Medicina nuclear y radioterapia

Background: Positron emission tomography (PET) with fluorine 18 fluorodeoxyglucose (¹⁸F-FDG) has been widely used for pediatric tumors. In order for ¹⁸F-FDG to confer utility in its widespread use, normal ranges encompassing physiologic variation must be elucidated, such that abnormal variation can be identified. Normal physiologic activity may vary according to age group. Previous studies investigating physiologic uptake in the head and neck organs have not focused on a pediatric population.

Objective: The purpose of this study is to elucidate the normative values for ¹⁸F-FDG uptake for head and neck structures in a pediatric population known to have no head or neck pathology.

Materials and methods: A total of 97 patients (43 females) ranging in age from 2.7 to 18 years old who had no history of head or neck malignancy/pathology (either at the time of study acquisition or on follow-up) were included in this retrospective study. PET/CT imaging was acquired approximately 60 minutes after injection of 5.18 MBq/kg (0.14 mCi/kg) ¹⁸F-FDG. Twelve head and neck structures including 1. Lingual tonsils, 2. Adenoid tissues (pharyngeal tonsils), 3. Palatine tonsils, 4. Soft palate, 5. Tongue, 6. Sublingual glands, 7. Parotid glands, 8. Submandibular glands, 9. Spine at the level of C1, 10. Thyroid gland, 11. Inferior concha and 12. Vocal cords were evaluated in this study. The region of interest (ROI) was drawn over these structures in the axial view and the maximum and mean SUV normalized to body weight were recorded.

Results: Lingual tonsils, adenoid tissues, palatine tonsils and sublingual glands showed the highest uptake. The mean SUVmaxes ± SD were 2.67 ± 1.25 in lingual tonsils, 3.99 ± 1.56 in palatine tonsils, 3.04 ± 1.91 in adenoid tissues, and 3.00 ± 1.21 in the sublingual glands. Other head and neck organs showed a mild ¹⁸F-FDG uptake; the mean SUVmax was 1.52 ± 0.43 for inferior concha, 1.51 ± 0.45 for soft palate, 1.33 ± 0.38 for tongue, 1.45 ± 0.52 for parotid glands, 1.97 ± 0.66 for submandibular glands, 1.68 ± 0.51 for the spine at the level of C1, 1.24 ± 0.40 for the thyroid gland, and 1.55 ± 0.87 for the vocal cords.

Conclusion: Adenoid tissues, lingual and palatine tonsils and sublingual glands show a moderate to high ¹⁸F-FDG uptake (mean SUVmax>2.5), while other small head and neck organs show a mild ¹⁸F-FDG activity.

Objective: To compare conventional Four-Field Pelvic Radiotherapy (4FRT), Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) in cervical cancer using increasing Clinical Target Volume (CTV) to Planning Target Volume (PTV) margins to account for daily variability in uterine position and to assess whether dosimetric advantages with advanced planning techniques continue with larger PTV margins.

Methods: Twenty patients with locally advanced cervical cancer previously treated with definitive radiation therapy were selected for the study. For each patient, computed tomography planning scans were obtained and PTVs were created with CTV to PTV uterine margins of 1.5 cm, 2 cm, 2.5 cm and 3 cm if anatomically feasible. 4FRT, IMRT and VMAT plans were generated and evaluated for target coverage, conformity, and homogeneity, dose to Organs at Risk (OAR), Total Monitor Units (MU) and delivery time.

Results: A total of 136 plans were generated. Target coverage was excellent for all plans generated regardless of technique. VMAT and IMRT were associated with significantly reduced dose to OAR compared with 4FRT for all CTV to PTV uterine margins (p ≤ 0.05). VMAT plans were associated with lower rectum V40 (p ≤ 0.02) and bowel V40 (p ≤ 0.04) for the smaller uterine margins and lower MU and delivery time (p ≤ 0.01).

Conclusion: VMAT and IMRT result in significantly lower doses to OAR compared with 4FRT in cervical cancer, regardless of CTV to PTV margins used to account for uterine motion. VMAT was associated with shorter delivery times compared with IMRT, which is useful in cervical cancer radiation therapy where intrafraction uterine motion may result in target under coverage if inadequate uterine margins are used. Accurate localization of the uterus with daily image guidance is critical when considering these advanced planning techniques for definitive radiation therapy in cervical cancer.