Development of a quasi-humanoid phantom to perform dosimetric and radiobiological measurements for out-of-field doses from external beam radiation therapy

Abstract

Our understanding of low dose, out-of-field radiation and their radiobiological effects are limited, in part due to the rapid technological advances in external beam radiotherapy, especially for non-coplanar and dynamic techniques. Reliable comparisons of out-of-field doses produced by advanced radiotherapy techniques are difficult due to the limitations of commercially available phantoms. There is a clear need for a functional phantom to accurately measure the dosimetric and radiobiological characteristics of out-of-field doses, which would in turn allow clinicians and medical physicists to optimize treatment parameters. We designed, manufactured, and tested the performance of a quasi-humanoid (Q-H) adult phantom. To test the physics parameters, we used computed tomography (CT) scans of assembled Q-H phantom. Static open field and dynamic techniques were measured both in- and out-of-field with ionization chambers and radiochromic films for two configurations (full solid and with water-filled containers). In the areas simulating soft tissues, lung, and bones, median Hounsfield units and densities were, respectively: 129.8, -738.7, 920.8 HU and 1.110, 0.215, 1.669 g/cm³ . Comparison of the measured to treatment planning systems (TPS) in-field dose values for the sample volumetric arc therapy (VMAT) (6 MV flattening filter-free (FFF)) plan, 96.4% of analyzed points passed the gamma evaluation criteria (L2%/2 mm, threshold (TH) 10%) and less than 1.50% for point dose verification. In the two phantom configurations: full poly(methyl) methacrylate (PMMA) and with water container, the off-axis median doses for open field, relative to the central axis of the beam (CAX) were similar, respectively: 0.900% versus 0.907% (15 cm distance to CAX); 0.096% versus 0.120% (35 cm); 0.018% versus 0.018% (52 cm); 0.009% versus 0.008% (74 cm). For VMAT 6 MV FFF, doses relative the CAX were, respectively: 0.667% (15 cm), 0.062% (35 cm), 0.019% (52 cm), 0.016% (74 cm). The Q-H phantom meets the International Commission on Radiation Units and Measurements (ICRU) and American Association of Physicists in Medicine (AAPM) recommended phantom criteria, providing medical physicists with a reliable, comprehensive system to perform dose calculation and measurements and to assess the impact on radiobiological response and on the risk of secondary tumor induction.

Keywords: humanoid phantoms; out-of-field doses; radiobiological response; risk modeling.

FIGURE 1

Design (a) and implementation (b) of quasi‐humanoid (Q‐H) phantom for dosimetric and radiobiological measurements. The phantom consists of slices with inhomogeneities simulating those present in human tissues, with gold fiducial markers for setup and inserts (containers) to irradiate cell flasks

FIGURE 2

The out‐of‐field dose measured with dosimetric films (EBT3) in both configurations of phantom: full poly(methyl) methacrylate (PMMA) inserts and during simultaneous irradiation of the flasks in the water container.

FIGURE 3

Water‐filled containers with numerous spacers and stabilizers to permit irradiation of cell flasks/detectors in any required position. The stabilizing bars with attachment hooks enable simultaneous irradiation of multiple flasks containing biological material anywhere in the container. The crossbars prevent the flasks from moving (a). The scheme of flask with simultaneous control through dosimetric films (orange line) with five regions of interest (ROIs), placed under the bottle (the distance between the film and cells is approximately 1 mm) (b)

FIGURE 4

Illustration of quasi‐humanoid (Q‐H) phantom used to determine the effects of low out‐of‐field doses. The image shows a 3D visualization based on computed tomography (CT) scans, with an example of dose distribution for pelvic irradiation and selected measurement points simulating the localizations of organs in the Q‐H phantom

FIGURE 5

An example of the dose distribution planned for the volumetric arc therapy (VMAT) (6 MV flattening filter‐free (FFF)) technique plan for the quasi‐humanoid (Q‐H) phantom in the coronal plane (a) with a comparison of dose profiles measured using Gafchromic EBT‐XD films and the planned distribution in the field (b)

FIGURE 6

An example of the out‐of‐field dose measured (EBT3 film) at distances ≥15 cm from the central axis of the beam (CAX) for the open field (532 Gy to CAX, 6 MV flattening filter‐free (FFF)) for two configurations: poly(methyl) methacrylate (PMMA) insert and water container (a). Measurements for an exemplary dynamic treatment plan with an appropriately scaled dose (22 fractions of volumetric arc therapy (VMAT), 10 Gy per fraction, 6 MV FFF) for a phantom with a water container (b)