volumetric - modulated arc therapy ( vmat ) has been characterized as intensity - modulated radiotherapy ( imrt ) in a single gantry arc , in which gantry speed , dose rate and leaf speed of the multileaf collimator ( mlc ) are varied during gantry rotation . in contrast , 180 partial - arc vmat has been employed for lung cancer treatment to reduce doses in contralateral healthy organs . dosimetric and delivery efficiency comparisons between 360 single - arc and 180200 partial - arc vmat for lung cancer have been made , reporting that partial - arc vmat significantly reduced mean dose to the contralateral lung with decreased delivery time . to our knowledge , however , no further comparisons of full - arc and half - arc vmat for tumors other than lung cancer have been reported . we speculated that a regular - shaped peripheral cancer such as a maxillary cancer may benefit from half - arc vmat due to its location . the purpose of this paper is comparison of dosimetric and delivery characterizations between full - arc and half - arc vmat for maxillary cancer . delivery was characterized by recording gantry angle error and gantry acceleration during vmat delivery in a log file . the patients were helically scanned on an aquilion lb ( toshiba , ootawara , japan ) computer tomography unit with a gantry rotation time of 0.5  s and the images were reconstructed with a slice thickness of 2  mm . gross tumor volume ( gtv ) was defined as a visible tumor at the head window settings . clinical target volume ( ctv ) planning target volume ( ptv ) was further defined by adding an isotropic margin of 5  mm to the ctv to account for setup uncertainty and mechanical inaccuracy . the normal brain , brainstem , spinal cord , optic chiasm , ipsilateral and contralateral eyes , and ipsilateral and contralateral optic nerves were contoured as organs at risk ( oars ) . the tumor sizes and positions for the six cases are shown in table  1 . table  1.tumor sizes , positions and gantry angle ranges used for half - arc vmat deliveriescase 1case 2case 3case 4case 5case 6ptv ( cm)280.1129.8347.9239.7302.8217.5ctv ( cm)181.276245.1150.7199.3129.4gtv ( cm)100.832.2142.280.5112.662.8lesion positionrlrlllgantry angle range ( )2254530012022545315135315135315135r : right side , l : left side . tumor sizes , positions and gantry angle ranges used for half - arc vmat deliveries r : right side , l : left side . vmat plans were created using the monaco 3.0 ( elekta , maryland heights , missouri , usa ) treatment planning system ( tps ) with gantry rotation angles of 360 ( full - arc vmat ) and 180 ( half - arc vmat ) . gantry angle ranges used for the half - arc vmat deliveries are shown in table  1 . the collimator and couch angles were fixed at 0. a dose of 66  gy in 33 fractions was prescribed to 95% of the ptv , and maximum dose was restricted to 110% of the prescribed dose . dose constraints for oars were given as follows : brainstem max dose 50  gy , spinal cord max dose 45  gy , optic chiasm mean dose , contralateral eye max dose and contralateral optic nerve mean dose 45  gy . ipsilateral eye and optic nerve maximum dose were minimized while maintaining the target d95 prescription . the monaco tps provides monte carlo dose calculations with a grid size of 3  mm and variance of 3% . dosimetric comparisons between the full- and half - arc vmat were performed with identical isocenter positions and dose constraints . for the ptv , a homogeneity index ( hi ) was calculated using the following formula : 
 ( 1 ) 
  where d2% , d98% and d50% are doses that cover 2% , 98% and 50% of the ptv , respectively . a conformity index ( ci ) was calculated using the following formula : 
 ( 2 ) 
  where vri was the volume of the reference isodose and tv was the target volume . ci95% , ci80% and ci50% were calculated , for which the reference isodose percentages were 95% , 80% and 50% , respectively . with regard to oar , a 6-mv photon beam with an mlc leaf width of 10  mm was used for vmat delivery using the synergy linear accelerator ( elekta , crawley , uk ) . treatment plans were transferred from the tps to a desktop pro 7.01 linac controller via a mosaiq v1.6 ( elekta , sunnyvale , california , usa ) record and verify system . total monitor units ( mu ) , mean dose rate , delivery time , gantry angle error and gantry acceleration were evaluated using a log file that recorded cumulative mu as well as planned and actual gantry angles every 250  ms . the gantry angle error was calculated by subtracting the planned gantry angle from the actual gantry angle for each treatment time . data were analyzed using the wilcoxon signed - rank test with statistical significance set at p  <  0.05 . statistical analysis was performed with spss v.19.0 ( ibm , chicago , il , usa ) . figure  1 shows representative dose distributions and dose volume histograms of full - arc and half - arc vmat for the same patient . compared with the full - arc vmat plan , the half - arc plan provided substantially lower doses to the normal brain and brainstem while maintaining a nearly identical dvh for the ptv . figure 1.dose distributions calculated by ( a ) full - arc and ( b ) half - arc vmat planning for a patient with maxillary cancer ; ( c ) comparison of dose volume histograms ( dvhs ) between full - arc ( solid line ) and half - arc ( dashed line ) vmat plans for the same patientthe ptv is shown as a translucent pink region . compared with the full - arc plan , the half - arc plan provided substantially lower doses to the normal brain and brainstem while maintaining a nearly identical dvh for the ptv . dose distributions calculated by ( a ) full - arc and ( b ) half - arc vmat planning for a patient with maxillary cancer ; ( c ) comparison of dose volume histograms ( dvhs ) between full - arc ( solid line ) and half - arc ( dashed line ) vmat plans for the same patient table  2 compares the hi of the ptv , ci and mean doses to the oar between the full- and half - arc vmat plans , with data shown as group averages with ranges ( n  =  6 ) . the hi , ci95% , ci80% , the mean doses to the spinal cord , optic chiasm , ipsilateral and contralateral eyes , and ipsilateral and contralateral optic nerves did not significantly differ between the full- and half - arc vmat plans ( p  >  0.05 ) . in contrast , mean doses to the normal brain and brainstem in the half - arc vmat plans were on average 16% and 17% lower than those in the full - arc plans , with these differences being statistically significant ( p  <  0.05 ) . table  2.dosimetric comparisons between full - arc and half - arc vmat plans(n  =  6)full - archalf - arcp valueptvhi0.075  mean0.190.21  range0.140.320.150.31ci95%0.345  mean1.11.1  range0.91.21.01.2ci80%0.249  mean1.61.6  range1.41.81.41.9ci50%0.028  mean2.82.6  range2.53.42.33.2normal brain dmean ( cgy)0.028  mean918.7768.3  range405.51623.2346.81335.0brainstem dmean ( cgy)0.028  mean2344.71943.8  range1805.92655.61403.12273.2spinal cord dmean ( cgy)0.080  mean1129.8890  range161.62341.9164.01709.5optic chiasm dmean ( cgy)0.463  mean2226.22117.5  range568.73224.3615.43344.8ipsilateral eye dmean ( cgy)0.280  mean3442.63254.3  range1487.45157.21338.44943.6ipsilateral optic nerve dmean ( cgy)0.600  mean4256.54212.3  range2548.46088.72274.66134.2contralateral eye dmean ( cgy)0.753  mean989.51006  range712.11512.8769.31542.0contralateral optic nerve dmean ( cgy)0.345  mean2843.62724  range1572.73454.31721.23378.1the wilcoxon signed - rank test resulted in a statistically significant mean dose reduction to the normal brain and brainstem in the half - arc vmat plans ( p  <  0.05 ) . dosimetric comparisons between full - arc and half - arc vmat plans the wilcoxon signed - rank test resulted in a statistically significant mean dose reduction to the normal brain and brainstem in the half - arc vmat plans ( p  <  0.05 ) . figure  2 shows plots of gantry acceleration and gantry angle errors as a function of treatment time during full- and half - arc vmat deliveries for a patient study set . full - arc vmat led to a larger gantry acceleration and larger gantry angle error than the half - arc vmat . in addition , gantry acceleration and gantry angle error appeared to show a considerable correlation . figure 2.plots of gantry acceleration ( black line ) and gantry angle errors ( gray line ) as a function of treatment time for a patient study set , with ( a ) full - arc and ( b ) half - arc vmat . on average , the full - arc vmat led to larger gantry acceleration and a larger gantry angle error than the half - arc vmat . plots of gantry acceleration ( black line ) and gantry angle errors ( gray line ) as a function of treatment time for a patient study set , with ( a ) full - arc and ( b ) half - arc vmat . on average , the full - arc vmat led to larger gantry acceleration and a larger gantry angle error than the half - arc vmat . gantry acceleration as a function of arc angle divided by total mu during each of the full - arc and half - arc vmat deliveries . gantry angle acceleration was linearly related to arc angle divided by total mu with an r value of 0.69 . figure 3.relationship between full - arc ( gray square ) and half - arc ( black diamond ) vmat deliveries.(a ) r.m.s . gantry acceleration during each delivery ( n  =  6 ) , and ( b ) r.m.s . gantry acceleration as a function of arc angle divided by total mu for each delivery . relationship between full - arc ( gray square ) and half - arc ( black diamond ) vmat deliveries . table  3 compares total mu , mean dose rate , delivery time , r.m.s . gantry acceleration and r.m.s . gantry angle error between the half - arc and full - arc vmat plans . on average , the half - arc plans resulted in 11% less total mu , 20% shorter delivery time , 12% higher mean dose rate , 30% less r.m.s . differences in all parameters were statistically significant according to the wilcoxon signed - rank test ( p  <  0.05 ) . table  3.comparison of delivery parameters between full - arc and half - arc vmat plans(n  =  6)full - archalf - arcp valuetotal mu0.028  mean506.2449.2  range430.0570.0378.0541.0mean dose rate ( mu / min)0.046  mean135.6152.7  range116.7164.3135.0175.9delivery time ( s)0.028  mean223.2178.3  range179.0264.0143.0210.0r.m.s . gantry angle error ( )0.028  mean0.280.21  range0.190.340.160.31group averages with ranges in parentheses are shown ( n  =  6 ) . the wilcoxon signed - rank test resulted in statistically significant differences ( p  <  0.05 ) for all parameters shown below . comparison of delivery parameters between full - arc and half - arc vmat plans group averages with ranges in parentheses are shown ( n  =  6 ) . the wilcoxon signed - rank test resulted in statistically significant differences ( p  <  0.05 ) for all parameters shown below . in this study , we demonstrated that half - arc vmat plans provided significantly reduced doses to the normal brain and brainstem compared with full - arc vmat plans , while maintaining nearly identical dose homogeneity and conformity in the ptv . for other oars , no significant dvh differences were observed between the full - arc and half - arc vmat plans , possibly because these oars were located closer to the ptv . these results suggest that half - arc vmat planning provides a clinical advantage in maxillary cancer . in contrast , conventional 3d plans for a maxillary cancer resulted in much higher doses to oars , whereas four- and nine - field imrt plans provided target coverage and sparing ability similar to our half - arc plans . our half - arc vmat plan may provide a better treatment option due to significantly decreased delivery time compared with the imrt and 3d plans . the dosimetric findings in the present study were consistent with those in a previous report in patients with lung cancer . we confirmed that the delivery time and total mu required for half - arc vmat delivery for maxillary cancer were less than those for full - arc delivery . the reduced mu may decrease the risk of secondary cancer . with regard to gantry acceleration and gantry angle error during vmat delivery , plots of r.m.s . gantry acceleration during the half - arc and full - arc vmat deliveries showed an approximately linear correlation . the greater gantry angle acceleration during full - arc delivery may be due to increased arc angle per total mu . values in half - arc deliveries were smaller than those in the full - arc deliveries . this finding might suggest that larger acceleration of the gantry and the accompanying greater inertia might result in a larger gantry angle error . we have demonstrated that half - arc vmat has clinical advantages over full - arc vmat for maxillary cancer . advantages of the half - arc vmat planning include improved oar sparing with nearly identical hi and ci for the ptv . the advantages of the half - arc vmat delivery include reductions in total mu and delivery time , thereby leading to better patient comfort and treatment throughput .
we compared the efficiency and accuracy of full - arc and half - arc volumetric - modulated arc therapy ( vmat ) delivery for maxillary cancer . 
 plans for gantry rotation angles of 360 and 180 ( full - arc and half - arc vmat ) were created for six maxillary cancer cases with the monaco treatment planning system , and delivered using an elekta synergy linear accelerator . 
 full - arc and half - arc vmat were compared with regard to homogeneity index ( hi ) , conformity index ( ci ) , mean dose to normal brain , total monitor units ( mu ) , delivery times , root mean square ( r.m.s . ) 
 gantry accelerations ( /s2 ) , and r.m.s . 
 gantry angle errors (  ) . 
 the half - arc vmat plans achieved comparable hi and ci to the full - arc plans . 
 mean doses to the normal brain and brainstem with the half - arc vmat plans were on average 16% and 17% lower than those with the full - arc vmat plans . 
 for other organs at risk ( oars ) , no significant dvh differences were observed between plans . 
 half - arc vmat resulted in 11% less total mu and 20% shorter delivery time than the full - arc vmat , while r.m.s . 
 gantry acceleration and r.m.s . 
 gantry angle error during half - arc vmat delivery were 30% and 23% less than those during full - arc vmat delivery , respectively . 
 furthermore , the half - arc vmat plans were comparable with the full - arc plans regarding dose homogeneity and conformity in maxillary cancer , and provided a statistical decrease in mean dose to oar , total mu , delivery time and gantry angle error . 
 half - arc vmat plans may be a suitable treatment option in radiotherapy for maxillary cancer .