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Dose-Volume Dependence of Targets and Critical Organs On the Dose Calculation Resolution in Prostate VMAT Plan

J Chow

J Chow1*, R Jiang2 , (1) Princess Margaret Cancer Centre, Toronto, ON, (2) Grand River Regional Cancer Center, Kitchener, ON


SU-I-GPD-T-414 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: This study evaluated the dose-volume dependences of targets and critical organs when the dose calculation grid size was changed between 1 and 5 mm in a prostate VMAT plan.

Methods: For 10 patients treated with the prostate VMAT with prescription 78Gy/39 fractions using the 6 MV photon beam, the grid size was changed to 1, 2, 3, 4 and 5 mm from the original one (2.5 mm) in the treatment plan. The dose-volume histograms for the CTV, PTV, rectum, rectal wall, bladder and bladder wall were recalculated using the Eclipse treatment planning system (ver.11.0.31). Variations of dose-volume points such as D99% of the CTV and PTV, D30% and D50% of the critical organs were determined for evaluation of the dose-volume change due to the dose calculation resolution.

Results: For variations of D99% (ΔD99%) of the targets, it is found that ΔD99% of the PTV (328.8 cGy) was much larger than that of the CTV (46.6 cGy). This shows that though the grid size effect was significant in the PTV, it did not affect the CTV with the same extent. The ΔD30% and ΔD50% of the rectum were 443.9 and 356.6 cGy which were smaller than those of the rectal wall (1817.6 and 846.1 cGy). It is seen that the rectal wall was more sensitive to the grid size than the rectum. However, this is not true for the bladder as the bladder wall and bladder had similar ΔD30% (236.4 cGy vs. 278.9 cGy) and ΔD50% (303.3 cGy vs. 321.8 cGy).

Conclusion: Dose-volume dependence of PTV on the grid size is more significant than CTV. For critical organs, though bladder and bladder wall have similar ΔD30% and ΔD50%, the ΔD30% and ΔD50% of the rectal wall are larger than the rectum when the grid size of the VMAT plan varies.

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