Normal tissue complication probabilities (NTCP) for modified reverse hockey stick technique (MRHS)

Abstract

Background

Several treatment techniques are used for irradiation of patients with breast cancer after mastectomy. There is no one technique accepted as the “gold standard”. In the Holycross Cancer Centre a novel technique – the modified reverse hockey stick technique – is used.

Aim

Evaluation of the risk of heart and lung injury in patients treated with the MRHS technique. Comparison of the risk for MRHS and tangential techniques.

Materials/Methods

The 3D CT based dose distributions for 25 left-sided and 25 right-sided patients after mastectomy were calculated. For each patient before the NTCP was calculated all physical doses were converted into biological doses according to an α/β model with an α/β value of 3Gy. The NTCP for the lung was for each patient calculated with generalized Lyman model with two parameters: the biological mean dose and the volume above a biological threshold dose of 13Gy (V13). For the heart the NTCP was calculated using the seriality model. The parameters of the models were taken from the literature. For the heart, for each patient the partial volume of the heart receiving more than 30Gy (V30) was also obtained. The correlation between NTCP and V30 for the heart and between the mean physical dose and the mean biological dose for the lungs were determined.

Results

For all left-sided patients but two for the MRHS technique the NTCP for the heart was smaller than 0.01. For tangential technique, the calculated risk of heart injury was higher. A very high correlation between V30 and NTCP was obtained for both techniques. Larger NTCPs were obtained for both techniques for patients treated on the right side. About two times higher values of NTCP were obtained if calculated with the V13 parameter. Based on the mean dose on the left side NTCP is always smaller than 0.05. For some patients treated on the right side, NTCP exceeded 0.1. NTCP values for patients treated either on the left or the right lungs for both techniques were similar. There was a high correlation between the mean physical and biological doses.

Conclusions

For left sided-patients, the MRHS technique is safe for the heart and is superior to the tangential technique. The risk of lung injury is higher for patients treated on the right side. Regarding the risk of lung injury, there is no difference between the two techniques. There is a very high correlation between NTCP and V30 for the heart and between the mean physical and biological doses for the lungs.