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patients (15 catheters in total). The shift of the catheters related to the clips is visualized in Figure 1, sorted by the day of the additional CT scan. Only shifts within the CTV are considered. On the second day, the maximum shift was 5 mm, while the shifts were much larger on the third and fourth day. On 6 occasions we decided to make a new plan: once due to 200% isodose bridges, four times due to large shifts of the catheters, and once due to both. Conclusions: Position verification of the catheters was found to be necessary to achieve sufficient quality control. The decision was therefore made to extend the length of active dwell positions in the original plan by 4 mm at both sides, in order to compensate for shifts of the catheters on the second day. As a consequence, we decided to omit the second day CT evaluation as we are convinced that the treatment can safely be continued. However, on the third day, a CT scan has to be performed followed by a new plan if necessary. This procedure is now a regular part of each treatment.

PURPOSE To report on the single-catheter high-dose-rate brachytherapy treatment of a 21-month-old girl child with an embryonal, botryoid-type, rhabdomyosarcoma limited to the external auditory canal (EAC). METHODS AND MATERIALS A 2.4-mm diameter catheter was inserted into the right EAC and placed against the tympanic membrane. A computed tomography simulation scan was acquired. A brachytherapy treatment plan, in which 21 Gy in seven fractions was prescribed to a 1-mm depth along the distal 2 cm of the catheter, was generated. Treatments were delivered under anesthesia without complication. A dosimetric comparison between this plan and an intensity-modulated radiation therapy (IMRT) plan was then conducted. A clinical target volume (CTV), which encompassed a 1-mm margin along the distal 2 cm of the catheter, was delineated for both plans. Given positioning uncertainty under image guidance, a planning target volume (PTV = CTV + 3-mm margin) was defined for the IMRT plan. The IMRT plan was optimized for maximal CTV coverage but subsequently normalized to the same CTV volume receiving 100% of the prescription dose (V100) of the brachytherapy plan. RESULTS The IMRT plan was normalized to the brachytherapy CTV V100 of 82.0%. The PTV V100 of this plan was 34.1%. The PTV exhibited dosimetric undercoverage within the middle ear and toward the external ear. Mean cochlea doses for the IMRT and brachytherapy plans were 26.7% and 10.5% of prescription, respectively. CONCLUSIONS For rhabdomyosarcomas limited to the EAC, a standard brachytherapy catheter can deliver a highly conformal radiation plan that can spare the nearby cochlea from excess radiation.