SU‐F‐BRD‐12: Prostate Cancer Dose Escalation Including Lymph Nodes: A Planning Study

Abstract

Purpose:Dose escalation improves local control for prostate cancer but not overall survival (OS), only systemic therapy such as ADT can improve OS. One explanation is that occult diseases in the lymph nodes (LNs) are either not treated, or treated at inadequate level. This study explores dose escalation using IMRT with reduced margins.Methods:14 previously treated patients were selected. The CTV1 includes prostate and seminal vesicles, and CTV2 includes pelvic LNs. OARs include rectum, bladder, small bowel, femoral heads, and penile bulb. Reference plan (RP) was designed to follow RTOG‐0521, with 10 mm margin for PTVs. In phase I, both PTVs are prescribed to 1.8 Gy/fx for 26 fractions. In phase II, PTV1 is boosted for another 16 fractions. In dose escalation plans (DEPs), margins from 0 to 10 mm are added to form PTVs, which are prescribed to 1.8 Gy/fx for 42 fractions. 95% of PTVs are covered by prescription.Results:The CTV1/CTV2 has volume of 63±20 and 415±132 cc. To keep bladder Dmean same as in RP, margin must be ≤6 mm in DEPs. Similarly, margin needs to be ≤7 mm for rectum Dmean. For small bowel, even at margin of 0, Dmean and EUDs on DEPs are higher than RP, because RP was also IMRT. Instead, QUANTEC criteria of “45Gy volume<195cc” was adopted. 6/14 patients fail this criteria in RP. For the remaining 8 patients, margins of ≤7(3) mm are necessary for 6(2) patients, respectively.Conclusion:To escalate LNs to full dose, planning margins need to be ≤6 mm to achieve same tolerance for rectum and bladder. A large variation in patient anatomy exists for small bowel and dose escalation was possible for half of patients with ≤6 mm margin. Investigations of whether and how ≤6 mm margin can be achieved through image guidance are needed.