Does the apex optimization line matter for single‐channel vaginal cylinder brachytherapy planning?

Abstract The objective of this study is to test the impact of the use of the apex optimization line for new vaginal cylinder (VC) applicators. New single channel VC applicators (Varian) that have different top thicknesses but the same diameters as the old VC applicators (2.0 cm diameter, 2.3, 2.6, 3.0, and 3.5 cm) were compared using phantom studies. Old VC applicator plans without the apex optimization line were also compared to the plans with an apex optimization line. The apex doses were monitored at 5 mm depth doses (eight points) where a prescription dose (Rx) of 6 Gy was prescribed. VC surface doses (eight points) were also analyzed. The new VC applicator plans without apex optimization line presented significantly lower 5‐mm depth doses over the Rx (on average −31 ± 7%, P < 0.00001) due to thicker VC tops (3.4 ± 1.1 mm thicker with the range of 1.2–4.4 mm) than the old VC applicators. Old VC applicator plans also showed a statistically significant reduction (P < 0.00001) due to the Ir‐192 source anisotropic effect at the apex region, but the percent reduction over the Rx was only −7 ± 9%. However, by adding the apex optimization line to the new VC applicator plans, the plans improved 5‐mm depth doses (−7 ± 9% over Rx) that were not statistically different from old VC applicator plans (P = 0.923), along with apex VC surface doses (−22 ± 10% over old VC vs −46 ± 7% without using apex optimization line). The use of the apex optimization line is important in order to avoid significant additional cold doses (−24 ± 2%) at the prescription depth (5 mm) of the apex, specifically for the new VC applicators that have thicker tops. A template‐based vaginal cylinder planning reduced the intra‐ and inter‐planner variations of manual generation of apex optimization line, along with treatment time.


| INTRODUCTION
Endometrial cancer is the most common gynecologic cancer in the United States and worldwide. In 2015, an estimated 54,870 women were diagnosed with endometrial cancer resulting in an estimated 12,900 deaths in the United States alone. 1 After hysterectomy and bilateral salpingo-oophorectomy, the vagina is the most frequent site of recurrence for endometrial cancer. 2 Post-operative vaginal brachytherapy (BT) without external beam radiotherapy (EBRT) was found to be as effective as EBRT by ensuring vaginal control with few gastrointestinal toxic effects when treating high to intermediate risk endometrial cancer. 2 The estimated 5-year recurrence rates after treatment with either vaginal BT or EBRT 2,3 were similar (1.8% and 1.6%) and showed no significant difference in 5-year locoregional recurrence and distant metastases rates. The American Brachytherapy Society (ABS) survey reported that vaginal BT is a common recommendation for post-operative adjuvant therapy for endometrial cancer. 3 Following surgery, the vaginal canal for most patients is roughly cylindrical, and the ABS recommends a properly sized, single-channel vaginal cylinder applicator (VC) for BT treatment. 4 The VC is the most common applicator used for high-dose-rate (HDR) BT 3 and is ideal for patients with a narrow vagina. 4 The region including the vaginal cuff accounts for about 75% of recurrences in endometrial cancer patients. 2,4,5 It is important to generate a radiation dose distribution that best conforms to the vaginal cuff region through optimization during treatment planning. The most recent ABS recommendations (released in 2012), define optimization as the manipulation of the HDR BT dwell positions, dwell times, or both. 4 The ABS recommends using an optimization line at the upper apex or vaginal cuff as well as the lateral areas in order to avoid unacceptably high doses to the vaginal apex and any overlying portion of the small bowel. 4,5 Delivering a radiation dose to the vaginal cuff that receives the prescription dose (Rx) as much as possible is desirable. At least its considerably cold doses should be avoided during planning procedure as the risk of recurrence at the vaginal cuff is greater than 70%. 2,4,5 Our institution had not previously applied the practice of using an apex optimization line, and consequently significant under-doses to the apex region were observed by a physician on a VC HDR plan. This study demonstrates how a new VC applicator, from the same vendor with the same diameter, can cause significant under doses at the vaginal cuff where the risk of recurrence is greatest. Additionally, we introduce a commissioning process using template-based VC planning in order to avoid errors incurred during the manual generation of the apex optimization lines.

2.A | New and previously used VC applicators
This planning study compares vaginal cuff doses using plans generated from both a new and discontinued VC applicator. All new and discontinued VC applicators used in our clinic are single channel applicators from the same vendor (Varian Medical Systems, Inc., Palo Alto, CA).
New VC applicators have the same diameter as the discontinued applicators but have a different top thickness, (see summary in Table 1). The top thickness values of both VC applicators in Table 1 are  prescribed to a depth of 5 mm from the VC applicator surface.
Active dwell-position lengths are typically 5 cm. All cases in this phantom study have a 5-cm active dwell-position length. As a matter of routine in our VC HDR workflow, a single treatment plan is generated after taking a computerized tomography (CT) scan after a VC insertion. This single plan is used for three distinct VC HDR deliver- As the apex 5-mm depth doses are dependent on the VC size and not on patient anatomy, this study was performed as a phantom planning study.

2.C | Commissioning a template-based VC plan
Because the apex optimization line requires the manual generation of eight points at a 5-mm depth in the apex region, inter-and even intra-user variations are inevitable. To avoid any user-induced variation, a template-based VC plan was commissioned using CT or x-ray images as the primary treatment planning dataset. Backup x-ray image-based VC planning templates were generated in case of CT scanner malfunction. Each new VC applicator was originally designed to be used with a flexible plastic probe with a tip thickness of is used, in accordance with ABS guidelines. 4  Single-channel VC applicators do have limitations such as the difficulty of sculpting dose away from the OARs. Due to its radially symmetrical dose distribution, the single-channel method offers fewer possibilities to shape the isodose lines. To generate conformal isodose lines, a multi-channel applicator has been developed 19 that, unlike its single-channel counterpart, is able to significantly reduce dose to OARs while optimizing target coverage. 19 A study using a 13 channel Capri applicator (Varian Medical Systems, Inc.), showed similar target coverage to the VC applicator. 19 However, the Capri applicator significantly decreased dosage to OARs (P < 0.00011) while optimizing target coverage. 19 The additional channels at the periphery of the applicator may allow better dosimetry and reduce the unnecessary dosage to the bladder and rectum compared with a single-channel applicator. 19 For institutions using multi-channel applicator-based planning, it is still essential that a prescribed dosage is properly delivered to the apex region.

| CONCLUSIONS
The use of apex optimization lines in treatment planning is important in order to avoid significant additional cold doses (À24 AE 2%) at the prescription depth (5 mm) of the vaginal apex, specifically for the new VC applicators that have thicker tops. A template-based vaginal cylinder planning method reduced the intra-and inter-planner variations inherent with the manual generation of the apex optimization line as well as reducing the treatment planning time.

CONFLI CT OF INTEREST
No conflict of interest.