Medical Dosimetry, Vol. 22. No. 2, pp. 139- 145. 1997 Copyright 0 1997 American Assocmtion of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/97 $17.00 + .OO ELSEVIER PI1 SO958-3947( 96) 00046-5 MIDLINE SHIELD FOR RADIATION THERAPY OF CARCINOMA OF THE UTERINE CERVIX: SHOULD IT BE “MIDLINE” OR “INDIVIDUALIZED”? NILOY RANJAN DATTA, M.D., D.N.B., K. J. MARIA DAS, M.Sc ., S. HALDER, M.D., and S. AYYAGARI, M.D. Department of Radiotherapy, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, India Abstract-This study attempts to evaluate the advantage of individualized midline shield (IMLS) con- structed 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 2 4.87) and 22.42-35.72% (28.12 2 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 2 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. 0 1997 American Association of Medical Dosimetrists Key Words: Cervical cancer, Radiotherapy, Brachytherapy, Midline shield. INTRODUCTION Radical radiation therapy for carcinoma of the uterine cervix involves a judicious combination of external beam therapy and intracavitary brachytherapy. Differ- ent treatment protocols have incorporated various com- binations of teletherapy and brachytherapy in terms of time, dose, fractions and techniques to optimize the dose to the target organ.‘-” Some centers have used a midline shield (MLS) during the entire course or a part of external beam radiotherapy-either to optimize the dose to the prescription point A, shield a part of the rectum and bladder or to deliver an additional boost to the parametrium.‘-*~““5.‘6~‘9~20.22 Such central shields are usually in the form of a 3 -5 cm midline rectangular block, or a step wedged block with the possible advan- tage of matching the pear shaped isodose distribution of the intracavitary application. 14~18720 However, in most of the cases, the shields are usually at the midline, irrespective of the normal uterine orientation which might have a varying degree of 0 angle23 tilt or lateral shift of the external OS from the midline. Although such midline shields would possibly shield a part of Mailing address: Dr. Niloy Ranjan Datta, Dept. of Radiother- apy, Sanjay Gandhi Post Graduate Institute of Medical Sciences. RaeBarelli Road. Lucknow-226014, (U.P.), India. the bladder and the rectum, the dose to the point A- both right (AR) and left (AL) might be inhomogenous depending on the fl angle of the uterus, lateral shift of the OSfrom the midline and the corresponding position of the dose prescription points, i.e., AR and AL. Fabrication of individualized midline shields (IMLS) based on the anatomical orientation of the uterus and the co-ordinates of the AR and AL points may therefore be considered. This study examines the utility of such IMLS over standard midline shield (SMLS) by making a comparative assessment of the variation of dose to points-AR and AL. MATERIAL AND METHODS Twenty consecutive patients of histopathologi- tally proven carcinoma of the uterine cervix were in- cluded in this study. Following the clinical assessment and routine work up, they were all assigned to be treated with radical radiotherapy using teletherapy and intracavitary brachytherapy according to the standard treatment protocol outlined below. While most of the patients were FIG0 stage II ( 13120) or III (6/20), one had stage Ib disease. Radiologically, as per the sonography or CT scan of abdomen and pelvis, all the patients had disease limited to the pelvis. The follow- 139