Rectal dose and toxicity dosimetric evaluation for various beam arrangements using pencil beam scanning protons with and without rectal spacers

Transcription

1 Rectal dose and toxicity dosimetric evaluation for various beam arrangements using pencil beam scanning protons with and without rectal spacers 2015 MAC-AAPM Annual Meeting, Baltimore, MD Heeteak Chung, Ph.D.*, Jerimy Polf, Ph.D., Minesh Mehta, MBChB., and Michael Choung, M.D.

2 Transperineal Implantation Pre implant space 3 month persistence 6 month absorption Rectal Spacer Application PEG hydrogel chemistry Axial T2 MRI images Injected: Solidifies: Persistence: Absorbs: 18G, liquid hydrogel precursors within 10 seconds lasts about 3 months Within approx 6 months Gel polymerization Water based polymer Does not heat up during polymerization Breaks down via hydrolysis Flows through perirectal fat, locking it in place Courtesy from Space OAR

3 Purpose Dosimetric evaluate rectal dose and toxicity with and without the use of rectal spacer for the treatment of prostate cancer with pencil beam scanning proton therapy. Two different beam arrangements will be also evaluated Lateral Fields LAO/RAO Field

4 Materials and Methods 12 patients with rectal spacers were accrued from Moffit Cancer Center where the rectal spacers were placed. 8 patients with no rectal spacers are from University of Maryland. The planning CT from these patients were obtained for dosimetric evaluation using Varian Eclipse Treatment Planning System (version 13). Hydrogel used: DuraSeal TM (Covidien, Mansfield, USA) Water soluble polyethylene-glycol with reactive linkage Compound stable for approximately 6 months Patient # Volume (cm 3 ) PTV CTV (Prostate) Rectum Bladder WITH RECTAL SPACER WITHOUT RECTAL SPACER Mean Median

5 Proton Planning Total of 20 planning CT data set 12 planning CT with rectal spacer and 8 without rectal spacer Eclipse Treatment Planning System (version 13) Prostate (CTV) PTV = CTV + 6 mm Rectum Bladder

6 LAO/RAO (60 o and 300 o ) LAT (90 o and 270 o ) Proton Planning- Beam Arrangements and Margins Distal Margin = 3.5% x WET Distal + 2 mm Proximal Margin = 3.5% x WET Proximal + 2 mm Typical margin: 7 mm to 10 mm WET Distal WET Proximal

7 Proton Planning Prescriptions and DVH Criteria PTV: 79.2 Gy in 44 Fx At least 95% of the PTV receiving 100% of prescription Dose Volume Constraints Rectum: V75 15%, V70 25%, V65 35%, and V60 50% Bladder: V80 15%, V75 25%, V70 35% and V65 50% Femoral Head: Max Dose = 50 Gy

8 Rectal NTCP Lyman-Kutcher-Burman (LKB) methodology Describes normal tissue complication probability (NTCP) after uniform radiation of a fractional volume (v) of normal tissue to a dose (D) using equation: NTCP = geud = 1 2 π t e x2 2 t = geud TD 50 m TD 50 1 N voxels N voxels i=1 dx D i 1 n n To evaluate grade 2 rectal toxicity, QUATEC-recommended parameters were used. They are as follows; n = 0.09, m = 0.13, and TD50 = 73.7 Gy. TD 50 : Dose at which 50% probability of developing specified rectal complication

9 Results DVH Comparison: Mean dose volume (VXX) points for patients with and without a rectal spacer for LAT and LAO/RAO field arrangements CTV, Rectum and Bladder CTV: V40, V50, V60, V70, V80, and Dmax Rectum: V40, V45, V50, V55, V60, V65, V70, and V75 Bladder: V50, V55, V60, V65, V70, V75, and V80 NTCP Analysis Predicted rectal toxicity ( grade 2) for patients with and without a rectal spacer for LAT and LAO/RAO field arrangements

10 Volume (%) CTV DVH LAO/RAO + Spacer LAO/RAO + No Spacer LAT + Spacer LAT + No Spacer Dose (Gy)

11 Volume (%) Rectum DVH LAO/RAO + Spacer LAO/RAO + No Spacer LAT + Spacer LAT + No Spacer Dose (Gy)

12 Volume (%) Bladder DVH LAO/RAO + Spacer LAO/RAO + No Spacer LAT + Spacer LAT + No Spacer Dose (Gy)

13 V70 (%) V70 vs. Separation Distance LAO/RAO + Spacer LAO/RAO + No Spacer LAT + Spacer LAT + No Spacer Rectum-Prostate Separation Distance (cm)

14 NTCP NTCP vs. Separation Distance LAO/RAO + Spacer LAO/RAO + No Spacer LAT + Spacer LAT + No Spacer Rectum-Prostate Separation Distance (cm)

15 Discussion: LAT vs. LAO/RAO LAT is the standard practice to treat prostate cancer using proton therapy Rational for using anterior oblique angles (LAO/RAO) was to take advantage of distal fall of the Bragg peak with the increased rectumprostate separation distance. Not significantly different whether with or without rectal spacer. LAO/RAO may be more advantages for patients with prosthetic hips

16 Discussion: Rectum-Prostate Separation Rectal spacer reduced the dose and toxicity to the rectum Minimum improvement to the bladder Based on the study, 0.8 cm of rectumprostate separation distance showed significant improvement to rectal sparing. Christodouleas et al. reported 0.7 cm (dosimetric proton study) Susil et al reported 1 cm to 1.5 cm (dosimetric IMRT study) The magnitude of rectal sparing is dependent of proper insertion of rectal spacer. Christodouleas JP, et al. The effect of anterior proton beams in the setting of a prostate-rectum spacer. Med Dosim 2013;38: Susil RC, et al. Effects of prostate-rectum separation on rectal dose from external beam radiotherapy. Int J Radiat Oncol Biol Phys 2010;76:

17 Conclusion Rectal spacer dose improve rectal sparing for LAT and LAO/RAO field arrangements for the treatment of prostate cancer using proton therapy. Did not see significant dosimetric difference between LAT and LAO/RAO however for hip prosthesis, LAO/RAO is recommended. Based on our data, the rectum-prostate separation distance should be 0.8 cm or larger to achieve maximum rectal sparing.

18 Thank you