The Effects of Small Field Dosimetry on the Biological Models Used In Evaluating IMRT Dose Distributions

Gene Cardarelli,PhD, MPH

I. PURPOSE/OBJECTIVEThe proper modeling of small field distributions is essential in reproducing accurate dose for IMRT. This evaluation was conducted to quantify the effects of small field dosimetry on IMRT plan dose distributions and the effects on four biological model parameters. Four biological models employed are 1) the generalized Equivalent Uniform Dose (gEUD), 2) the Tumor Control Probability (TCP), 3) the Normal Tissue Complication Probability (NTCP) and 4) the Probability of uncomplicated Tumor Control (P+). These models are used to estimate local control, survival, complications and uncomplicated tumor control. Due to the nature of uncertainties in small field dosimetry and the dependence of biological models on dose volume information, this examination investigates the effects of small field dosimetry techniques on radiobiological models and suggests pathways to reduce the errors in using these models to evaluate IMRT dose distributions. This study shows the importance of valid physical dose modeling prior to the use of biological modeling.

II. METHODS Pinnacle3 (Philips Medical Systems) version 7.4 was used to

input and model measured dose profiles and depth dose data. Data were collected using two different types of radiation detectors. 1) PTW Pinpoint model N31006 (Figure1a) waterproof chamber measuring 0.015 cc in volume. This chamber was used to collect small field and multi-leaf collimator (MLC) defined field data. 2) Kodak EDR2 film (Figure 1b). Three different dose calculation models were commissioned. The first model, PINPOINT, was created using data collected with the 0.015 cc Pinpoint chamber for all MLC fields less than 5 cm. The second model, FILM, was created using data collected with film for all small field collimated fields as well as MLC defined fields. The third model, SMF, was created using data collected with a small chamber (Pinpoint) but forced fitting of small field MLC to a 3x3 open field size.

Figure 1: Detectors used to scan profiles.

• Figure 1a PINPOINT • Figure 1b EDR2 FILM

Equation 1. gEUD

aa

iiDvgEUD

1

II. METHODS (cont)

Three dose models were used to calculate IMRT plans. Each IMRT plan was calculated using the same objectives and number of fields. Each of the three dose models were calculated to deliver a fractional dose of 200 cGy using a Varian model 2100CD SN526 Linear Accelerator (LINAC) equipped with a 120 leaf MLC. The physical position of the isodose distribution was measured using Kodak EDR Film by placing a ready pack film between 2 slabs of the solid water phantom at a depth of 10 cm. The dose plot can be compared to an expected distribution by importing the actual planar dose distribution generated from the treatment planning computer. (Figure 2a, 2b). Figure 3 shows the distance to agreement of the measured vs. planned isodose distribution displayed in Figure 2, for the Pinpoint model and the Film model.

Figure 2: RIT Planar dose grid overlay of measured isodose vs. expected (a) Pinpoint, (b) Film.

2a PINPOINT 2b FILM

Figure 3: RIT Distance to Agreement (a) Pinpoint and (b) Film

3a PINPOINT • 3b FILM

II. METHODS (cont)The three dose models were evaluated using the Pinnacle3 biological evaluation software. The biological model comparison of EUD calculations was computed using the Eq. (1) for each structure and adjusted for different values of the parameter a. This parameter a was adjusted from 1 to -20 for tumors and 1 to +20 for critical structures. gEUD was computed and subsequently compared for each dose model. This biological model was evaluated using different calculation grids, assessment of MLC positioning errors, and tissue heterogeneities. The biological model comparison of TCP and NTCP was computed for each structure or target for each type of plan and adjusting different values of alpha, α, or beta, β, depending on the type of tissue. The final biological model comparison, P+ statistic, probability of uncomplicated tumor control, was calculated for the prostate. All three dose models were used to generate this statistic.

III. RESULTS

The Penumbra is more pronounced in the Film curves than the Ion chamber curves. (Figure 4) The EUD analysis between the three doses models was conducted by adjusting the parameter a recalculating the gEUD in Pinnacle3. The PTV comparison results are shown in Figures 5, 6 and 7. Figure 5 indicates a significant deviation (p=0.0238) among the FILM model and the PINPOINT model when “a” is less than -5. Figures 6 and 7 show significant deviations as well, p=0.001 and p=0.001 respectively.

Figure 4: Profile comparison of 2x2 field at dmax for 6 MV X-Rays using (blue) 0.125 cc ion chamber, (red) PINPOINT, (green) Film Transverse, (teal) Film Radial.

Figure 5: Dose Clouds IMRT PLAN for (a) Pinpoint (b) Film with associated (c) PTV comparison for gEUD for different values of parameter a

a. PINPOINT b. FILM c. PTV Comparison

Figure 6: Dose Clouds IMRT PLAN Dose Grid change (a) 0.4 cm Grid (b) 0.2 cm Grid (c) PTV comparison for gEUD for different values of parameter a

a. 0.4Grid b. 0.2 Grid c. PTV Comparison

IV. DISCUSSION The comparison with grid size for gEUD was quite remarkable.

Figure 6 shows a dose cloud comparison between calculation grids of 0.2 and 0.4 cm. The graph shown in Figure 6 supports the conclusion that there is a statistical difference (p=0.001) in gEUD due to this calculation matrix change. These differences can be as high as 24%. Both models tested were equally affected with the FILM model showing a slightly increased effect at a = -20. This is logical because the equation for gEUD includes a voxel component. An increase in the number of voxels will change the result and will be inflated by the parameter a which is an exponential term in the equation for gEUD. Figure 7 shows the results of the MLC offset. It is obvious from the dose cloud that the effects of incorrect MLC position affects the dose distribution. The associated graph in Figure 7 further demonstrates the sensitivity of gEUD on small field parameters and the importance of proper MLC QA. This phenomenon should not be overlooked when performing IMRT converted plans.

Figure 7: Dose clouds IMRT plan for (a) MLC correct, (b) MLC offset, (c) PTV Comparison for gEUD for different values of parameter a

a. MLC Correct b. MLC Offset• c. PTV Comparison

IV. CONCLUSION

This study showed that proper beam modeling is essential in delivering accurate IMRT treatments. The use of film to document the effects of small field penumbra is more accurate than using a small ion chamber (PINPOINT). This study has demonstrated that film should be used for all field sizes less than 3x3 cm. The data requirements for Pinnacle3 should be altered to allow true out of field and in field corrections. Prior to using biological models to make decisions about patients’ treatments, an extensive review of accurate small field beam data must be done. The discrepancies documented in this study can have significant consequences when using biological dose optimization or integrating functional biological data into IMRT.