Radiation and Cancer Physics

PD 06 - Physics 2 - Poster Discussion - Treatment Delivery

1053 - Precision of Two Low-Dose Abdomen/Pelvis CBCT Protocols for Alignment to Bone and Soft Tissue in Pediatric Patients Receiving Image-Guided Radiation Therapy

Monday, October 22
11:33 AM - 11:39 AM
Location: Room 217 C/D

Precision of Two Low-Dose Abdomen/Pelvis CBCT Protocols for Alignment to Bone and Soft Tissue in Pediatric Patients Receiving Image-Guided Radiation Therapy
A. D. Rao1, J. Lee2, W. Fu2, S. E. Nicholas2, S. R. Alcorn3, J. Moore3, M. Ladra4, M. Mahadevappa2, S. Bartolac2, and S. A. Terezakis5; 1Department of Radiation and Molecular Radiation Sciences, Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins School of Medicine, Baltimore, MD, 2Johns Hopkins University School of Medicine, Baltimore, MD, 3Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, MD, 4Massachusetts General Hospital, Boston, MA, 5The Johns Hopkins Hospital, Department of Radiation Oncology and Molecular Radiation Sciences, Baltimore, MD

Purpose/Objective(s): Recommendations for low-dose cone-beam computed tomography (LD-CBCT) protocols for pediatric image-guided radiation therapy (IGRT) are lacking. We aim to evaluate the precision of two LD-CBCT protocols to align to bone and soft tissue landmarks for pediatric patients receiving IGRT to the abdomen and pelvis (A/P).

Materials/Methods: 858 CBCTs from 46 pediatric patients who received IGRT to the A/P starting January 2015 –December 2017 were reviewed. Image quality of CBCTs over this time guided the development of two significantly dose-reduced protocols, LD-CBCT1 (180º rotation, 120 kV, 63 mAs total, 315 frames, S20 collimator) and LD-CBCT2 (180º rotation, 100 kV, 31.5 mAs total, 315 frames, S20 collimator). Eight patients with at least 1 CBCT from both protocols during the course of IGRT were selected for a simulation study, and a representative LD-CBCT1 and LD-CBCT2 scan was registered to the simulation CT (sim-CT) separately aligning to bone and soft tissue. To simulate the process of correcting a setup error to a bone and soft tissue landmark using each protocol, 18 blinded random offsets (max translation 10 mm any x/y/z direction) were applied to each LD-CBCT1 and LD-CBCT2 from a starting registration to bone and a soft tissue structure. Each of the resultant 504 simulated set-up errors was then blindly corrected using rigid registration by a radiation oncologist. The vector magnitude difference (VM= √(x2 + y2 + z2)) between the random offset and final registration attempt was calculated, adjusting for any systematic error in the initial sim-CT↔LD-CBCT1/2 registration. A one-sided paired T-test was used to evaluate if LD-CBCT1, delivering higher dose, was superior to the lower dose LD-CBCT2 for bone and soft tissue alignment.

Results: Age and patient size (using abdominal area) ranges of the eight sample patients with representative LD-CBCT1 and LD-CBCT2 scans were 1-5-9.2 years and 516-954 cm2, respectively. Comparing 288 registrations to a bone landmark across 8 patients, there was no difference in the vector magnitude of the offsets using LD-CBCT 1 (mean [̄x̄], 0.73 mm; standard deviation [σ], 0.39 mm) and LD-CBCT2 (̄x̄, 0.74 mm; σ, 0.40 mm) (p=0.425). Comparing 216 registrations to a soft tissue landmark across 6 patients, alignment using LD-CBCT2 (̄, 1.55 mm; σ, 1.08 mm) resulted in larger differences in the vector magnitude of the offsets compared to LD-CBCT1 (̄, 1.37 mm; σ, 0.74 mm) (p=0.049).

Conclusion: Given the particular importance of reducing dose to normal tissue in pediatric patients and the increasing use of IGRT, clinics treating pediatric patients should consider a protocol mirroring LD-CBCT2 validated in this study for A/P IGRT aligned to bone. Delivering higher dose, LD-CBCT1 was slightly superior to LD-CBCT2 for soft tissue alignment, but further investigation is needed given that this study design evaluated the utility in alignment to a kidney structure as the representative soft tissue structure.

Author Disclosure: A.D. Rao: Employee; Johns Hopkins School of Medicine. J. Lee: None. W. Fu: None. S.E. Nicholas: None. S.R. Alcorn: None. J. Moore: President-Elect of Mid-Atlantic Chapter; AAPM. M. Ladra: None. M. Mahadevappa: None. S.A. Terezakis: Research Grant; Elekta. Honoraria; Elekta. Chair of Pediatric Scientific Committee; ASTRO.

Avani Rao, MD

Johns Hopkins School of Medicine

Disclosure:
Employment
Johns Hopkins School of Medicine: MD-housestaff: Employee

Presentation(s):

Send Email for Avani Rao


Assets

1053 - Precision of Two Low-Dose Abdomen/Pelvis CBCT Protocols for Alignment to Bone and Soft Tissue in Pediatric Patients Receiving Image-Guided Radiation Therapy



Attendees who have favorited this

Please enter your access key

The asset you are trying to access is locked. Please enter your access key to unlock.

Send Email for Precision of Two Low-Dose Abdomen/Pelvis CBCT Protocols for Alignment to Bone and Soft Tissue in Pediatric Patients Receiving Image-Guided Radiation Therapy