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Dosimetric Comparison of Volumetric Modulated Arc Therapy, Step-And-Shoot, and Sliding Window IMRT for Prostate Cancer

E Schnell*, T De La Fuente Herman, J Young, K Hildebrand, O Algan, E Syzek, T Herman, S Ahmad, Oklahoma Univ. Health Science Ctr., OKLAHOMA CITY, OK

SU-E-T-616 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate treatment plans generated by Step-and-Shoot (SS), Sliding Window (SW) and Volumetric Modulated Arc Therapy (VMAT) in order to assess the differences in dose volume histograms of planning target volume (PTV) and organs at risk (OAR), conformity indices, radiobiological evaluations, normal tissue dose, and plan quality for prostate cancer cases.

Methods: Ten prostate cancer patients treated in our center were selected for this retrospective study. Treatment plans were generated with Eclipse version 8.9 using 10 MV photon beams. For VMAT, Varian Rapid Arc with 1 or 2 arcs, and for SS and SW IMRT, 7-9 coplanar fields were used. Each plan had three PTVs with prescription doses of 81, 59.4, and 45 Gy to prostate, to prostate and lymph nodes, and to pelvis, respectively. Doses to PTV, OAR, normal tissues, and the conformal indices (COIN) were compared among three techniques. The equivalent uniform dose (EUD), tumor control probability (TCP), and normal tissue complication probability (NTCP) were calculated and compared.

Results: The average mean doses to the PTV prostate were 82.8 Gy and the average differences of mean dose among all techniques were below 0.3 Gy. For bladder and rectum, the percent differences of average mean dose among all techniques were below 2.2%, though high dose rectal volumes were reduced for VMAT. The COIN slightly favoured VMAT. The average TCP varied by less than 0.001%. The average NTCP for the rectum was 3.55%, 3.75% and 3.87% for VMAT, SS and SW, respectively, while the differences in the average NTCP for the bladder was negligible. The full body volumes receiving 20 to 30 Gy were reduced in VMAT treatments.

Conclusion: Data indicates similar target coverage and OAR sparing in general. Additional patients will continue to be added to the study. Variation in normal tissue distribution was less than expected.

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