Intraoperative Radiotherapy With INTRABEAM: Technical and Dosimetric Considerations

Abstract

Purpose: We evaluate dose characteristics and clinical applications of treatment accessories used in intraoperative radiotherapy (IORT) and make site-specific recommendations for their optimal use.

Methods and materials: Dose measurements were performed for a low energy (50 kV) X-ray INTRABEAM source. For spherical, flat, surface, and needle applicators, the following dosimetric parameters were measured: depth-dose (DD) profiles, surface dose (Ds), output factors (OF), and target dose homogeneity (DH). Optical density versus exposure calibration films were employed to obtain 2-dimensional dose distributions in planes parallel and perpendicular to beam direction. Film results were verified via repeat dose measurements with a parallel-plate ionization chamber in a custom water tank. The impact of applicator design on dose distributions was evaluated.

Results: Spherical applicators are commonly used for treating the inner-surface of breast lumpectomy cavity. Flat and surface applicators provide uniform planar dose for head and neck, abdomen, and pelvis targets. Needle applicators are designed for kypho-IORT of spinal metastasis. Typically, larger applicators produce a more homogeneous target dose region with lower surface dose, but require longer treatment times. For 4-cm diameter spherical, flat, and surface applicators, dose rates (DR) at their respective prescription points were found to be: 0.8, 0.3, and 2.2 Gy/min, respectively. The DR for a needle applicator was 7.04 Gy/min at 5 mm distance from the applicator surface. Overall, film results were in excellent agreement with ion-chamber data.

Conclusion: IORT may be delivered with a variety of site-specific applicators. Appropriate applicator use is paramount for safe, effective, and efficient IORT delivery. Results of this study should help clinicians assure optimized target dose coverage and reduced normal tissue exposure.

Keywords: dosimetry; flat; intrabeam; spherical; surface applicators.

Figures

Figure 1

Intrabeam stand and X-ray source shown with spherical applicator attachment. The floor stand provides full flexibility of movement for the X-ray tube with millimeter precision in six dimensions. The floor stand weighs 275 kg and has dimensions of 74 cm × 194 cm × 150 cm (W × H × L) in transport position. Also shown are internal components of the X-ray source (dimensions: 7 cm × 11 cm × 17 cm; weight: 1.6 kg).

Figure 2

Measurement setup for dose output factor, OF and depth dose profiles, DD. (A) Water tank showing positions of ion chamber and X-ray source; (B) PTW34013A thin-window parallel plate ion chamber used for dose measurements; (C) a schematic diagram of the parallel plate ion chamber showing beam entrance window (thickness = 0.22 mm).

Figure 3

Determination of film sensitometric (H&D) curve. (A) Measurement setup showing X-ray source, phantom slabs, and EBT3 films; (B) a batch of EBT3 films irradiated in the dose range: 0–400 cGy; (C) H&D curve showing variation of film optical density with dose.

Figure 4

Dose distribution produced by a 4 cm diameter spherical applicator. Doses are shown at the applicator surface and at 5, 10, and 20 mm distance from it. Dose homogeneity was evaluated in a 1 cm thick spherical shell surrounding the applicator. Applicator shown in inset.

Figure 5

Spherical applicator depth-dose (DD) profiles for a range of diameters (3–5 cm) plotted as a function of the distance from the X-ray source. Note that each depth dose profile begins at the applicator surface, for example, DD profile for the 3 cm applicator begins at 1.5 cm from the X-ray source, etc.

Figure 6

Dose distribution produced by a 4 cm flat applicator. Dose is prescribed at 5 mm depth in phantom. Also indicated is 5 mm superficial layer used to evaluate dose homogeneity. Applicator shown in inset.

Figure 7

Flat applicator depth-dose (DD) profiles for a range of diameters (1–6 cm) as measured with an ion chamber. Larger applicators are characterized by superior dose homogeneity, lower surface dose, smaller output factor, and longer treatment times. Notice a lack of measured data at shallow depths (

Figure 8

Film vs. ion chamber depth dose comparison for a 4 cm diameter flat applicator.

Figure 9

Dose distribution from a 4 cm surface applicator. Dose is prescribed at the surface of the phantom. Also indicated is 5 mm superficial layer used to evaluate dose homogeneity. Applicator shown in inset.

Figure 10

Surface applicator depth-dose (DD) profiles for a range of diameters (1–4 cm). Larger applicators are characterized by superior dose homogeneity, smaller output factor, and longer treatment times. Notice a lack of measured data at shallow depths (

Figure 11

Film vs. ion chamber depth dose comparison for a 4 cm diameter surface applicator. Discrepancy between film and ion chamber data could be related to X-ray spectral changes resulting from steep dose gradient associated with surface applicators.

Figure 12

Dose distribution from a needle applicator. Dose is prescribed at 5 mm from the applicator surface or 8 mm from the source. Needle applicator shown in inset.

Figure 13

Depth-dose (DD) profiles for the Needle applicator (diameter 4.4 mm). Inset figure shows a prescribed dose of 8 Gy delivered at 8 mm from the source.

Figure 14

Depth-dose profile comparison for 4 cm diameter flat vs. surface applicators. Notice the steep dose-fall of with depth for the surface applicator resulting in inferior dose homogeneity, larger output factor, and shorter treatment times.