International Journal of Radiation Oncology*Biology*Physics
Clinical InvestigationsSpatial Reproducibility of the Ring and Tandem High-Dose Rate Cervix Applicator
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.
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