Midline shield for radiation therapy of carcinoma of the uterine cervix: should it be "midline" or "individualized".

ABSTRACT

This study attempts to evaluate the advantage of individualized midline shield (IMLS) constructed on the basis of uterine geometry and applicator position in terms of the dosimetric consequences to points A-right (AR) and left (AL) as compared to standard midline shield (SMLS) in radiation therapy of carcinoma cervix. Twenty consecutive patients of carcinoma cervix (Stage I, II and III) were treated by external beam radiotherapy (EBRT) (50 Gy/5 weeks/25 fractions) and high-dose rate intracavitary brachytherapy (24 Gy/4 weeks/4 fractions) prescribed at point A. At the completion of 40 Gy by EBRT (phase I), IMLS (5 cm wide) at isocenter was fabricated on the basis of uterine geometry as ascertained by a dummy intracavitary application. The remaining 10 Gy of EBRT was delivered using IMLS (phase II) to effectively minimize and optimize the dose to point A. The dose profiles of IMLS were compared against the corresponding dose profile of a 5 cm SMLS and were found to be dependent on the positional variation of AR and AL with respect to the midline. With IMLS, the dose to AR and AL for the 10 Gy of phase II varied between 21.7-38.87% (30.01 +/- 4.87) and 22.42-35.72% (28.12 +/- 3.79) respectively. However with SMLS, the AR and AL doses would have ranged from 22.03% to 77.26% (34.55 +/- 15.94) for AR and from 20.59% to 96.2% (46.93 +/- 28.15) for AL leading to considerable inhomogeneity. Thus, in protocols incorporating midline shield for radiotherapy of carcinoma cervix, IMLS in place of SMLS could be preferred for achieving a definitive and homogeneous dose to the points AR and AL.