Feasibility of safe ultra-high (EQD(2)>100 Gy) dose escalation on dominant intra-prostatic lesions (DILs) by Helical Tomotheraphy

PURPOSE: to verify the possibility of using Helical Tomotherapy to safely escalate dose to single or multiple highly radioresistant dominant intra-prostatic lesions (DILs) as assessed by functional magnetic resonance imaging (MRI). MATERIAL: in seven intermediate/high risk patients, T2WI, T1WI and DWI MRI imaging showed evidence of one DIL in four patients and two DILs in three patients in the peripheral zone of the prostate. The planning strategy was to deliver median doses of 80, 90, 100 and 120 Gy to PTVDIL while delivering 71.4 Gy/28 fractions (EQD(2)=75 Gy) to the remaining portion of PTV. A higher priority was assigned to rectal constraints relative to DIL coverage. Rectal NTCP calculations were performed using the most recently available model data. RESULTS: the median dose to DIL could safely be escalated to at least 100 Gy (EQD(2,α/β=10)=113 Gy) without violating safe constraints for the organs at risk. Typical rectal NTCP values were around or below 1-3% for G3 toxicity and 5-7% for G2-G3 toxicity. For the 100 Gy DIL dose boost strategy, mean D95% of DIL and PTVDIL were 98.8 Gy and 86.7 Gy, respectively. The constraints for bladder, urethra and femoral heads were always respected. CONCLUSIONS: IGRT by Helical Tomotherapy may permit the safe escalation of EQD(2,α/β=10) to at least 113 Gy to DILs without significantly increasing rectal NTCP compared to plans without dose escalation. A Phase I-II clinical study is warranted