Assessment of target dose delivery in anal cancer using in vivo thermoluminescent dosimetry

Purpose: To measure anal dose during external beam radiotherapy (EBRT) usin g in vivo dosimetry, to study the difference of measured from prescribed do se values, and to evaluate possible associations of such differences with a cute and late skin/mucosal toxicity and anorectal function. Materials: Thirty-one patients with localized anal carcinoma underwent in v ivo measurements during the first EBRT session. Themoluminescent dosimeters (TLD) were placed at the center of the anal verge according to a localizat ion protocol. No bolus was used. Patients received a median dose of 39.6 Gy (range: 36-45 Gy) by anteroposterior opposed AP/PA pelvic fields with 6 or 18 MV photons, followed by a median boost dose of 29 Gy (range: 13-24 Gy). Concomitant chemotherapy (CCT), consisting of 1-2 cycles of continuous inf usion 5-fluorouracil (5-FU) and bolus mitomycin-C (MMC), was usually admini stered during the first weeks of the pelvic and boost EBRT courses. Acute a nd late skin/mucosal reactions were recorded according to the Radiation The rapy Oncology Group (RTOG) toxicity scale. Anal sphincter function was asse ssed using the Memorial Sloan Kettering Cancer Center (MSKCC) scale. Results: TLD anal doses differed by a mean of 5.8% (SD: 5.8) in comparison to the central axis prescribed dose. Differences of at least 10% and at lea st 15% were observed in eight (26%) and three (9.7%) patients, respectively . TLD doses did not significantly correlate with acute or late grade 2-3 sk in or mucosal toxicity. However, patients having good-fair MSKCC anal funct ion had a significantly,greater mean difference in anal TLD dose (10.5%, SD : 5.9) than patients having excellent function (3.8%, SD: 4.6) (P = 0.004). Prescribed dose values, length of follow-up, and age at diagnosis did not correlate with late sphincter function. Conclusions: These data show that AP/PA fields using megavoltage photons de liver adequate dose to the anal verge. However, in about one quarter of pat ients treated with this technique the anal dose varied from the prescribed dose by at least 10%. The observed correlation of TLD values and late sphin cter function suggests that direct measurement of the dose delivered to the anal verge might be clinically relevant. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.