The Benefit of Individualized Custom Bolus in the Postmastectomy Radiation Therapy:Numerical Analysis with 3-D Treatment Planning

Cho, Jae Ho; Cho, Kwang Hwan; Keum, Kichang; Han, Yongyih; Kim, Yong Bae; Chu, Sung Sil; Suh, Chang Ok

Abstract

PURPOSE: To reduce the irradiation dose to the lungs and heart in the case of chest wall irradiation using an oppositional electron beam, we used an individualized custom bolus, which was precisely designed to compensate for the differences in chest wall thickness. The benefits were evaluated by comparing the normal tissue complication probabilities (NTCPs) and dose statistics both with and without boluses.
MATERIALS AND METHODS
Boluses were made, and their effects evaluated in ten patients treated using the reverse hockey-stick technique. The electron beam energy was determined so as to administer 80% of the irradiation prescription dose to the deepest lung-chest wall border, which was usually located at the internal mammary lymph node chain. An individualized custom bolus was prepared to compensate for a chest wall thinner than the prescription depth by meticulously measuring the chest wall thickness at 1 cm2 intervals on the planning CT images. A second planning CT was obtained overlying the individualized custom bolus for each patient's chest wall. 3-D treatment planning was performed using ADAC-Pinnacle3 for all patients with and without bolus. NTCPs based on "the Lyman-Kutcher" model were analyzed and the mean, maximum, minimum doses, V50 and V95 for the heart and lungs were computed.
RESULTS
The average NTCPs in the ipsilateral lung showed a statistically significant reduction (p<0.01), from 80.2+/-3.43% to 47.7+/-4.61%, with the use of the individualized custom boluses. The mean lung irradiation dose to the ipsilateral lung was also significantly reduced by about 430 cGy, from 2757 cGy to 2,327 cGy (p<0.01). The V50 and V95 in the ipsilateral lung markedly decreased from the averages of 54.5 and 17.4% to 45.3 and 11.0%, respectively. The V50 and V95 in the heart also decreased from the averages of 16.8 and 6.1% to 9.8% and 2.2%, respectively. The NTCP in the contralateral lung and the heart were 0%, even for the cases with no bolus because of the small effective mean radiation volume values of 4.4 and 7.1%, respectively.
CONCLUSION
The use of an individualized custom bolus in the radiotherapy of postmastectomy chest wall reduced the NTCP of the ipsilateral lung by about 24.5 to 40.5%, which can improve the complication free cure probability of breast cancer patients.