Positional Stability in Breast Cancer Radiotherapy: Assessing the Effect of Thermoplastic Immobilization Casts

Abstract

Purpose: Adjuvant external beam radiotherapy (EBRT) plays a pivotal role in the management of breast cancer by reducing the risk of local recurrence and improving disease-free survival following surgery. Accurate radiation delivery depends on reproducible patient positioning throughout treatment. Thermoplastic immobilization casts are commonly used to enhance setup reproducibility; however, their routine use remains controversial. This study aimed to evaluate the effect of thermoplastic immobilization casts on positional stability during adjuvant EBRT for breast cancer patients.

Methods: This prospective comparative study included 30 patients with histologically confirmed breast cancer who had undergone either modified radical mastectomy (MRM) or breast-conserving surgery (BCS) and subsequently received adjuvant radiotherapy between May 2023 and December 2024. Patients were randomly assigned to two treatment arms. In Arm A, simulation and treatment were performed using thermoplastic immobilization casts, whereas in Arm B, patient positioning relied solely on skin markings without cast immobilization. Daily cone beam computed tomography (CBCT) was used for image-guided verification and correction of setup errors along the X (lateral), Y (longitudinal), and Z (vertical) axes before each treatment fraction. Positional variations were recorded and compared between the two groups.

Results: A statistically significant reduction in lateral (X-axis) setup variation was observed in patients treated with thermoplastic casts compared with those treated without casts (p = 0.02). No significant differences were identified in the Y- or Z-axis setup variations between the two groups. Within Arm B, patients who underwent breast-conserving surgery exhibited significantly greater longitudinal (Y-axis) variation than those treated with modified radical mastectomy (p = 0.04). This difference was not observed in Arm A. In the non-cast group, body weight and body mass index (BMI) demonstrated moderate positive correlations with X-axis variation (r = 0.48 and r = 0.50, respectively), suggesting increased lateral setup deviations among heavier patients. No significant correlations were identified in the cast group.

Conclusion: Routine use of thermoplastic immobilization casts may not be necessary for all patients undergoing adjuvant breast radiotherapy. However, their use appears beneficial for patients undergoing breast-conserving surgery and for those with a higher BMI, as they improven setup reproducibility and may enhance treatment accuracy.