**3. Advantages and limitations of the wedge technique in clinical applications**

#### **3.1. Wedge property uncertainty during treatment**

Tumor motion (i.e., intrafractional organ motion) is an important consideration during radiotherapy [24]. Intrafractional motion can be caused by the respiratory, skeletal muscular, cardiac, and gastrointestinal systems. Respiratory motion in particular affects all tumor sites in the thorax and abdomen; the disease of most relevance in this case is lung cancer, as shown in **Figure 3**. Of note, respiratory motion is just one potential source of error in radiotherapy [25]. Chen et al. reported that lung tumor motion varies from 0 to 5 cm [26]; Shirato et al. reported

**Figure 2.** (a) Profile curves and (b) percentage depth-dose (PDD) curves of 10 MV X-ray beams in the treatment planning system (TPS) and measurements. The green line indicates TPS data, the red dots indicate measurement data, and the black line indicates the percentage of error between the TPS and actual measurements.

that the average amplitude of liver tumor motion was up to 1.9 cm [27], while Hamlet et al. reported that the larynx elevates approximately 2 cm while swallowing [28].

Intrafractional organ motion can result in two types of effect. The first is the "dose-blurring effect," which results in the over/under dosage of the tumor with radiation. The second is

**Figure 3.** The change over time of tumor motion in the lung between exhalation and inhalation. The red line indicates the contour of the tumor during exhalation; the tumor moves up and down markedly during breathing.

termed the "interplay effect," which is only a problem in the case of dynamic delivery of intensity-modulated radiation therapy or dynamic treatments with nonphysical wedges. This effect is the result of interplay between the moving tumor and the motion of the radiation beam as defined by the nonphysical wedges [29, 30] and can result in dose discrepancy.

The respiratory-based interplay effects for nonphysical wedges have previously been studied; it was reported that approximately 50% of the organ receives a dose 5–15% higher than that prescribed when the collimator is moved from the caudal to the cranial direction. Conversely, collimator movement in the opposite direction results in under-dosing [29]. Moreover, Kakakhel et al. estimated the interplay effects for nonphysical wedges in a phantom study and reported that more than 90% of the area of the target region was covered by the prescribed dose when the phantom was rested. However, for a moving phantom, less than 70% of the target region was covered by the prescribed dose [24].

For the reasons stated above, nonphysical wedges should be considered with caution before utilization for treatment in cases of respiratory organ motion. On the other hand, physical wedges have limited field sizes, densities, and composition materials; hence, they create more lowenergy electrons and photon-scattering radiation than nonphysical wedges [31]. Furthermore, the dose outside the field using nonphysical wedges is half that of physical wedges [32].

#### **3.2. Appropriate choice whether physical or nonphysical wedge at several irradiation situations**

that the average amplitude of liver tumor motion was up to 1.9 cm [27], while Hamlet et al.

**Figure 2.** (a) Profile curves and (b) percentage depth-dose (PDD) curves of 10 MV X-ray beams in the treatment planning system (TPS) and measurements. The green line indicates TPS data, the red dots indicate measurement data, and the

Intrafractional organ motion can result in two types of effect. The first is the "dose-blurring effect," which results in the over/under dosage of the tumor with radiation. The second is

reported that the larynx elevates approximately 2 cm while swallowing [28].

black line indicates the percentage of error between the TPS and actual measurements.

256 Radiotherapy

The choice of physical vs. nonphysical wedges is critical in several clinical situations. As mentioned above, nonphysical wedges have more liabilities than physical wedges for the treatment of moving tumors. In contrast, physical wedges create more secondary radiation than nonphysical wedges. Petrovic et al. reported that the peripheral dose of the nonphysical wedge field is half that of the physical wedge field; this is owing to scatter outside the physical wedge field that arises from the interaction of the beam with the material of the physical wedge (such interactions include Compton scattering).

Clinically, this provides an advantage to the nonphysical wedge field [32]. The effect of secondary radiation outside the field is an important consideration for breast cancer treatment. For example, **Figure 4** shows how the low-dose area was expanded to the opposite breast when using physical wedges; such secondary radiation exposure may precipitate the development of another tumor. Warlic et al. reported that the average dose outside of the field with a nonphysical wedge was 2.7–2.8%, whereas the dose was 4.0–4.7% with a physical wedge. The nonphysical wedge is hence a practical advance that improves the dose distribution in patients undergoing breast conservation while simultaneously minimizing the dose to the contralateral breast, thereby reducing potential carcinogenic effects [33].

**Figure 4.** The dose distributions of radiotherapy in a breast cancer patient using (a) physical wedges or (b) nonphysical wedges. Each line indicates the dose corresponding to each treatment intensity planning.

Nonphysical wedges have significant benefits for both the therapists and patients. Saminathan et al. reported that the number of MUs used to deliver a particular dose using a nonphysical wedge field is less than that used for a physical wedge field [2]. Moreover, Njeh reported that using nonphysical wedges results in significant dose reductions to areas outside of the treatment field [34]. The reduction of MUs can also result in minimizing treatment times; this benefits patients who have worse performance statuses.
