Dose Shadowing and Prosthesis Involvement for Megavoltage Photon In vivo Diode Dosimetry

Abstract

Aim: The aim of the study is to investigate the photon beam perturbations induced by an in vivo diode in combination with prosthesis involvement in a human-like phantom.

Materials and methods: Beam perturbations for 6 MV and 10 MV photons caused by an EDP-20^3G in vivo diode in combination with prosthesis involvement were studied in a unique water-equivalent pelvic phantom, equipped with bony structures and Ti prosthesis using single fields between 2 × 2 and 15 cm × 15 cm as well as 10 MV lateral opposing fields and a four-field plan. Dose distributions were measured with Gafchromic EBT3 films with and without the diode included in the beams on the prosthesis (prosthetic fields) and non-prosthesis (non-prosthetic fields) sides of the phantom. Differences between prosthetic and non-prosthetic field dose data were determined to assess the effect of the prosthesis on the diode-induced beam perturbations inside the phantom.

Results: Photon beam dose perturbations ranged from 2% to 7% and from 5% to 12% for prosthetic and non-prosthetic fields, respectively, with relative differences between 2% and 4%. In addition, d₅₀ depths ranging from 8.7 to 11.5 cm and from 11.5 to 15 cm were acquired in the phantom for prosthetic and non-prosthetic fields, respectively, with relative differences between 2% and 5%.

Conclusion: On the basis of accuracy requirements in radiotherapy noting that a small underdose to tumors could yield a decrease in the probability of tumor control, the diode-induced beam perturbations in combination with prosthesis involvement in the photon fields may affect treatment outcome, as there would be a reduction in the prescribed target dose during treatment delivery.

Keywords: Beam perturbations; in vivo diode; pelvic phantom; photons; prosthesis.

Figure 1

The human-like prosthesis phantom oriented in the supine position with inserted Gafchromic EBT3 film piece for measurements (film is in direct contact with the prosthesis). When the phantom is placed in the upright position, the measurement plane is at a depth of 11 cm from the top of the phantom

Figure 2

Single AP beam setups in the phantom slice for 6 MV and 10 MV photon beam dose distribution measurements with and without an EDP-20^3G in vivo diode placed on the beam's central axis

Figure 3

Multi-beam setups for a 10 MV photon beam including lateral opposing fields (a) and four-field box (b) plan. The Ti prosthesis and region of interest representing the prostate are also indicated. Film was inserted in the measurement plane and placed over and in direct contact with the prosthesis as indicated in Figure 1

Figure 4

(a and b) Depth dose data acquired in the human-like pelvic phantom for a 5 cm × 5 cm 6 MV photon beam. Data are shown for (a) prosthetic (with prosthesis), and (b) non-prosthetic (without prosthesis) fields, with and without the EDP-20^3G in vivo diode on the beam's central axis. Also shown are plots for the ratio of film dose without the diode to film dose when using it. The error bars indicate the random fluctuations of the dose ratios due to radiation scatter and measurement uncertainties

Figure 5

(a and b) A comparison of 10 MV photon beam dose distributions acquired in the pelvic prosthesis phantom with and without an EDP-20^3G in vivo diode on the beam's central axis. Also shown are plots for discrepancies between dose data obtained without the diode and when using the diode. Data are shown for lateral opposing fields (a) and four-field box (b) treatment plans. Beam setups for these multifield plans including the positions of the diode on the phantom are indicated in Figure 3

Figure 6

Relative response values (ratio of film dose without the EDP-20^3G in vivo diode to film dose when using the diode) for prosthetic and non-prosthetic (without prosthesis) fields. Also shown are plots for relative differences between the prosthetic and non-prosthetic field data. Data are shown for: (a) a 5 cm × 5 cm 6 MV beam; and (b) a 15 cm × 15 cm 10 MV beam. The error bars indicate the random fluctuations of the dose ratios due to radiation scatter and measurement uncertainties