Faculty Bibliography

Purpose: Recurrent or primary vaginal disease located in the rectovaginal septum presents a challenging clinical location for interstitial brachytherapy given reported complications of rectovaginal fistulas posttreatment. This study evaluates the dosimetry of conformal boost to persistent gross recurrent disease in the vagina with a non-invasive, balloon based, multi-channel (MC) CapriTM vaginal brachytherapy applicator in patients with rectovaginal gynecologic cancer following whole pelvis radiation. Materials and Methods: All patients had fiducial markers placed at the site of gross disease, and were treated with whole pelvis radiation to a dose of 45 Gy in 25 fractions. Following whole pelvis radiation, patients were treated with Ir-192 HDR vaginal brachytherapy. Patients underwent sizing using the balloon-based, MC CapriTM applicator (Varian Medical Systems, Palo Alto, CA, USA) expanded to maximum capacity with saline to a median maximum horizontal cross sectional diameter of 4.50 cm. A CT scan was performed with the applicator in place. Target volume was defined as the gross tumor volume delineated by fiducial markers and expanded by 3 mm to create the planning target volume (PTV). Using Brachytherapy Planning in the Eclipse treatment planning system (Varian, Palo Alto, CA, USA), dosimetry of the conformal boost plan was manually optimized to achieve 100% coverage of PTV coverage using a prescription of 4 Gy per fraction using the minimum number of catheters and dwell positions closest to the target volume. Maximum dose to 2 cm³ (D2cm³) to bladder and rectum, as well as vaginal mucosal surface points ipsilateral and contralateral to PTV 400 cGy and biologically equivalent dose in 2Gy equivalents (EQD2) for bladder and rectum were calculated using a prescription of 4Gy x 6 fractions with alpha/beta of 3. Results: Five plans were evaluated from 5 patients with vaginal cancer (2) and endometrial cancer (3) with gross vaginal cuff recurrences. Each patient was planned with separate plans representing each individual insertion. The average GTV and PTV taken from the 14 plans was 5.00 cm³ and 6.30 cm³, respectively with a PTV average depth of 1.1 cm. The median number of loaded catheters used in each optimized plan was 3 (range, 2-4 catheters). The average D99, D95 to PTV for all plans was 3.89 Gy and 3.92 Gy, respectively. Average D2cm³ to bladder was 3.02 Gy (range 1.01- 4.36 Gy), and average D2cm³ to rectum was 3.22 Gy (range 2.23-5.08 Gy). The average maximum dose to ipsilateral vaginal mucosa was 12.79 Gy (range 2.04- 33.0 Gy,) while the average maximum dose to the contralateral vaginal mucosa was 9.8 Gy (range 0.99 Gy- 7.46 Gy.) Average EQD2 total for 45 Gy external beam and 6 fractions of 4 Gy for bladder and rectum was 67.5 Gy and 69.5 Gy, respectively. Conclusions: Although ABS Consensus Guidelines for vaginal cancer suggest that lesions with a thickness exceeding 5 mm cannot be adequately treated with a vaginal cylinder, we demonstrate that with the Multichannel Balloon CapriTM applicator, PTV depths averaging 1cm from vaginal surface were able to be prescribed using a multicatheter approach with clinically acceptable bladder, rectal and vaginal doses. Preliminary data suggests that the use of the Multicatheter CapriTM applicator may be an alternative approach to using an interstitial implant for selected patients with small vaginal recurrences located at the rectovaginal septum, however, a larger cohort of patients with long-term follow-up is required to assess long-term clinical outcomes using this technical approach. (Figure Presented)

PURPOSE: Maximum dose to the left anterior descending artery (LADmax) is an important physical constraint to reduce the risk of cardiovascular toxicity. We generated a simple algorithm to guide the positioning of the tangent fields to reliably maintain LADmax <10 Gy. METHODS AND MATERIALS: Dosimetric plans from 146 consecutive women treated prone to the left breast enrolled in prospective protocols of accelerated whole breast radiation therapy, with a concomitant daily boost to the tumor bed (40.5 Gy/15 fraction to the whole breast and 48 Gy to the tumor bed), provided the training set for algorithm development. Scatter plots and correlation coefficients were used to describe the bivariate relationships between LADmax and several parameters: distance from the tumor cavity to the tangent field edge, cavity size, breast separation, field size, and distance from the tangent field. A logistic sigmoid curve was used to model the relationship of LADmax and the distance from the tangent field. Furthermore, we tested this prediction model on a validation data set of 53 consecutive similar patients. RESULTS: A lack of linear relationships between LADmax and distance from cavity to LAD (-0.47), cavity size (-0.18), breast separation (-0.02), or field size (-0.28) was observed. In contrast, distance from the tangent field was highly negatively correlated to LADmax (-0.84) and was used in the models to predict LADmax. From a logistic sigmoid model we selected a cut-point of 2.46 mm (95% confidence interval, 2.19-2.74 mm) greater than which LADmax is <10 Gy (95% confidence interval, 9.30-10.72 Gy) and LADmean is <3.3 Gy. CONCLUSIONS: Placing the edge of the tangents at least 2.5 mm from the closest point of the contoured LAD is likely to assure LADmax is <10 Gy and LADmean is <3.3 Gy in patients treated with prone accelerated breast radiation therapy.