A novel approach for superficial intraoperative radiotherapy (IORT) using a 50 kV X‐ray source: a technical and case report

I. INTRODUCTION

The use of intraoperative radiotherapy (IORT) as a treatment modality for patients with close or positive margins has increased over the past decade. It is commonly done with high voltage electrons, ¹ , ² HDR brachytherapy ³ , ⁴ , ⁵ or particularly with low kV X‐ray IORT. ⁶ , ⁷ , ⁸ , ⁹ Low kV sources are of interest because of minimal radiation protection requirements. ¹⁰ , ¹¹ The XRS 4 (INTRABEAM System) has been mainly used for intraoperative tumor bed treatment of breast cancer ⁽⁶⁾ and spinal metastasis ¹² , ¹³ , ¹⁴ using dedicated spherical or needle applicators. Retroperitoneal cancer, such as sarcomas, can also benefit from IORT ⁽¹⁵⁾ because postoperative external beam radiotherapy (EBRT) is limited by the tolerance of the surrounding healthy tissues. ⁽¹⁶⁾ Hu and Harrison ⁽¹⁷⁾ summarize an improvement in local control with an IORT to the flat tumor bed using HDR brachytherapy with a Harrison‐Anderson‐Mick (HAM) applicator or high‐energy electron treatment with an electron cone. Similar effects are discussed by Dubois et al. ⁽¹⁸⁾ who performed IORT of locally advanced or recurrent rectal cancer with high‐energy electrons to a flat target volume with a diameter of 5‐10 cm and a thickness of 1‐5 cm. However, using HDR brachytherapy or high‐energy electrons requires substantial radiation protection measure, such as a specially shielded operating room. The patient has to be transported to or operated in that OR, and the staff is not allowed to stay in the room during radiotherapy. Alternatively, low kV X‐rays can be used, as described in Guo et al. ⁽¹⁹⁾ They described comparable rates of toxicity, local recurrence, and survival rates treating locally advanced or recurrent rectal cancer with the INTRABEAM system. They treated areas of concern for a close or involved radial margin of initial tumor sizes of 0‐10 cm. To get an almost flat dose distribution, they chose mostly a 5 cm spherical applicator (prescription of 5 Gy to a depth of 1 cm). Small lead sheets were used to protect the surrounding tissue.

Those studies demonstrated that IORT is beneficial for many treatment sites, but an applicator is needed to treat flat areas. Thus new applicators for superficial treatment with the INTRABEAM system were developed. Here we report our evaluation of the dosimetric characteristics, such as homogeneity of the dose distribution and the depth dose, of these new applicators and their first clinical use.

II. MATERIALS AND METHODS

III. RESULTS

IV. DISCUSSION

Film dosimetry is a common tool in evaluation of dose distributions in radiotherapy. ²⁵ , ²⁶ Contrary to many advantages of using GAFCHROMIC films (such as it may be handled in visible light, there is no variation because of wet processor performance, and better tissue equivalence than radiographic film), one disadvantage is the nonlinearity of pixel value (grey value) to dose. ²⁷ , ²⁸ , ²⁹ In order to evaluate the dose homogeneity, we had to convert pixel values into absolute dose. This calibration curve, developed previously, ⁽¹¹⁾ was based on absolute dose measurements using a soft X‐ray ionization chamber (Type 23342, Physikalisch‐Technische Werkstaetten), which has a bigger sensitive volume (0.02 cm³) than the chamber used in that study to measure the depth dose (0.0053 cm³). This volume difference influences the depth dose up to 30% (depending on applicator and size). This will affect the ratio of pixel value to dose of the calibration which could slightly affect the ratio of center dose to edge dose of the profiles, which would influence the uniformity ratios (max. 4% for the worst uniformity: 4 cm surface applicator in 5 mm depth). However, due to the nonlinearity of pixel value to dose, a relative comparison of the profiles (without absolute calibration) would lead to an underestimation of the uniformity ratio up to 30%.

In this report, we made the assumption that the dose distribution is radial symmetric because of the manufacturing tolerances of the applicator (main body and flattening filter), which allows a reproducible position of source to applicator. However, the source itself shows a deviation of $<3\%$ in the radial plane, which will also slightly influence the uniformity values estimated based on profile measurements.

The absolute dosimetry with ionization chambers depends on the beam quality/the energy spectrum of the system. With decreasing the flattening filter thickness, the mean energy of the system is also decreasing. This was shown by Eaton and Duck ⁽³⁰⁾ who evaluated the half value layers (HVLs) for the INTRABEAM system with different spherical applicators, which are made of the same material as the flattening filters. They evaluated HVLs of 0.85‐1.3 mm Al and compared it to a reference value of 0.64 mm Al, as measured by Zeiss using a 1 cm solid water buildup. For the surface applicators, the filters are even thinner, which further decreases the HVL/mean energy. However, this variance in mean energy results in a maximum difference in the beam quality factor of 3.6%, which is minor compared to the variances caused by geometrical misplacements (about $10\%–40\%\hspace{0.17em}{\text{mm}}^{-1}$ depending on applicator size and treatment depth).

The flat applicators each demonstrate a unique depth at which the maximum homogeneity was observed. To get a small uniformity ratio ($<1.03$ along the profile) in the same depth (e.g., 5 mm) for all applicators, the flattening filter must be changed. But this would also influence the uniformity at the surface of the applicator. For example, a 4 cm flat applicator actually shows the minimum uniformity ratio in the dhmax of 3 mm depth. The edge dose in 5 mm depth is 13% lower, while the edge dose in 1 mm depth is 31% higher than the central axis dose (see Fig. 6). If the flattening filter would be changed to have a minimum uniformity in 5 mm depth, the edge dose at the surface of the applicator would be even larger.

A challenge is to evaluate the absolute dose and the uniformity at the surface of the applicators. If a film is used as shown in this report, the artifacts based on the scanner and the resolution do not make it possible to evaluate the first millimeter of the film. Even if the film is positioned horizontally (parallel with the applicator surface), the minimum distance from the surface of the applicator to the active layer of the film is 0.2 mm. If a soft X‐ray ionization chamber is used to measure the absolute dose and the homogeneity in a water tank, a solid water holder has to be used which inhibits measurements close to the surface. However, to evaluate only the absolute dose, the chamber can be used in a solid water phantom (sensitive volume in plane with the phantom surface), ⁽²⁰⁾ such as Gammex RMI457 which has the best water equivalency using low kilovolt X‐rays. ⁽³¹⁾ Further research is needed to investigate the uniformity at the surface of the applicators, perhaps by Monte Carlo simulations, as shown by Clausen et al. ⁽³²⁾ and Nwankwo et al. ⁽³³⁾

V. CONCLUSIONS

Many previous studies (e.g., Guo et al. ⁽¹⁹⁾ ), described that a superficial kV IORT is beneficial and needed for many treatment sites. The present work shows that the INTRABEAM system using the newly developed flat or surface applicators can be used for those paradigms. The reported uniformities and dose distributions were judged to be clinically acceptable for the case described. The provided data, which helps the clinician and medical physicist to choose the approach for the individual patient, and the additional shielding effect of the applicator's main body, provides new application possibilities for use of the INTRABEAM system.

Supporting information