Clinical Investigation
Conformal Brachytherapy Planning for Cervical Cancer Using Transabdominal Ultrasound

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Purpose

To determine if transabdominal ultrasound (US) can be used for conformal brachytherapy in cervical cancer patients.

Materials and Methods

Seventy-one patients with locoregionally advanced cervix cancer treated with chemoradiation and brachytherapy were included in this study. The protocol consisted of US-assisted tandem insertion and conformal US-based planning. Orthogonal films for applicator reconstruction were also taken. A standard plan was modified to suit the US-based volume and treatment was delivered. The patient then underwent a magnetic resonance imaging (MRI) scan with the applicators in situ. Retrospectively, individual standard (STD), US, and MRI plans were extrapolated for five fractions and superimposed onto the two-dimensional sagittal MRI images for comparison. Doses to Point A, target volume, International Commission on Radiation Units and Measurements (ICRU) 38 bladder and rectal points, and individualized bowel points were calculated on original implant geometry on Plato for each planning method.

Results

STD (high-dose-rate) plans reported higher doses to Point A, target volume, ICRU 38 bladder and rectal points, and individualized bowel point compared with US and MRI plans. There was a statistically significant difference between standard plans and image-based plans—STD vs. US, STD vs. MRI, and STD vs. Final—having consistent (p ≤ 0.001) respectively for target volume, Point A, ICRU 38 bladder, and bowel point. US plan assessed on two-dimensional MRI image was comparable for target volume (p = 0.11), rectal point (p = 0.8), and vaginal mucosa (p = 0.19). Local control was 90%. Late bowel morbidity (G3, G4) was <2%.

Conclusions

Transabdominal ultrasound offers an accurate, quick, accessible, and cost-effective method of conformal brachytherapy planning.

Introduction

Brachytherapy has proven efficacy in treating cancer of the cervix (1). Its effectiveness rests on the technical quality of the implant 2, 3. Yet, most cervix brachytherapy is applied without image guidance and only planned with the aid of two-dimensional (2D) X-ray imaging. Computed tomography (CT) is slowly replacing traditional simulators, but often the pear-shaped distribution based on applicator size and standard loadings continues to be used to cover the paracervical triangle 4, 5, 6.

The introduction of image-based planning for brachytherapy has raised questions about the volume and nature of the tissues incorporated into the brachytherapy field. Definition of a target is a new concept in cervix brachytherapy being investigated by the Groupe European de Curietherapie and the European Society for Therapeutic Radiology and Oncology (GEC-ESTRO) group through an international study. The volumes of normal tissue within the radiation field become apparent on imaging and may shed some light on historical toxicity rates and late complications.

Although residual tumor volumes cannot always easily determined by CT or ultrasound (US), these modalities can be used to verify the position of the tandem in the uterus, and to assess dose to surrounding normal structures. Radiotherapy centers are slowly adapting these imaging modalities.

Transabdominal US has been used to assist applicator placement in this institution since 1985. Magnetic resonance imaging (MRI) was introduced in 2001 to assess tumor response to external beam treatment. The next logical step was to assess brachytherapy dosimetry on the MRI scan and develop an image-based planning and treatment protocol. Over time a strong correlation between the size and shape of the uterus and cervix, containing residual tumor, as seen on the US and MRI images, was observed (Fig. 1). It became apparent that US could give an accurate depiction of the applicator within the cervix and uterus, and the cervix and uterus within the pelvis. US was progressively incorporated into the protocol to avoid uterine perforation and to assist with the dosimetric coverage of the cervix, residual tumor, and uterus (7).

This retrospective study aimed to validate conformal US-based planning. In the period under review, 2D planning was the norm. Infrastructure and facilities did not support three-dimensional (3D) planning. In an effort to improve on standard dosimetry-based on 2D X-rays, an imaging modality was sought that could provide sufficient anatomical information to help define a target and ensure target coverage and sparing of surrounding normal tissues. The primary aim was to compare isodose distributions resulting from high-dose-rate (HDR) standard (STD) plans, US-derived plans, and 2D MRI derived plans with the final dosimetry achieved based on the combination of planning methods used in treating patients.

Section snippets

Materials and Methods

This was a retrospective comparative study of 71 patient's treatment plans. This study was approved by the Peter MacCallum Cancer Centre Ethics committee. All patients were newly diagnosed with locally advanced cervix cancer, had pretreatment MRI available, and received chemoradiation and brachytherapy with curative intent between October 2002 and July 2005.

Results

Data of 71 patients were examined in this study. The characteristics of the patients are described in Table 1. The median age of patients was 58 years. The pretreatment tumor volumes were 56 mL (mean), 44 mL (median), with a range of 2–270 mL. Residual tumor volumes at first brachytherapy insertion as assessed on MRI were 15.7 mL (mean), 11.7 mL (median), with range of 0–107 mL.

Discussion

This study suggests that using US imaging in cervix brachytherapy can substantially reduce doses to organs at risk while not compromising dose to the target volume.

Incorporation of soft-tissue imaging into gynecological brachytherapy programs has been developing for many years. Confirmation of the spatial accuracy of anatomy and applicator reconstruction, improved organ visualization, more accurate assessment of dose to organs, and the merits and disadvantages of each modality are well

Conclusion

This study suggests that important improvements can be made to current treatments based on standardized 2D X-ray image-based planning. It has been shown that US can identify an effective target volume. By using 2D US, it is possible to improve technical accuracy, visualize organ boundaries, and, with experience, plan conformal treatments that by definition spare organs at risk. US is easily accessible, portable, and inexpensive. Use of US allows for delivery of safe treatment in a simple

References (17)

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