Dosimetric comparison of extended dose range film with ionization measurements in water and lung equivalent heterogeneous media exposed to megavoltage photons

Abstract

In this study, a dosimetric evaluation of the new Kodak extended dose range (EDR) film versus ionization measurements has been conducted in homogeneous solid water and water-lung equivalent layered heterogeneous phantoms for a relevant range of field sizes (up to a field size of 25x25 cm2 and a depth of 15 cm) for 6 and 15 MV photon beams from a linear accelerator. The optical density of EDR film was found to be linear up to about 350 cGy and over-responded for larger fields and depths (5% for 25x25 cm2 at depth of 15 cm compared to a 10x10 cm2, 5 cm depth reference value). Central axis depth dose measurements in solid water with the film in a perpendicular orientation were within 2% of the Wellhöfer IC-10 measurements for the smaller field sizes. A maximum discrepancy of 8.4% and 3.9% was found for the 25x25 cm2 field at 15 cm depth for 6 and 15 MV photons, respectively (with curve normalization at a depth of 5 cm). Compared to IC-10 measurements, film measured central axis depth dose inside the lung slab showed a slight over-response (at most 2%). At a depth of 15 cm in the lung phantom the over-response was found to be 7.4% and 3.7% for the 25x25 cm2 field for 6 and 15 MV photons, respectively. When results were presented as correction factors, the discrepancy between the IC-10 and the EDR was greatest for the lowest energy and the largest field size. The effect of the finite size of the ion chamber was most evident at smaller field sizes where profile differences versus film were observed in the penumbral region. These differences were reduced at larger field sizes and in situations where lateral electron transport resulted in a lateral spread of the beam, such as inside lung material. Film profiles across a lung tumor geometry phantom agreed with the IC-10 chamber within the experimental uncertainties. From this investigation EDR film appears to be a useful medium for relative dosimetry in higher dose ranges in both water and lung equivalent material for moderate field sizes and depths.

Figure 1

A schematic view of the experimental setup of the layer‐lung [(a), full‐slab and (b), tumor] geometry used in the measurement of dose.

Figure 2

Sensitometric curves for EDR2 films for 6 and 15 MV photons, for a $10\times 10{ \text{cm}}^2$ field size at 5 cm depth in the phantom.

Figure 3

Normalized net optical densities for EDR film at a dose of 50 cGy as a function of field size and depth in solid for (a) 6 MV, and (b) 15 MV photons. The optical densities were normalized to the value for a $10\times 10{ \text{cm}}^2$ field size at a depth of 5 cm. The uncertainties were estimated to 2% (1σ).

Figure 4

Depth dose comparison in homogeneous solid water between the EDR film in perpendicular orientation and the IC‐10 ionization chamber for different field sizes ($2\times 2$, $3\times 3$, $10\times 10$, and $25\times 25{ \text{cm}}^2$) from (a) 6 MV, and (b) 15 MV photon beams. Depth dose curves for both dosimeters are normalized to a common value at a depth of 10 cm for each field size.

Figure 5

Profile comparison between the EDR film in homogeneous solid water and the IC‐10 ionization chamber in a water tank for different field sizes ($2\times 2$, $3\times 3$, $10\times 10$, and $25\times 25{ \text{cm}}^2$) from (a) 6 MV, and (b) 15 MV photon beams. Profiles are normalized to their respective central axes. The IC‐10 profiles are subsequently rescaled to the central axis value of the EDR profiles for a given field size and energy.

Figure 6

Depth dose comparison between the EDR film in the perpendicular orientation and the IC‐10 ionization chamber for different field sizes ($2\times 2$, $3\times 3$, $10\times 10$, and $25\times 25{ \text{cm}}^2$) from (a) 6 MV, and (b) 15 MV photon beams in heterogeneous solid water and full‐slab lung. Depth dose curves for a given dosimeter are normalized to 2 cm depth of their analogous homogeneous depth doses for the respective field sizes.

Figure 7

Lung correction factors vs depth for $2\times 2$, $3\times 3$, $10\times 10$, and $25\times 25{ \text{cm}}^2$ field sizes. A comparison between the EDR film in the perpendicular orientation and the IC‐10 ionization chamber (a) 6 MV, and (b) 15 MV photon beams in heterogeneous solid water and full‐slab lung. The correction factor at a given depth, field size, and energy is defined as the ratio of the dose in the heterogeneous phantom to the dose in the homogeneous phantom.

Figure 8

Profile comparison between the EDR film and the IC‐10 ionization chamber in heterogeneous full‐slab lung geometry for $2\times 2{ \text{cm}}^2$ and $10\times 10{ \text{cm}}^2$ fields at depth of 8 cm (in the lung) and 12 cm (beyond the lung). Profiles are shown for (a) 6 MV and (b) 15 MV photon beams. Profiles are normalized to their respective central axes. The IC‐10 profiles are subsequently rescaled to the central axis value of the EDR profiles for a given field size and energy.

Figure 9

Profile comparison between the EDR film and the IC‐10 ionization chamber in the heterogeneous lung tumor geometry for $2\times 2$, $5\times 5$, 10* 10, and $20\times 20{ \text{cm}}^2$ fields. The depth for measurement is 8 cm in the phantom; at this depth lung/tumor/lung interfaces are encountered laterally. Two photon beams are illustrated: 6 and 15 MV. Profiles are normalized to the 2 cm depth dose value for a $2\times 2{ \text{cm}}^2$ field of the homogeneous phantom depth dose for fixed energy and field size.