Justin Tang, MD, MS
Montefiore Medical Center
PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s): Given that majority of prostate cancer is found in the peripheral zone, it is increasingly likely for a targetable prostate lesion to be found at the prostatic-rectal interface (PRI). We hypothesized that a simultaneous integrated boost (SIB) technique can increase the dose to peripheral nodules while maintaining rectal dose tolerance.
Materials/Methods: Ten prostate cancer patients that underwent SBRT with a radiation treatment planning MRI pelvis from 2016-2017 were identified. A 3D volumetric T2-weighted non-contrasted 3T MRI prostate was obtained in the treatment planning position and registered to the planning CT. Dominant Intra-prostatic lesions (DIL) were contoured on the MRI. The PTV was defined as the prostate and proximal seminal vessels with a 2mm margin. A PTVEval, defined as the PTV minus 1 or 2mm margins from overlapping OARs was used for plan normalization. Treatment plans with a prescription dose of 9Gy x 5 were generated for three scenarios: PTVEval with and without ensuring adequate coverage of the DIL, and a SIB of the DIL to 50Gy. For all scenarios rectal dose constraints were given a higher priority than DIL dose coverage constraints. Planning objectives were PTVEval coverage: D95%≥45Gy and rectal wall: Dmax (0.03cc)<42.75Gy, D1cc< 36Gy, and D50%<22.5Gy. Student t-test was used to assess PSA and dosimetric differences. Patients with collected PSA values were treated to PTVEval with 45Gy in 5 fractions.
Results: The median age of the cohort was 64.5 years (IQR: 56-69.5) and 8 patients had Gleason 7 disease while 2 had Gleason 6. Median follow-up was 3 months (IQR 1-6), mean PSA at time of diagnosis was 9.9ng/mL (STD 5.4), 1-month post-treatment increased to 13.2ng/mL (STD 8.8), and then dropped to 6.9ng/mL (STD 3.2) at 3-months. No statistically significant change in rectal wall Dmax or D1cc was found between planning scenarios. All patients had identifiable DIL on MRI and 6 of the patients had DIL at the PRI. For patients with peripheral DIL, additional optimization objectives produced an increase in the volume of the DIL receiving 45Gy (average V45Gy increased from 78.3% to 94.3%). For SIB plans, the mean DIL dose was 51.7Gy with V50Gy = 86.4%. Patients who had DIL at the PRI had significantly higher 3-months post-RT PSA than patients who did not (8.2 vs 4.0 ng/mL, p=0.04). However, PSA in both groups continued to trend downwards. The closest distance between the DIL and rectal wall ranged from 0.3 to 2.8mm between patients. Increased distance allows for greater sparing of the rectal wall or increased DIL dose escalation.
Conclusion: Our study showed feasibility of SIB to DIL without significant increase in rectal dose. The BED of 45Gy and 50Gy using α/β=2 are 247.5 vs 300 Gy2 respectively and the increased BED may translate to higher tumor control probability. Utilization of spacer gels to create extra separation between peripheral DIL and rectum may provide better rectal wall sparing and dose escalation.
Montefiore Medical Center
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