A list of CTVs and GTVs with designations that are matched precisely to the labels on the respective contours in the treatment planning system.
The margin(s) to be added to each GTV or CTV to generate a final PTV.
The desired dose and dose per fraction for each PTV.
Normal tissue constraints with emphasis on areas of particular concern.
A suggested field arrangement (e.g., four-field, AP-PA, or appositional electrons) or inverse planning approach (e.g., IMRT).
Information about previous treatments or areas that may receive additional exposure from sequential boosts or future brachytherapy.
If the skin is to be included in the target volume, this should be specified and the need for bolus discussed.
Uncertain boundaries of gross disease due to ambiguous diagnostic findings or distortion and degradation of reconstructed images. In general, these uncertainties are addressed in designation of the GTV.
Uncertainties about the extent of microscopic spread to lymph nodes or other tissues at risk for tumor involvement; these are addressed in the CTV.
Uncertainties caused by internal motion of target structures. These are usually incorporated in an ITV based on diagnostic findings and an understanding of the nature of internal organ motion (Chapter 5).
Uncertainties caused by inconsistent positioning of the patient (setup error). A PTV margin is typically added to the GTV, CTV, or ITV to account for these uncertainties.
FIGURE 6.2 With IMRT, it is possible to achieve radiation dose distributions that conform tightly to unrealistic target volumes. This patient was referred for an infrarenal paraaortic node recurrence of endometrial cancer 2 years after completing adjuvant pelvic IMRT for a stage IAG2 endometrioid cancer. Only three central axis views of the doses from her previous treatment were available. However, this axial view suggests that her treatment was planned without sufficiently considering possible positioning uncertainties. The vaginal target was designated with no margin for internal organ motion or patient setup error. The patient also was simulated with a rigid obturator in the vagina; this is not recommended because such devices cause substantial vaginal distortion (Chapter 4). The distal obturator nodes also appear to have been excluded from the nodal target volume. Fortunately, the patient did not have a pelvic recurrence and has no evidence of disease 4 years after definitive treatment for her paraaortic node recurrence.
60 Gy; with these changes, the rate of infield paraaortic nodal recurrence dropped dramatically to <5%.4 These results also suggest that there is a very steep relationship between radiation dose and the likelihood of tumor control.
Body habitus. In their analysis, Kim et al.5 reported a significant correlation between a patient’s body habitus and the magnitude of positional errors.
Field size. Studies of pelvic setup reproducibility have focused on the translational shifts that are required to bring the isocenter into the correct position. However, as demonstrated in Figures 7.5 and 7.6, tilt errors and rotational errors can be equally important sources of errors in the setup of patients who have large fields. A mere 2-degree right-to-left tilt error will cause a shift of 5 mm at 15 cm from the isocenter and a shift of 7 mm at 20 cm (Fig. 6.5). Right-to-left rotational errors (Fig. 7.6) can also result in underdosage, particularly of presacral or inguinal targets. Because these errors are much more difficult to detect and correct than translational errors, they remain a significant problem even when treatments are set up using daily image guidance.
Immobilization method. In a comparison of several different pelvic immobilization methods, Malone et al.8 reported very little difference between the mean isocenter shifts required to achieve pubic alignment of prostate cancer patients treated in generic leg support devices (similar to that shown in Fig. 4.10a) or molded devices (similar to that shown in Fig. 4.10c). However, patients who were immobilized in HipFix thermoplastic devices were less likely to require large error shifts than patients immobilized in the other two types of devices. The ability to mark the isocenter directly on this type of device is a relative advantage of the thermoplastic device. The authors of this Canadian study did not report whether the device was equally effective for obese or thin patients.
Image guidance. The practical application of image guidance is addressed in Chapter 7. Daily image guidance is an important tool that significantly reduces the magnitude of alignment errors. However, image guidance does not eliminate all setup errors for several reasons:
Even the most acute observer cannot always detect 1- to 2-mm errors in isocenter alignment and will occasionally miss even 3- to 4-mm errors, particularly if the isocenter is not close to a clearly visualized boney landmark.
The relatively poor image quality and low resolution of reconstructed pelvic images and cone beam CTs often makes it difficult to accurately identify the borders of key landmarks.
As discussed above, rotations and tilts are important sources of potential errors in gynecologic treatments; these are easily missed during review of onboard images that have a limited field of view (Fig. 6.5). These complex positioning errors can also be difficult and time consuming to correct, particularly if the entire spine and pelvis must be brought into alignment; this process can easily exceed the patient’s tolerance.
surrounding the GTV is less if the GTV is surrounded by a larger CTV-PTV that receives only a slightly lower dose than the GTV. This is also the rationale for occasional use of nested GTVs to treat large tumors that are close to critical structures (Chapter 5; CS 10.11, 14.1, 14.8, 15.1, 15.2).
TABLE 6.1 Recommended CTV/GTV/ITV to PTV Margins for Radiation Therapy Planninga
For most studies, a single CT was used to calculate organ doses and volumes. These calculations do not accurately represent the doses delivered to mobile critical structures during a protracted course of radiation treatment.
Inconsistent methods have been used to designate normal tissue volumes.
Results that were based on site-specific multimodality treatments for nongynecologic cancers may not be broadly applicable to gynecologic cancer patients.
Many factors other than dose and volume contribute to the risks of side effects (Chapter 9).
The level of risk that is considered acceptable may vary according to the clinical situation (i.e., the potential benefit of a treatment).