volume comparison of ptv3d , ptvgated and ptv4d with different ctv expansion the motion of tumor in peak to peak amplitude was found with a variation of 0.2 cm ( min ) and 1.2 cm ( max ) . in ca lung patients , the tumor motion during breathing was found to be varying widely in all directions ( first five patients in table 3 ) , but for abdominal cancer patients , tumor motion was dominant in the superior to inferior ( si ) direction only ( last five patients in table 3 ) . to determine the impact of motion on position of ptvs , overlapping of ptv4d1.5 cm and ptv3d was studied . it was noticed that in only one patient , ptv4d1.5 cm was completely overlapped by ptv3d , as shown in table 2 . because a 2 cm margin was used for ctv3d to get ptv3d and a 1.5 cm margin was used for ctv4d to get ptv4d1.5 cm , ptv4d1.5 cm should be completely overlapped by ptv3d . tumor motion and ptv overlapping as shown in figure 2 , abdomen tumor ptvs were not completely overlapped in the si direction because tumor motion was in the same direction , i.e. in the si direction . as tumor motion was very complex in lung patients , nonoverlapping of ptvs was also in a different direction . this clearly indicates that the nonpatient 's specific margin of 2 cm to ctv is also not sufficient if we do not know the tumor motion . table 4 shows dose to critical structures . among the three ptvs plan ( ptv3d , ptv4d , ptvgated ) , the ptvgated plan has the least lung mean dose and lung v20dose . for the ptvgated plan , v20 lung dose and lung mean dose was lesser than ptv3d by 24% and 25% , respectively . similarly , for the ptv4d1.5 cm plan , the lung v20 dose and lung mean dose were smaller than the ptv3d plan by 9.87% and 13.2% . the averages doses in terms of percentages of prescribed dose for normal tissues for three different ptvs the same pattern of dose reduction was also observed for abdominal tumors . mean kidney dose reduction of 12% for the ptvgated plan and 16% for ptv4d1.5 cm as compared with the ptv3d plan were observed . ptvgated has the smallest volume among the three ptvs and it has less overlapping with organ at risk . there are no general patterns of respiratory behavior that can be assumed for a particular patient prior to observation and treatment . the individual characteristics of breathing , like quiet versus deep , chest versus abdominal , healthy versus compromised , etc . , and many motion variations associated with tumor location and pathology lead to distinct individual patterns in displacement , direction and phase of tumor motion . therefore , it is important to know the exact tumor motion to account for i m . generalized i m increases the probability of not missing target and treating extra normal tissue . the ptv must account for all interfractional as well as intrafractional variations throughout a treatment course . several sources of errors potentially remain , including ( 1 ) residual imaging artifacts , ( 2 ) setup variations , ( 3 ) delineation variations , ( 4 ) interfractional means tumor position variations and ( 5 ) treatment - induced changes in target size and motion patterns . the 4d target design eliminates systematic errors in ct imaging introduced by imaging a nonrepresentative target position during a standard helical scan . patient dose from 4dct is a major concern because the total radiation dose for a 4dct scan is eight - times the dose from a single conventional scan . this variability suggests that the use of 4dct can improve the definition of the treatment target . assuming an accurate internal motion on 4dct images , ims could be reduced from 1.0 cm to 0.5 cm . this will result in reduction of ptv volume and reduced chance of missing target and dose to critical structures . with proper knowledge of trajectory of tumor motion , respiratory - gated treatment can reduce the irradiated volume and can spare more normal structures .
in this study , four dimensional computed tomography ( 4dct ) scanning was performed during free breathing on a 16-slice positron emission tomography pet /computed tomography ( ct ) for abdomen and thoracic patients . 
 images were sorted into 10 phases based on the temporal correlation between surface motion and data acquisition with an advantage workstation . 
 gross tumor volume gross tumor volume ( gtv ) s were manually contoured on all 10 phases of the 4dct scan . 
 gtvs in the multiple ct phases were called gtv4d . 
 gtv4d plus an isotropic margin of 1.0 cm was called ctv4d . 
 two sets of planning target volume ( ptv ) 4d ( ptv4d ) were derived from the ctv4d , i.e. ptv4d2 cm = ctv4d plus 1 cm setup margin ( sm ) and 1 cm internal margin ( i m ) and ptv4d1.5 cm = ctv4d plus 1 cm sm and 0.5 cm i m . ptv3d was derived from a ctv3d of the helical ct scan plus conventional margins of 2 cm . 
 ptvgated was generated only selecting three ct phases , with a total margin of 1.5 cm . 
 all four volumes were compared . to quantify the extent of the motion , we selected the two phases where the tumor exhibited the greatest range of motion . 
 we also studied the effect of different ptv volumes on dose to the surrounding critical structures . 
 volume of ctv4d was greater than that of ctv3d . 
 we found , on an average , a reduction of 14% volume of ptv4d1.5 cm as compared with ptv3d and reduction of 10% volume of ptvgated as compared with ptv4d1.5 cm . 
 we found that 2 cm of margin was inadequate if true motion of tumor was not known . 
 we observed greater sparing of critical structures for ptvs drawn taking into account the tumor motion .