A QUANTITATIVE ASSESSMENT OF STANDARD VS CUSTOMIZED MIDLINE SHIELD CONSTRUCTION FOR INVASIVE CERVICAL-CARCINOMA

Purpose: An individualized midline shield (MLS) has been advocated for delivering homogeneous radiotherapy for patients with invasive cervic al carcinoma. Yet, many radiation oncologists continue to employ a sta ndard block, In the latter instance, any deviation of the cranial-caud al central axis of the tandem from the patient's midline could result in dose inhomogeneity to tumor., A retrospective review of a single un iveristy medical center's experience with constructing the MLS was ini tiated to determine the outcome of using a standard block vs, a custom ized block that conforms to the ''Point A'' isodose line. In addition, participating radiation oncologists associated with the Gynecologic O ncology Group (GOG) were polled to assess if there exists a consensus regarding midline block utilization in the management of cervical canc er patients which could be compared to the institutional study. Method s and Materials: From January 1, 1990 through December 31, 1992, 32 pa tients with invasive cervical carcinoma who underwent low dose rate br achytherapy at a single institution were identified, Patients were gro uped as having a standard block (18 cases), customized block (5 cases) , or no block (9 cases), The ''Point A'' isodose distribution from the implant was superimposed onto the whole pelvic simulation film and qu antitatively compared to the actual or a hypothetical standard block o utlined on the same radiograph, In September of 1995, 56 member and af filiated institutions in the GOG were surveyed concerning their use of a MLS, and the results were tabulated in December of 1995. Results: A pproximately 72% of all cases (23 out of 32) at the single institution had tandem deviation ranging from 0-230 with a median of 50. This tra nslated into a median percent overdosage to ''Point A'' Right of 15% a nd ''Point A'' Left of 12.5%. Although overall survival and incidence of chronic complications have not been affected by type of shielding, patient follow-up is limited with a median of 17.7 months (range: 4.2- 58.9 months), Of the 56 surveyed radiation facilities in the GOG, 34 ( 61%) responded, One center was subsequently excluded as it performs on ly high dose rate brachytherapy. Of the evaluable respondents, 88% (29 out of 33) utilize a MLS in treating their patients with invasive cer vical carcinoma, Of the latter group, 76% (22 out of 29), 21% (6 out o f 29), and 3% (1 out of 29) employ a standard block, customized block, and a ''step-wedge,'' respectively, For those using a standard block, 77% (17 out of 22) align the central cranial-caudal axis of the MLS a long the corresponding midplane of the patient's pelvis on an anteropo sterior radiograph rather than along the superior-inferior central axi s of the tandem. Conclusions: This study suggests that the use of a st andard midline shield could result in potential tumor dose inhomogenei ty and should be avoided, A national survey of major academic centers further suggests that the majority of these facilities also utilize a rectangular central block that is not positioned with respect to possi ble tandem deviation, Further investigation concerning the techniques of midline shield construction should be considered. Copyright (C) 199 7 Elsevier Science Inc.