We performed a review of 99 patients treated for tumors located in different regions (locations) of the body. All the plans were created according to the clinical protocol followed in the department without any deviation. A CT (Computed Tomography) scan based radiographic volume data set was used for the definition of target volumes, organs at risk, and other structures of interest. The target volumes like Gross Tumor Volume (GTV), Clinical Target Volume (CTV) and PTV were defined as per their definitions in International Commission on Radiation Units and Measurements (ICRU) Report 50.[10 ]
The treating radiation oncologist determined the target volumes and the treatment dose. In most cases, the planning was done with the following aims: minimum dose greater than or equal to 95% and the maximum dose less than or equal to 107% of the prescribed dose. The normalization of each plan followed the recommendation of ICRU report 50. The ICRU point was used for normalization and was set at 100%. This point dose was also the prescribed dose, resulting in a dose variation across the target from 95% to 107% of the prescription dose in most cases. The target volume selected for calculating the HI was PTV. In cases, where two or more target volumes were present, the primary/larger target volume was selected for analysis. The leaf edge to PTV distance (target margin) was kept 7 mm in all cases. The DVH of each plan was generated and it was evaluated by radiation oncologist along with medical physicist until an acceptable plan was obtained.
The HI was calculated using the five different formulae given below.
Formula A : D5/D95; where D5 = minimum dose in 5% of the target volume and D95 = minimum dose in 95% of the target volume. The ideal value is 1 and it increases as the plan become less homogeneous.[11 (link)]
Formula B: Dmax/Dmin; where Dmax and Dmin represent the maximum and minimum point dose in the target volume, respectively. This formula has not been used in literature but it represents the classical definition of HI i.e., the ratio of maximum and minimum dose. This formula was used to represent the philosophy behind the definition of HI. It may not be technically correct to use this formula in practice as the doses may be very high or very low, if only point doses are considered. The ideal value is 1 and it increases as the plan becomes less homogeneous.
Formula C: D1–D98/Dp × 100; where D1 and D98 are the minimum dose in 1% and 98% of the target volume and Dp is the prescribed dose. This is the formula used for calculating HI in our department which is a slight modification of the formula (D1 instead of D2 making it more sensitive) described by Wu, et al.[4 (link)] The ideal value is Zero and increase as homogeneity decreases.
Formula D: D5–D95/Dp×100; where D5 and D95 are the minimum dose in 5% and 95% of the target volume and Dp is the prescribed dose. The ideal value is zero when D5 and D95 are equal. This formula is also a modification of formula by Wu et al., where D2 and D98 has been replaced by D5 and D95.[4 (link)]
Formula E: Dmax/Dp; where Dmax is the maximum point dose and Dp is the prescribed dose to the target volume i.e., the prescription isodose line chosen to cover the margin of the tumor.[8 (link)12 (link)] This was first described by RTOG and the ideal value is 1.
The patients were then divided into five groups, based on the prescribed dose for treatment, volume of target and the location of the target. This resulted in 15 different groups. The mean HI was calculated for each group using all the formulae. The data was then analyzed to find out the relationship between HI and various parameters like prescribed dose, target volume and location of the target (CTV or PTV).
The data was compiled using microsoft excel software and mean and median values of HI were calculated using statistical methods. Pearsons Chi-square test (SPSS Vs 16.02) was used to test the association between the mean value of HI calculated by a particular formula and the prescribed dose, volume of target and the location of target in the body. The same test was performed separately for each group using one formula at a time.
The treating radiation oncologist determined the target volumes and the treatment dose. In most cases, the planning was done with the following aims: minimum dose greater than or equal to 95% and the maximum dose less than or equal to 107% of the prescribed dose. The normalization of each plan followed the recommendation of ICRU report 50. The ICRU point was used for normalization and was set at 100%. This point dose was also the prescribed dose, resulting in a dose variation across the target from 95% to 107% of the prescription dose in most cases. The target volume selected for calculating the HI was PTV. In cases, where two or more target volumes were present, the primary/larger target volume was selected for analysis. The leaf edge to PTV distance (target margin) was kept 7 mm in all cases. The DVH of each plan was generated and it was evaluated by radiation oncologist along with medical physicist until an acceptable plan was obtained.
The HI was calculated using the five different formulae given below.
Formula A : D5/D95; where D5 = minimum dose in 5% of the target volume and D95 = minimum dose in 95% of the target volume. The ideal value is 1 and it increases as the plan become less homogeneous.[11 (link)]
Formula B: Dmax/Dmin; where Dmax and Dmin represent the maximum and minimum point dose in the target volume, respectively. This formula has not been used in literature but it represents the classical definition of HI i.e., the ratio of maximum and minimum dose. This formula was used to represent the philosophy behind the definition of HI. It may not be technically correct to use this formula in practice as the doses may be very high or very low, if only point doses are considered. The ideal value is 1 and it increases as the plan becomes less homogeneous.
Formula C: D1–D98/Dp × 100; where D1 and D98 are the minimum dose in 1% and 98% of the target volume and Dp is the prescribed dose. This is the formula used for calculating HI in our department which is a slight modification of the formula (D1 instead of D2 making it more sensitive) described by Wu, et al.[4 (link)] The ideal value is Zero and increase as homogeneity decreases.
Formula D: D5–D95/Dp×100; where D5 and D95 are the minimum dose in 5% and 95% of the target volume and Dp is the prescribed dose. The ideal value is zero when D5 and D95 are equal. This formula is also a modification of formula by Wu et al., where D2 and D98 has been replaced by D5 and D95.[4 (link)]
Formula E: Dmax/Dp; where Dmax is the maximum point dose and Dp is the prescribed dose to the target volume i.e., the prescription isodose line chosen to cover the margin of the tumor.[8 (link)12 (link)] This was first described by RTOG and the ideal value is 1.
The patients were then divided into five groups, based on the prescribed dose for treatment, volume of target and the location of the target. This resulted in 15 different groups. The mean HI was calculated for each group using all the formulae. The data was then analyzed to find out the relationship between HI and various parameters like prescribed dose, target volume and location of the target (CTV or PTV).
The data was compiled using microsoft excel software and mean and median values of HI were calculated using statistical methods. Pearsons Chi-square test (SPSS Vs 16.02) was used to test the association between the mean value of HI calculated by a particular formula and the prescribed dose, volume of target and the location of target in the body. The same test was performed separately for each group using one formula at a time.