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Should the Machine-Related Range Shifter (MRS) Be Replaced by a Patient-Related Range Shifter (PRS) to Preserve the Scanning Beam Dosimetric Advantages?

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H Lin

H Lin1*, J Shen2 , J McDonough1 , Z Tochner1 , S Both1 , (1) University of Pennsylvania, Philadelphia, PA, (2) Mayo Clinic Arizona, Phoenix, AZ


SU-E-T-692 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:To present and evaluate the impact of PRS relative to MRS on PBS spot size.

Methods:Three PRS: U-shape bolus (USB), anterior-lateral bridge bolus (ALBB), couch top bolus (CTB)) and MRS mounted on the nozzle are currently employed in our institution. Homogeneous materials with different densities were employed for each device. In-air spot sizes were simulated and compared for 6 conditions: open field, MRS with 15cm, 20cm, and 35 cm air gaps and PRS with 2cm and 8cm air gaps to reproduce clinical treatment conditions. The pros and cons of MRS are evaluated against PRS.

Results:Spot sizes increased with greater air gaps, which are more significant for low energy protons. PRS can be consistently placed close to the patient surface; therefore spot sizes don’t change significantly relative to the open beam. MRS may have the same effect for small air gaps. While PRS is overall superior in avoiding patient collision, its impact on the quality of the verification imaging increases as Z increases, in particular for CTB. However, lower Z-material CTB may increase collision and still affect CBCT quality. The main advantages of the PRS are spot size preservation (<2mm change for air gaps of 2 to 8cm), increased uniformity and conformality of the dose distributions and patient collision avoidance. This cannot be achieved consistently for Brain and HN treatment using MRS as the air gap may increase greater than 15cm (maximal increase of 4.5 mm on spot size) due to clearance issues.

Conclusion:The spot size and PBS dosimetrical advantages can be maintained with minimized air gap for both PRS and MRS. However; the PRS non-removable device (CTB) affects CBCT imaging quality. Therefore, an ideal approach would be a combination between the USB for the brain and HN treatments and an automated MRS for body and extremities.

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