intensity - modulated radiation therapy ( imrt ) is one of the treatment choices for localized prostate tumor irradiations .
imrt is often delivered with low - energy photons , viz . , below 10 mv .
these deep - seated prostate tumors were earlier treated with 3d - conformal radiation therapy ( 3d - crt ) using high - energy photons ( 15 to 25 mv ) .
utilization of low - energy photons in imrt is known to have certain advantages , like high conformity , negligible neutron contamination , etc .
but , there are indications provided by certain investigations , that these low - energy photons may deliver high doses in regions far from the ) target . also , there are some indications in literature of better target coverage and very low risk of induction of secondary malignancies due to neutron contamination in high - energy beams.[14 ] hence , a complete analysis of the influence of the photon energy on the quality of the imrt plans is necessary . for this study , a cohort of 8 prostate patients was selected , and 6-mv and 15-mv step and shoot imrt plans were compared by calculating relative efficacies of several physical indices , including primitive h - index , modified h - index , primitive hi - index , modified hi - index , s - index , conformity index and mean dose volume histogram ( dvh ) .
two mean dvh 's were generated and were considered as completely representative for 6-mv and 15-mv plans , respectively .
at our center , the radiation therapy is delivered using elekta precise high - energy linear accelerator .
inverse imrt planning is done using segmental inverse optimizer of elekta preciseplan planning system , version 2.03 .
sample imrt plans were created for all 8 prostate patients , and the dose prescription was 80 gy in 40 fractions with 6-mv photons .
five co - planar fields at gantry angles of 255 , 315 , 45 , 105 , 180 degrees were used . gross tumor volume ( gtv ) , planning target volume ( ptv ) , rectum , bladder and femoral heads were contoured .
the dose constraints used for these plans were in accordance with radiation therapy oncology group ( rtog ) protocol and are shown in table 1a .
thus , the advantage is that , segments too can be kept constant between 6-mv and 15-mv plans . in order to study the effect of energy on the quality of the plans ,
competing plans were also carried out using 15-mv beams for all patients . to ensure that the similarity or difference was due to energy alone , all other parameters like beam angles , number of beams , number of segments , shapes of the segments , dose constraints , etc .
all the patients image sets were chosen such that , there was not much variation in their anatomy .
all the patients antero - posterior ( ap ) and lateral dimensions were very close .
the ap diameters were between 22 and 23 cm , and the lateral diameters were between 33 and 34 cm .
ptv was drawn by expanding gtv by 1 cm in all directions except in the posterior direction , where 0.6 cm was used .
rect_imrt ( a portion of rectum ) was derived from rectum such that it extended only 3 mm beyond ptv in superior and inferior directions . for the coplanar beams used , this definition of rect_imrt was chosen , as it is more sensitive to the optimization and less sensitive to the anatomy .
the following parameters were evaluated for comparison : mean dvh , primitive h - index , modified h - index , primitive hi - index , modified hi - index , s - index and conformity index .
dose volume histograms ( dvh - s ) are two - dimensional representations of complex 3d dose distributions inside the patient .
evaluation of homogeneity of dose inside the ptv using dvh - s is of paramount importance , as the cold spots can negatively affect the tumor control probability ( tcp ) .
all the 6-mv plans were compared with corresponding 15-mv plans on a one - to - one basis . to facilitate dvh comparison ,
all the dvh - s were normalized such that 95% volume of the ptv was covered by the prescribed dose of 80 gy .
in addition , a mean dvh for each contoured organ ( ptv , rect_imrt , bladder , rt .
mean dvh can be found by averaging the volumes in each dose bin and re - plotting the data as a function of dose .
bioplan software developed by nahum et al . , was used to generate the mean dvh - s .
but , many of them have their limitations and fail to predict the homogeneity correctly . some of them used in this study are defined below . the conventionally used homogeneity index ( h - index )
is defined as the ratio of the maximum dose ( dmax ) in the ptv to the prescription dose ( dp ) , with a value closer to 1 indicating better homogeneity .
the h - index can be made more robust by using d5 instead of dmax ( modified h - index ) . as an improvement to h - index and modified h - index ,
another index called hi - index has been defined as where d2 and d98 represent doses to 2% and 98% volume of ptv , respectively .
this hi - index can be further improved by using d5 and d95 instead of d2 and d98 ( modified hi - index ) .
this improvisation can be realized , as these new substitutions clip the usually small - volume high and low dose extremities in the cumulative dvh of the ptv and get only the steeper portion of the curve into the calculation .
hence , other parameters were also used in an attempt to find out the appropriate energy .
a new index called sigma index ( s - index ) , which gives better representation of homogeneity , was introduced by them .
this index uses the differential dvh , unlike other indices , which use cumulative dvh .
s - index is a measure of the standard deviation of the doses ( entire curve ) about the mean dose .
dmean is the mean dose of the target ( ptv in this study ) curve .
conformity index is defined as the ratio of the volume of the body receiving the prescription dose to the volume of the ptv receiving the same dose .
as stated above , all physical indices were evaluated for the two mean dvh - s of 6 mv and 15 mv , respectively . in figure 1 , 6-mv and 15-mv
figure 1a shows the mean dvh - s of ptv for both 6 mv and 15 mv .
as mentioned earlier , the ptv dvh - s were normalized , to facilitate comparison , thus resulting in very similar curves for both 6 mv and 15 mv . in the mean dvh - s of rectum , shown in 
 figure 1b , for doses greater than 80% ( approximately > 64 gy ) , 2% to 4% more volume received higher dose in 6 mv in comparison to 15 mv .
there was no significant difference between 6-mv and 15-mv for bladder , as clearly evident from figure 1c .
hence , the graphs were magnified and the values were measured . a : mean dose volume histogram of planning target volume b : mean dose volume histogram of rectum c : mean dose volume histogram of bladder d : mean dose volume histogram of body e : mean dose volume histogram of left femoral head f : mean dose volume histogram of right femoral head in figure 1d , the mean dvh - s of the body are shown . for 20 % to 60 % of doses ,
1% to 1.5% more volume was found to receive high dose with 6-mv beam than with 15-mv beam . for 15% to 20% of doses , approximately 0.5% more volume was found to receive high dose with 15-mv beam than with 6-mv beam . since , the magnitude of the dose delivered to ptv was very high in this study , the slight differences at very low doses ( less than 10% ) are not reflected in this figure .
but , from the data used to plot this graph , it is clear that greater volume received high dose with the 6-mv beam than with the 15-mv beam . as evident from figures 1e and 1f ,
the mean dvh - s of femoral heads favor 6 mv in comparison with 15 mv . for 42 % to 78% of doses ,
4% to 5% more volume of femoral heads received high dose with the 15-mv beam .
the different homogeneity indices for all the prostate patients and for both 6 and 15-mv energies are summarized in the table 1 , table 2 and table 3 .
the relative efficacy values for h - index , modified h - index , hi - index , modified hi - index and sigma index are 1.015 , 1.006 , 1.031 , 1.060 and 1.089 , respectively .
h and mod h - indices for eight patient intensity - modulated radiation therapy plans with 6 mv and 15 mv x - rays hi and mod hi - indices for eight patient intensity - modulated radiation therapy plans with 6 mv and 15 mv x - rays sigma indices for eight patient intensitymodulated radiation therapy plans with 6 mv and 15 mv x - rays conformity indices for eight patient intensity - modulated radiation therapy plans with 6 mv and 15 mv x - rays in figure 2 , the cumulative dvh - s of 6-mv plan for patient 1 and patient 3 are shown in red and green solid colors , respectively .
it is obvious that the dose homogeneity index of patient 1 is better than that of patient 3 .
but the h - indices are almost similar ( 1.128 and 1.126 ) , as evident from table 1 .
the s - indices are 2.233 and 5.14 for patient 1 and patient 3 , as evident from table 3 , predicting good accuracy .
the lower the value of the s - index , the better is the homogeneity .
cumulative dose volume histogram of patient 1(red ) and patient 3 ( green ) - 6 mv miften et al . , have demonstrated the use of target conformity index ( tci ) and normal tissue - sparing index ( ntsi ) to assist in the process of judging the merit of a clinical treatment plan .
but , in this work , the widely accepted conformity index was used to evaluate the conformity of the treatment plans .
the mean conformity index was 1.172 and 1.194 for 6 mv and 15 mv , respectively .
these small differences indicate that the plans are nearly identical in their conformity of dose to the target .
in figure 1 , 6-mv and 15-mv curves are shown in red and green color , respectively .
figure 1a shows the mean dvh - s of ptv for both 6 mv and 15 mv .
as mentioned earlier , the ptv dvh - s were normalized , to facilitate comparison , thus resulting in very similar curves for both 6 mv and 15 mv . in the mean dvh - s of rectum , shown in 
 figure 1b ,
for doses greater than 80% ( approximately > 64 gy ) , 2% to 4% more volume received higher dose in 6 mv in comparison to 15 mv .
there was no significant difference between 6-mv and 15-mv for bladder , as clearly evident from figure 1c .
a : mean dose volume histogram of planning target volume b : mean dose volume histogram of rectum c : mean dose volume histogram of bladder d : mean dose volume histogram of body e : mean dose volume histogram of left femoral head f : mean dose volume histogram of right femoral head in figure 1d , the mean dvh - s of the body are shown . for 20 % to 60 % of doses ,
1% to 1.5% more volume was found to receive high dose with 6-mv beam than with 15-mv beam . for 15% to 20% of doses , approximately 0.5% more volume was found to receive high dose with 15-mv beam than with 6-mv beam . since , the magnitude of the dose delivered to ptv was very high in this study , the slight differences at very low doses ( less than 10% ) are not reflected in this figure .
but , from the data used to plot this graph , it is clear that greater volume received high dose with the 6-mv beam than with the 15-mv beam . as evident from figures 1e and 1f ,
the mean dvh - s of femoral heads favor 6 mv in comparison with 15 mv . for 42 % to 78% of doses , 4% to 5% more volume of femoral heads received high dose with the 15-mv beam .
the different homogeneity indices for all the prostate patients and for both 6 and 15-mv energies are summarized in the table 1 , table 2 and table 3 .
the relative efficacy values for h - index , modified h - index , hi - index , modified hi - index and sigma index are 1.015 , 1.006 , 1.031 , 1.060 and 1.089 , respectively .
h and mod h - indices for eight patient intensity - modulated radiation therapy plans with 6 mv and 15 mv x - rays hi and mod hi - indices for eight patient intensity - modulated radiation therapy plans with 6 mv and 15 mv x - rays sigma indices for eight patient intensitymodulated radiation therapy plans with 6 mv and 15 mv x - rays conformity indices for eight patient intensity - modulated radiation therapy plans with 6 mv and 15 mv x - rays in figure 2 , the cumulative dvh - s of 6-mv plan for patient 1 and patient 3 are shown in red and green solid colors , respectively .
it is obvious that the dose homogeneity index of patient 1 is better than that of patient 3 .
but the h - indices are almost similar ( 1.128 and 1.126 ) , as evident from table 1 .
the s - indices are 2.233 and 5.14 for patient 1 and patient 3 , as evident from table 3 , predicting good accuracy .
the lower the value of the s - index , the better is the homogeneity .
cumulative dose volume histogram of patient 1(red ) and patient 3 ( green ) - 6 mv miften et al . , have demonstrated the use of target conformity index ( tci ) and normal tissue - sparing index ( ntsi ) to assist in the process of judging the merit of a clinical treatment plan .
but , in this work , the widely accepted conformity index was used to evaluate the conformity of the treatment plans .
the mean conformity index was 1.172 and 1.194 for 6 mv and 15 mv , respectively .
these small differences indicate that the plans are nearly identical in their conformity of dose to the target .
the mean dvh - s of ptv for both 6 mv and 15 mv show very little difference .
greater than 64 gy , 2 % to 4 % more rectum volume received a higher dose with 6-mv beam because of high entry dose .
femoral heads consistently received more dose with 15-mv beam for all patients as they received a higher depth dose .
but , it is unlikely that such small magnitudes of doses can produce any complications . except for minor differences ,
this work also assessed widely used homogeneity indices like h , hi , modified h and modified hi .
, was successfully used to predict correct homogeneity , as it takes the entire dvh curve into consideration .
s - index , was then , successfully used to evaluate the relative homogeneity between 6-mv and 15-mv groups .
thus , the s - index value of 1.089 , as seen in table 3 , should be a good estimate of relative homogeneity between 6 mv and 15 mv energies .
in addition , h - index , modified h - index , hi - index and modified hi - index were also used to evaluate the relative efficacy .
evaluation of all these parameters clearly depicts less homogeneity for 6 mv than 15 mv group .
the sample size was not a bad number , and h - index was also consistent with other parameters though they failed occasionally .
a relative conformity index value of 0.976 between 6-mv and 15-mv energies shows that 6-mv group is more conformal than 15-mv group
. in general , comparison of all above parameters showed that there was little difference between 6-mv and 15-mv groups . in practice
, much tighter constraints are maintained in low - energy treatment plans such that the complication probability is kept within 5% .
these tighter constraints might increase dose to other regions and other critical organs but are unlikely to increase their complication probabilities .
bladder is a relatively large organ when compared to rectum and also exhibits a higher partial volume effect .
usually , only the posterior part of bladder receives a high dose , and a small increase in dose is unlikely to increase its complication probability significantly .
similarly , femoral heads are at a clinically insignificant distance from ptv ( prostate and seminal vesicles ) and usually experience small doses .
hence the slight advantages of 15 mv in providing benefits of better normal - tissue sparing and better coverage can not be considered to outweigh its well - known risk of non - negligible neutron production .
 the goal of the present study was to study the effects of low- and high - energy intensity - modulated photon beams on the planning of target volume and the critical organs in cases of localized prostate tumors in a cohort of 8 patients . to ensure that the difference between the plans is due to energy alone , all other parameters were kept constant . 
 a mean dose volume histogram ( dvh ) for each value of energy and for each contoured structure was created and was considered as completely representative for all patients . to facilitate comparison between 6-mv and 15-mv beams , 
 the dvh - s were normalized . 
 the different parameters that were compared for 6-mv and 15-mv beams included mean dvh , different homogeneity indices , conformity index , etc . 
 analysis of several indices depicts more homogeneous dose for 15-mv beam and more conformity for 6-mv beam . 
 comparison of all these parameters showed that there was little difference between the 6-mv and 15-mv beams . for rectum , 2 to 4 % more volume received high dose with the 6-mv beam in comparison with the 15-mv beam , which was not clinically significant , since in practice much tighter constraints are maintained , such that normal tissue complication probability ( ntcp ) is kept within 5 % . 
 such tighter constraints might increase the dose to other regions and other critical organs but are unlikely to increase their complication probabilities . 
 hence the slight advantages of 15-mv beam in providing benefits of better normal - tissue sparing and better coverage can not be considered to outweigh its well - known risk of non - negligible neutron production . 
Introduction
Materials and Methods
Results and Discussion
Comparison of mean DVH-s
H, HI, modified H, modified HI, Sigma and conformity indices
Conclusions