Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_16_3267 - Dosimetric Variations Assessed With CBCT for Head and Neck Cancer Radiation Therapy

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Dosimetric Variations Assessed With CBCT for Head and Neck Cancer Radiation Therapy
J. Xue1, H. Wang1, T. Chen1, P. B. Schiff1, I. J. Das1, and K. S. Hu2; 1Department of Radiation Oncology, NYU Langone Health, New York, NY, 2Department of Radiation Oncology, NYU School of Medicine and Perlmutter Cancer Center, New York, NY

Purpose/Objective(s): Head & Neck (HN) cancers treated with conventional fractionations likely experience changes in the volume and shape of both targets and normal tissues over the long course of treatment. This study investigated the dosimetric variations of both targets and normal tissues related to the organ deformation throughout treatment.

Materials/Methods: Eleven H&N cancer patients treated to the oropharynx cancer were retrospectively included in the study. Treatment was delivered with IMRT to 70 Gy in 35 fractions for PTV/CTV. Standard image guided radiation therapy (IGRT) procedure was applied for daily setup corrections, which utilized cone beam computed tomography (CBCT) with the auto rigid registration to account for translational as well as rotational setup errors. For each patient, the daily CBCT image set acquired during setup from every other fraction of treatment were imported to a third party software, VelocityTM, for the deformable registration with the planning CT image set. The registration process transformed the plan dose to each CBCT image set and deformed it as the delivered fraction dose for each structure. The accumulated dose for both targets and normal tissues was calculated with the actual deformed dose to CBCT.

Results: It is observed that dose distribution for both targets and normal tissues has changed over the course of treatment. The variation of the accumulated dose from the plan dose is organ and patient dependent. The largest variation was up to 150% in minimum dose. Maximum dose and mean dose had less variations in general. The percent variation of minimum, maximum and mean dose is summarized in a table for CTV, GTV and normal tissues. The change in the shape and volume of each structure was also observed by the translational deviation of the DICOM center of a structure more than a centimeter in x, y or z direction for some patients. The weekly breakdown of the variations in accumulated dose and structural volumetric center presents a consistent deformation of some structures over the course of treatment.

Conclusion: Conventional fractionation H&N cancer radiotherapy in is a standard protocol. IGRT has been employed to correct the patient positioning by kV orthogonal or CBCT imaging. Considerable variations have been observed in dose distribution over the course of treatment for both targets and normal tissues due to the organ deformation. The results of this study can be used to determine whether or when adaptive therapy is needed throughout the treatment or to calculate any remedy necessary at the end of the course.
Structure Max Variation (%) relative to the plan
Dose Mean Min Max
CTV 70Gy -1.1 -42.0 -0.2
CTV 59.5 Gy -2.3 -68.8 -0.2
GTV -0.8 -39.4 -0.2
Brainstem 30.5 150.5 11.4
Spinal Cord 5.3 20.9 8.2
Mandible -8.2 -55.9 -2.4
Oral Cavity -11.4 -48.5 -2.6
Right Parotid 11.1 51.8 -4.3
Left Parotid 10.4 79.6 -2.5
Right Brachial plexus 10.8 -62.3 32.2
Left Brachial plexus -6.6 102.6 5.5
Right Submandibular -9.8 8.4 -8.5
Left Submandibular -3.5 -17.7 -1.2

Author Disclosure: J. Xue: None. H. Wang: None. P.B. Schiff: Independent Contractor; Guidepoint Global, Schlesinger Associates. I.J. Das: Honoraria; JASTRO, Japanese Society of Therapeutic Radiation. Speaker's Bureau; JASTRO, Japanese Society of Therapeutic Radiation. Travel Expenses; JASTRO, Japanese Society of Therapeutic Radiation. Committee Member and surveyor; ACR. Associate Editor; Br J Radiology, Medical Physics. examiner; ABR. Committe member and Chair; AAPM. K. Hu: Stock; Biogen Idec, Amgen, Johnson and Johnson, ZIOP Pharmaceutical. Treasurer; NY Head and Neck Society.

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