P05.04 Dosimetrical consequences of unilateral (left) hippocampus sparing WBRT
Authors: T. Kazda, M. Vrzal, T. Prochazka, P. Pospisil, T. Nikl, P. Dvoracek, A. Dziacky, P. Slampa, R. Jancalek
Journal: Neuro-Oncology (2017)

Introduction: Hippocampal (Hipp) sparing during whole brain radiotherapy (WBRT), in an effort to preserve cognition, is widely investigated radiotherapy (RT) technique for patients with brain metastases unsuitable for local treatment. Some reports suggest different post RT changes in the left or right Hipp. The most important neurocognitive domain affected by WBRT is mainly verbal memory, which formation seems to be more affected by RT injury of left (dominant) Hipp. Though still only at hypothetical level, unilateral Hipp sparing may further minimize side effects of WBRT. Dosimetrical consequences of potential unilateral (left) Hipp sparing are not known. We hypothesize that with only left Hipp avoiding (LHA) WBRT, it is possible to further decrease dose within spared Hipp and/or to improve dose coverage of the remaining brain. The aim of this retrospective planning study is to compare both HA WBRT plans with LHA WBRT ones. METHODS: Ten consecutive patients with brain metastases referred for RT in our institution and with available planning CT and MRI scans were eligible. Experimental RT plans for bilateral Hipp sparing WBRT were created in accordance to RTOG 0933 (phase II trial assessing bilat HA WBRT) study protocol criteria (online contouring atlas of Hipp; 90 % of whole brain planning target volume (PTV) receiving 10 x 3 Gy; PTV_D2 % ≤ 37.5 Gy, PTV_D98 % ≤ 25 Gy; Dmax of optic nerves or optic chiasm ≤ 37.5 Gy, Hipp D100 % ≤ 9 Gy and Hipp Dmax ≤ 16 Gy). These RT plans served as baseline for comparisons. Experimental plans with sparing of only left Hipp were prepared: 1) with the effort to maintain the same left Hipp dosimetry as in the baseline plan (“BEST PTV”) and 2) to maintain same dosimetry in PTV as in the baseline plan (“BEST left Hipp”). Dosimetrical characteristics were compared on the case by case basis. Results: Bilat HA WBRT plans met RTOG 0933 protocol criteria with mean Conformity index 1.09 and mean Homogeneity index 0.21. Mean right and left Hipp D100 % was 7.8 Gy and 8.5 Gy and mean Dmax 14.0 Gy and 13.8 Gy, respectively. For “BEST PTV” plan, with the same left Hipp dosimetry, PTV_D2 % decreased by 2.5 %, PTV_D98 % increased by 5 %, homogeneity index decreased by 32 % and optic pathway Dmax decreased by 1 Gy. For “BEST left Hipp” plan, left Hipp D100 % decreased by 12 % and Dmax by 11 %. DISCUSSION: “BEST PTV” plan describes improvement of PTV coverage while meeting RTOG 0933 criteria for left Hipp. Plan labeled as “BEST left Hipp” describes lowering of left Hipp dose while meeting RTOG 0933 criteria for PTV. Patients, who had lower Hipp D100 % in previous clinical trials performed significantly better in certain memory tests. Therefore, decreasing hippocampal dose to as low level as possible is clinically relevant. CONCLUSION: Unilateral Hipp sparing during WBRT could improve target coverage and/or further decrease dose at spared Hipp. Clinical relevance of suggested planning study remains to be determined.