In vivo dosimetry using Gafchromic films during pelvic intraoperative electron radiation therapy (IOERT)

Abstract

Objective: To characterize in vivo dose distributions during pelvic intraoperative electron radiation therapy (IOERT) for rectal cancer and to assess the alterations introduced by irregular irradiation surfaces in the presence of bevelled applicators.

Methods: In vivo measurements were performed with Gafchromic films during 32 IOERT procedures. 1 film per procedure was used for the first 20 procedures. The methodology was then optimized for the remaining 12 procedures by using a set of 3 films. Both the average dose and two-dimensional dose distributions for each film were determined. Phantom measurements were performed for comparison.

Results: For flat and concave surfaces, the doses measured in vivo agree with expected values. For concave surfaces with step-like irregularities, measured doses tend to be higher than expected doses. Results obtained with three films per procedure show a large variability along the irradiated surface, with important differences from expected profiles. These results are consistent with the presence of surface hotspots, such as those observed in phantoms in the presence of step-like irregularities, as well as fluid build-up.

Conclusion: Clinical dose distributions in the IOERT of rectal cancer are often different from the references used for prescription. Further studies are necessary to assess the impact of these differences on treatment outcomes. In vivo measurements are important, but need to be accompanied by accurate imaging of positioning and irradiated surfaces.

Advances in knowledge: These results confirm that surface irregularities occur frequently in rectal cancer IOERT and have a measurable effect on the dose distribution.

Figure 1.

Measuring conditions: (a) the setup used to obtain the surface dose distribution for a 7-cm-diameter applicator with a 45° bevel (7B45). The EBT3 film was placed on top of a solid water phantom. (b) EBT3 films wrapped in a plastic envelope for one- and three-point in vivo measurements. (c) The schematic representation of a 7B45 applicator position on a pre-operative CT scan. The film location is schematized and the direction from where the photograph is taken is given by the arrow. (d) Sacral phantom (SP) with three film pieces and the 7B45 applicator positioned in such a way as to leave a step-like surface inside the applicator. (e) CT image of the SP with film locations (arrows) and film positions relative to a hypothetical flat surface and to the reference point (R) for the expected dose on the flat surface.

Figure 2.

Surface dose distributions obtained in phantoms: (a) dose distribution on a flat surface for an 8B45 applicator and 9 MeV (applicator positioning is shown in Figure 1a), and the region of interest (ROI) used to obtain dose profiles. (b) surface dose profiles obtained along this ROI for 6B45, 7B45 and 8B45 applicators and 9 MeV electron beam, and comparison with surface doses measured on the sacral phantom (SP) with 7B45 applicator (Figure 1d). The arrows represent probable film locations during in vivo measurements. (c) Two-dimensional surface dose distributions from the three films placed on the SP.

Figure 3.

Surface types: schematic representation of typical intraoperative electron radiation therapy irradiation surface shapes, identified visually while performing in vivo measurements.

Figure 4.

Three examples of one-point measurement results: Procedure 9: Type 1 surface; Procedure 17: Type 2 surface; and Procedure 16: fluid build-up is visible after irradiation (the upper left photograph was obtained before the irradiation and the lower right photograph after the irradiation). Film positioning relative to the applicator and the surface is shown in the photographs. The corresponding two-dimensional dose distributions (obtained from the films) are presented, but their orientation may differ from film orientation in the photographs (this information was not registered for these procedures).

Figure 5.

Four examples of three-point measurement results, organized by groups: Procedure 3: Group 1; Procedure 7: Group 2; Procedure 4: type Ref. surface—Group 3; and Procedure 8: miscellaneous—Group 3. Film position and orientation in the two-dimensional dose distributions is the same as in the photographs. For bevelled applicators, the longest part of the applicator appears on the left side of the photographs, which corresponds to an inferior (deeper) position than the ones on the right side. This is easily understood by observation of Figure 1c.