A Dosimetric Issue for Intensity Modulated Radiation Therapy
Murshed Hossain*, M. Saiful Huq, James M. Galvin
Thomas Jefferson University Hospital
Philadelphia, PA

Rotational IMRT offers the possibility of dose conformality for irregularly shaped targets combined with rapid peripheral dose fall-off. Dose delivery is accomplished with a series of small beam elements whose orientations and intensities are determined by inverse planning. This new treatment modality presents some difficult dosimetry issues. The final dose is the sum of many tiny beams that do not achieve lateral electronic equilibrium. This situation causes a discrepancy between film and ion-chamber measurements when many such beams are superimposed. This difference is evident when the film is calibrated against the chamber for larger 10x10 cm fields. The extended size of the ion chamber will overestimate the dose in regions of positive curvature at the "low dose" region of the profile for a small beam and underestimate the dose in the central region where the curvature is negative. However, the magnitude of the overestimation does not equal the underestimation when the effect of area (or volume) is considered. The exact amount of net overestimation of the chamber-determined dose reading depends on the details of each delivery, but typically ranges from 5-20%. Film dosimetry does not suffer such anomaly because the grain size is sufficiently small. This paper discusses two simple cases of intensity modulated delivery using the NOMOS MIMiC collimator which demonstrate the delivery dependent anomaly. This phenomenon could lead to reconsideration of the use of ion chamber dosimetry for IMRT with many small beams.

Experiments were designed to test the agreement of film and ion chamber measurements for rotational IMRT (NOMOS system with MIMiC collimator) dose delivery. The MIMiC controller was programmed to cycle a certain leaf pattern while the linear accelerator (Philips SL 75/5) arced through an angle of 290 degrees and delivered 300 MU. The dose profile across the narrow dimension of the "slit" opening was determined with both film and an ionization chamber. The film was scanned with a densitometer with a 0.5 mm spot. The ion chamber (PTW 4.5 mm diameter by 5.0 mm length) profile was obtained by moving the long dimension of the chamber across the field opening in steps of approximately 3.0 mm. The profiles or these measurements are plotted together for two separate cases in Figures 1 and 2. In the first case (Fig. 1), six central leaves from one leaf bank (2 cm leaf opening mode) were cycled with a preassigned pattern. For the SL 75/5, the 2 cm mode results in a beam element size of 8x16 mm. The figure shows that the central dose measured by the ion chamber is about 6% higher than the dose measured with film. Calibration of film and ion chamber was performed with larger field sizes with full lateral equilibrium. For the second case, six additional leaves from the second bank were added to the modulation produced by the original leaf bank. Fig. 2 shows the comparison for this wider field opening. The ion chamber reading is now 9% higher than the film measurement at the field center. The penumbral dose contribution in the second case is increased relative to the first case. This anomaly can be explained by investigating what happens when profiles for single fields with small dimensions are determined with both film and ion chamber. This comparison is shown in Figure 3. The reading of the ion chamber is higher than the film where the profile curvature is positive and lower where the curvature is negative. These two opposing effects at any measurement point do not cancel when an area average over many fields is taken. The contribution of negative curvature effect is more than the positive effect. Figure 4b shows a schematic diagram to illustrate how a positive profile curvature makes the ion chamber read higher while a simple linear gradient (Fig. 4a) shows no discrepancy. The effect illustrated by the two examples used here will be greater when the 1.0 cm leaf mode is used. The 1.0 cm leaf mode results in an 8x8 mm opening for the Philips SL 75/5 accelerator used for this investigation.

Abstracts Submitted to 1999 Annual AAPM Meeting | Medical Physics Home