The use of bony landmarks to define gynecologic RT portals has been repeatedly shown to lead to inadequate target coverage and excess normal tissue irradiation compared to three-dimensional (3D) planning.²,³ Computed tomography (CT) provides more accurate and customized target delineation and permits dose calculations based on electron density, so CT has become the standard imaging technique used today. However, compared to CT, magnetic resonance imaging (MRI) provides superior soft tissue definition, permitting better delineation of the volume and extent of pelvic tumors.⁴,⁵ MRI is also superior to physical examination for assessing uterine involvement and tumor response and better predicts local control and disease-free survival.⁶, ⁷, ⁸ Therefore, although presently MRI is not routinely incorporated into external-beam RT planning, in select cases, registering or fusing an MRI with the planning CT can be useful to guide target delineation.

For assessing nodal metastasis, routine MRI and CT have similar accuracy.⁹ Several investigators have studied ultrasmall particles of iron oxide (USPIO) to improve nodal assessment with MRI. Rockall et al.¹⁰ found that USPIO-enhanced MRI increased the sensitivity of MRI, without loss of specificity, in endometrial and cervical cancers. A meta-analysis found that MRI enhanced with superparamagnetic iron oxide nanoparticles has a sensitivity of 88% and specificity of 96%, compared to 63% and 93%, respectively, for unenhanced MRI.¹¹ Taylor et al.¹² used USPIO-enhanced MRI to guide nodal delineation and found that a 7-mm margin with slight modifications was sufficient to encompass 99% of pelvic nodes (Fig. 16.1).

Numerous studies in cervical cancer have demonstrated that [¹⁸F] fluorodeoxyglucose (FDG) positron emission tomography (PET) (or PET/CT) detects cervical abnormalities and occult nodal disease with higher sensitivity than MRI or CT alone.¹³, ¹⁴, ¹⁵ Moreover, several reports have also shown that FDG-PET abnormalities are correlated with poorer outcomes.¹³,¹⁶,¹⁷ Others have found that FDG-PET improves diagnosis and staging of endometrial¹⁸ and vaginal cancer.¹⁹ However, although FDG-PET may be a useful tool to guide nodal clinical target volume (CTV) delineation for RT planning purposes, it may not have sufficient negative predictive value to obviate lymph node sampling or lymphadenectomy for patients with preoperative imaging negative for nodal metastasis.²⁰ FDG-PET can be used to measure tumor volume and even monitor response during treatment of cervical cancer.²¹,²² For patients with para-aortic lymph node metastases, FDG-PET may be useful to delineate the gross disease, which, at least theoretically, can be safely boosted to 59.4 Gy.²³,²⁴

A few studies have investigated applications of advanced imaging. Dynamic contrast-enhanced MRI can identify regions of tumor hypoxia²⁵ and ¹H-magnetic resonance spectroscopy (MRS) can be useful to differentiate tumor from normal tissue (Fig. 16.2).²⁶ Studies have also investigated PET with
alternative tracers, such as ¹¹C-choline, ¹¹C-methionine, and ⁶⁰Cu-diacetyl-bis(N4-methylthiosemicarbazone) (⁶⁰Cu-ATSM), to identify metabolic abnormalities or hypoxia.²⁷, ²⁸, ²⁹, ³⁰ Roeske et al.³¹ found that single-proton emission CT (SPECT) could be a useful adjunct for IMRT planning by imaging active bone marrow, which could be selectively avoided. To date, however, limited data exist to support the use of these sophisticated imaging approaches for RT planning in gynecology patients, and further study is required to define their role.

Many investigators have described advantages of 3D imaging and volumetric dosimetry over conventional imaging modalities and standard point-dose dosimetry for gynecologic brachytherapy.³², ³³, ³⁴ CT³⁴ and ultrasound³⁵ are useful and relatively inexpensive imaging techniques that can both guide and improve brachytherapy. More recently, MRI and PET have received attention as means of augmenting brachytherapy planning.

Studies have found that CT can overestimate the target volume compared to MRI.³⁶ Wachter-Gerstner et al.³⁷ reported that MRI-guided brachytherapy planning improves estimated dose delivery to the bladder and rectal walls compared to conventional techniques. High rates of local control and low toxicity have been reported using MRI-guided approaches.³⁸,³⁹ Investigators at the Medical University of Vienna have developed a specialized applicator for MRI-guided brachytherapy,³⁸ and in recent years, working groups have published recommendations for MRI-guided brachytherapy in cervical cancer.³²,⁴⁰,⁴¹ Although the majority of research in MRI-guided brachytherapy has focused on cervical cancer, promising results have also been reported using this technique to treat recurrent endometrial⁴² and inoperable uterine⁴³ cancers.

Investigators at Washington University have conducted several studies of PET-guided intracavitary brachytherapy for cervical cancer, demonstrating the feasibility and dosimetric advantages of this approach over conventional techniques.⁴⁴,⁴⁵ The procedure involves scanning patients after both intravenous delivery of [¹⁸F]FDG and insertion of tubes containing [¹⁸F]FDG into the tandem and colpostats. The PET image is then transferred to the 3D planning system, where volumes are delineated. Outcome data are awaited to assess the potential benefits of this approach.