Dose to Organ at Risk and its Characteristic Variation with the Clinically Used Different Prescription Levels for Early-stage Left-sided Breast Cancer.

ABSTRACT

AIMS: To evaluate the organ at risk (OAR) dose and its characteristic variation with different clinically usable prescription doses (RxD) for breast and chest wall radiotherapy in patients with early-stage left-sided breast cancer. MATERIALS AND METHODS: In total, 145 patients with early-stage breast cancers (T1N0M0-T2N0M0) on the left side were treated with radiotherapy after a modified radical mastectomy or breast conservation surgery, with a mean age of 45.1 ± 21.6 years. The patient received 4050 cGy of field-in-field (three-dimensional conformal radiotherapy) treatment limited to the breast or chest wall, excluding the supraclavicular node, axillary node and internal mammary chain, over 15 fractions. Additional plans of 5000 cGy/25 fractions, 4500 cGy/20 fractions and 2600 cGy/5 fractions were created with no or minor changes to the original plan. Mathematical modelling was used to study the distinctive change in the dose-volume characteristics for various OARs as a function of the RxD. OAR dosages, both absolute and normalised, were expressed in terms of the RxD. The mathematical (functional) relationship between OAR doses and different prescription levels was deduced by the least squares fit method. RESULT: The left lung mean dose, V5Gy (%), V10Gy (%) and V20Gy (%) and the heart mean dose, V10Gy (%) and V20Gy (%) were evaluated. The dose-volume parameters showed a parabolic variation (x2) with the RxD. Prescription normalised OAR doses showed a linear relationship with the RxD; relative dose increased with diminishing RxD. Normalised lung and heart mean doses exhibited saturation (linear relationship) with RxD variation. Paired sample t-test results between RxD versus all evaluated parameters were found to be statistically significant (P = 0.004). The Pearson correlation coefficient between different prescription levels for left lung mean dose (range 0.942-1.0), heart mean dose (range 1.0-1.0), left lung V5Gy (%) (range 0.987-1.0), left lung V10Gy (%) (range 0.991-0.999), heart V10Gy (%) (range 0.998-1.0). CONCLUSION: The functional form of absolute OAR dose-volume parameters versus RxD is parabolic and the RxD normalised OAR dose-volume parameter versus RxD is a straight line with a negative slope as RxD increases. This indicates an increase in the relative OAR dose-volume parameters if the RxD is reduced. This study is the first of its kind to compare the OAR doses as a function of clinically used degenerate prescription levels. These data will help to comprehend the OAR doses while adopting a new dose fractionation regimen and reviewing the radiotherapy treatment plans.