Jongmyung Kim MD, PhD, Presenter: Nothing to Disclose

Stephen Sozio, Abstract Co-Author: Nothing to Disclose

Venkat R. Narra PhD, Abstract Co-Author: Nothing to Disclose

Rihan Davis DMD, Abstract Co-Author: Nothing to Disclose

Madeera Kathpal DO, MA, Abstract Co-Author: Nothing to Disclose

Atif Jalees Khan MD, Abstract Co-Author: Nothing to Disclose

Ning Jeff Yue PhD, Abstract Co-Author: Nothing to Disclose

Sung Kim MD, Abstract Co-Author: Nothing to Disclose

Purpose/Objectives:Craniospinal Irradiation [CSI] typically involves 2 matches: upper lateral fields matched to an upper PA spine field, and upper spine field matched to a lower spine field. Historically, the spine fields are matched at the skin (conventional). Another method uses a couch kick and appropriate gantry rotation in the lower spine field to develop a perfect match with the upper spinal field divergence (couch kick). IMRT may also be used. Our hypothesis was that the downward angulation of the inferior spine field associated with the couch kick technique might increase the equivalent dose (and theoretical risk of secondary malignancy) to pelvic structures, and that the opposite was true for IMRT. This study compares the risk of secondary malignancy from a pelvic organ (bladder) associated with the couch kick and IMRT techniques compared to conventional.Materials/Methods:Four patients had three CSI plans (conventional, couch kick, IMRT) made with a prescription of 54 Gy to the posterior fossa and 36 Gy to the spine. Mean dose to bladder was recorded for each technique, as well as the minimum dose absorbed by the most highly radiated 2 cc of bladder (for a more conservative estimate). The equivalent dose to the bladder was determined by multiplying the mean dose by 1 Sv/Gy (radiation weighting factor for photons). Lifetime risk of fatal secondary malignancy from bladder was calculated by multiplying equivalent dose by 0.3 %/Sv (NCRP report 116 coefficient for probability of experiencing fatal cancer from the bladder for a whole population). To compare the risk of secondary malignancy from bladder using couch kick and IMRT compared to conventional techniques, the ratio of excess relative risk [RRR] was used. RRRcouch kick was defined as ERRcouch kick / ERRconventional where ERR (excess relative risk) is the risk of fatal secondary malignancy from bladder as calculated above.Results:The risk of secondary malignancy from bladder was increased by couch kick (RRRs were 3.98, 2.32, 0.95, 1.17) and decreased by IMRT (RRRs were 0.92, 0.84, 2.70, 0.50) compared to conventional CSI techniques. The results using minimum dose to most highly radiated 2cc of bladder was consistent with the results using mean dose.Conclusions:This study demonstrates that the couch kick technique increases dose (and theoretical risk of fatal secondary malignancy) to the bladder, compared with conventional CSI. Though it has the advantage of a perfect match at the lower junction, the downward angulation may increase dose to pelvic structures. Conversely, IMRT lowers the dose at the bladder in most cases. It should be noted that the absolute difference was in some cases minimal. The potential advantages/disadvantages of these CSI techniques should of course be tempered by factors such as total MU output and overall safety/reproducibility.

Kim, J, Sozio, S, Narra, V, Davis, R, Kathpal, M, Khan, A, Yue, N, Kim, S, Risk of Fatal Secondary Malignancies from the Bladder using 3 different CSI Techniques.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14042405.html