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Composite IMRT Planning: How Does the Initial Plan Dose Distribution Affect the Boost Plan Optimization?

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P Papanikolaou

S Stathakis , G Avila , P Mavroidis , C Ha , T Eng , Z Shi , P Papanikolaou*, University of Texas Health Science Center, UTHSCSA, San Antonio, TX


SU-E-T-597 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To perform a dosimetric and radiobiological comparison of the composite prostate plans when the boost plan was optimized with and without knowledge of the initial dose distribution.
Methods: Ten patients previously treated in our department were selected for this study. The planning target volume (PTV) included the prostate, seminal vesicles and pelvic lymph nodes (PTV1) while the prostate with margins was considered PTV2. All organs at risk (OAR) and target structures were contoured by the prescribing physician. The prescribed doses were 54Gy in 30 fractions to PTV1 and 78Gy to PTV2. All plans were optimized using the RayStation treatment planning system (TPS). Plans to deliver the prescribed dose to PTV1 were optimized first. Two boost plans and the respective composite plans were then created for each patient. First, the boost plan was optimized by treating the prescription of 24Gy to PTV2 as a standalone plan. Second, the boost plan for PTV2 was optimized taking into consideration the dose distribution of the initial plan. For each method the respective composite plans was created. The two final composite plans were compared by in terms of DVHs and isodose line distributions, as well as radiobiologically.
Results: In terms of PTV2 coverage the two methods were very similar with negligible differences. However, differences were observed for the overall coverage of PTV1, seminal vesicles and penile bulb. For these structures the mean doses were higher in the case of the independent boost plan. Similar trend was observed for the doses to bladder and rectum but the differences were smaller. The dosimetric differences were patient (OAR) dependent.
Conclusion: By including the initial dose distribution during boost plan optimization the final composite plan shoed that doses to most OAR can be reduced. The dosimetric differences are patient dependent and in some cases can be significant.

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