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Radiobiological Comparison of Planned and Delivered Prostate IMRT/VMAT Plans Using 3DVH Software

K Hedrick

K Hedrick*, L Rankine , R Chen , S Das , P Mavroidis , University of North Carolina, Chapel Hill, NC


SU-I-GPD-T-295 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: This study aims to compare two methods of IMRT QA for a cohort of prostate cancer patients: conventional gamma analysis and structure-specific DVHs. Furthermore, we investigate how these IMRT QA methods relate to the expected tissue response rates in order to estimate their clinical efficacy.

Methods: Twelve patients were studied and for each patient two dose distributions (planned and measured) were analyzed. Prescription dose varied from 62.5-66.6 Gy and structures investigated include the PTV, CTV, bladder, and rectum. The 3DVH software (Sun Nuclear Corporation) was used to calculate DVHs from ArcCHECK measurements and compare these to the treatment plan (RayStation 5.0.2, RaySearch Laboratories) by generating structure-specific gamma passing values. The TCP and NTCP values were also calculated for both measured and planned dose distributions, using the Relative Seriality models for the organs at risk (OARs) and the Poisson-LQ model for the targets. Additionally, the P+ index, which indicates the probability of achieving tumor control without normal tissue complications, was calculated.

Results: The proposed plan evaluation shows that patient plans systematically deliver more dose to OARs and less dose to targets than the treatment planning software predicts. Although all patient plans passed global gamma analysis with 3%/3mm criteria, structure-specific differences averaged -4.2±5.5% for bladder, -1.6±4.6% for rectum, -6.7±8.9% for CTV, and -8.9±6.4% for PTV. Differences in NTCP averaged 0.6±0.4% for bladder and 1.4±1.1% for rectum; while differences in TCP averaged -1.3±0.8% for CTV, -0.6±0.6% for PTV. Additional analysis showed no correlation between radiobiological parameters and structure-specific gamma values.

Conclusion: These findings suggest that current IMRT QA practices using global gamma analysis may obscure significant dose discrepancies in specific structures. Both global and structure-specific gamma analysis also appear having no correlation with the corresponding TCP, NTCP or P+ values.

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