Clinical Investigations
Spatial Reproducibility of the Ring and Tandem High-Dose Rate Cervix Applicator

https://doi.org/10.1016/S0360-3016(98)00026-1Get rights and content

Abstract

Purpose: High-dose rate (HDR) applicators for uterine cervix brachytherapy are used with increasing frequency. Because multiple HDR fractions are required for treatment, the applicator position reproducibility is of most importance. To study this effect, the clinical data from patients with uterine cervix cancer were examined retrospectively to evaluate the interfraction geometric variation of the HDR applicator and its potential treatment impact.

Methods and Materials: Eighteen patients with invasive cervical cancer who were treated with definitive radiotherapy at William Beaumont Hospital were included in the study. Patients were treated with 45–50.4 Gy megavoltage external beam to the pelvis, and 35 Gy to the prescription point A from 7 fractions of HDR brachytherapy. The 3-dimensional (3D) interfraction geometrical variation of the ring and tandem (R & T) applicator was measured using predefined reference points in the 7 sets of orthogonal simulation films obtained prior to each HDR application. Spatial reproducibility of the R & T insertion and time-trend of the R & T position variation related to patient’s anatomy during the treatment course were analyzed with respect to different groups of patients who had either early or advanced disease.

Results: The translational variation of the applicator position for all patients was 6.5, 5.9, and 7.7 mm (one standard deviation), respectively, in the patient’s superior-to-inferior (SI), right-to-left lateral (RL), and anterior-to-posterior (AP) direction. The rotational variation was 3.4, 4.6, and 6.0 degrees (one standard deviation) in the patient’s coronal, transverse, and sagital planes. When the patients were grouped based on early disease or advanced disease, the latter demonstrated substantially larger variation (factor of 2) in the applicator position than the former. Furthermore, the time-trend of position variation was observable for both groups of patients. The variations occurred primarily during the first 3 fractions.

Conclusions: Based on the good spatial reproducibility observed in our study, the current clinical procedure for the HDR R & T applicator placement is reliable. Positional reproducibility of the R & T applicator is highly dependent upon the size of tumor volume, which, in turn, deviates the applicator during the early course of HDR brachytherapy. Attention to the construction of the midline block is of paramount importance.

Introduction

For most patients with invasive cervical cancer, treated primarily by radiotherapy, gynecologic brachytherapy remains an integral component of the treatment program. To deliver a high cumulative dose to the primary tumor while sparing adjacent healthy tissues, the proper placement of the intracavitary applicator is paramount to the success of the treatment. Using continuous low-dose rate (LDR) gynecologic brachytherapy, one of the most significant drawbacks is the displacement and motion of the applicator due to the prolonged treatment time and the lack of rigid applicator fixation. Ljunggren et al. [1]found significant motion of the applicator relative to the pelvis or bladder in terms of rotation and/or translation (+10 degrees and +8 mm, respectively). This degree of motion was found in 87.5% of their studied patients. Others have shown that there is a substantial interfraction motion of the reference points as defined by ICRU #38 [2]relative to the bony pelvis between the first and second intracavitary applications. This movement results in a variation in the dose to the prescription point from −33% to +35% [3].

High-dose rate (HDR) brachytheraphy for gynecologic cancer requires the use of multiple fractions (4 to 12 applications). Therefore, the geometrical reproducibility of the applicator placement becomes essential. Studies have shown that there are considerable geometric variations between multiple applications in the Fletcher–Suit HDR systems 4, 5. These variations were mainly associated to the tissue relaxation due to the use of general anesthesia prior to each fraction and the actual packing, which could be operator-dependent. Therefore, the exclusion of anesthesia and packing might influence the reproducibility of HDR applicator placement. To verify this conjecture, a retrospective study was designed to measure the interfraction placement variation of the Nucletron HDR ring and tandem (R & T) applicator (Nucletron B. V. Corporation, Veenendaal, The Netherlands) used at William Beaumont Hospital for patients with invasive cervical cancer. Current computerized 3D graphics technology allowed us to measure the interfraction geometric variation of the ring and tandem applicator and potential effects on dose delivered. This is the first report measuring spatial applicator geometric variation based on bony anatomy and its impact on dose delivered using the 3D methodology for the ring and tandem HDR applicator.

Section snippets

Methods and Materials

Since 1991, we have switched from LDR gynecologic brachytherapy to HDR brachytherapy for our patients with invasive cancer of the cervix. The linear quadratic formulation [6]was utilized to convert a standard LDR treatment, with a total dose of 50 Gy divided in 2 fractions 2 weeks apart, to the HDR treatment, with a total dose of 35 Gy delivered by 7 fractions within 2 weeks. Patients were first treated with 10 or 18 MV external beam using a 4-field pelvis technique with a daily dose of 1.8 Gy

Results

From 18 consecutive patients, 119 orthogonal pairs of films were studied. Among them, 15 pairs of films were not included in the analysis due to impaired quality. Therefore, 104 pairs of films were evaluated to measure the geometrical variation in the R & T applicator placement. Of the 18 patients, 14 were treated as per protocol with 7 fractions of brachytherapy, and 4 patients were treated with 4, 4, 5, and 8 fractions, respectively. The specifications R & T applicators used during the

Discussion

No information is available in the literature regarding spatial reproducibility of the HDR applicator position when measured with regard to bony anatomy. In addition, the paucity of reports dealing with applicator position shifting for multifraction HDR treatments using different measurement methodologies makes comparison of our data with others rather difficult. Results from the present study reveal that interfraction spatial variation of the R & T applicator placement occurs in the process of

Acknowledgements

The authors thank Vicky Dykes for secretarial assistance in the preparation of this manuscript. Also, we thank Dr. John Wong for the scientific input and support of this paper.

References (14)

There are more references available in the full text version of this article.

Cited by (48)

  • Quantitative CT assessment of a novel direction-modulated brachytherapy tandem applicator

    2018, Brachytherapy
    Citation Excerpt :

    Numerous applicator systems are available, which can be loaded with radioactive sources and used for cervical cancer intracavitary brachytherapy. They are typically made of plastic, stainless steel, or titanium (5–16). Visualization of the applicator and the treatment region has been traditionally accomplished using anterior–posterior and lateral plain-film X-ray images.

View all citing articles on Scopus
View full text