A retrospective physical and radiobiological dose evaluation in field-in-field technique and step-and-shoot intensity modulated radiotherapy to organs-at-risk in left-breast cancer

Abstract

Background: Breast cancer has become the most dominant cancer affecting women in Hong Kong since 1993according to the Hong Kong Cancer Registry. Different radiotherapy (RT) techniques such as field-in-field (FIF) and step-and-shoot intensity modulated radiotherapy (IMRT) have been adopted to treat breast cancer in the Hong Kong Baptist Hospital. For radiation-induced complications after RT to left breast being a great concern, a suitable RT technique should be selected from the current practice to minimize the complications. Objectives: This study aims to retrospectively evaluate the physical and the radiobiological dose together with normal tissue complication probability (NTCP) of organs-at-risk (OARs) in left-breast cancer patients who have been respectively treated with FIF or step-and-shoot IMRT techniques. Method: RT treatment plans of twenty-two patients who had received lumpectomy or mastectomy, treated with1cm multi-leaf collimator (MLC) on left breast/chest wall, supraclavicular fossa (SCF) and ipsilateral internal mammary chain (IMC) from January 2011 to June 2015, were retrospectively evaluated. OARs including heart, independent lung, spinal cord, contralateral breast, 2mm and 3mm skin rind were contoured. Various physical point doses and volume doses recommended by ICRU Report 50 and 62 of different OARs were collected and analyzed. The NTCP of heart and lungs using pericarditis and pneumonitis as end point were also calculated by a modified program in excel VBA format based on Gay and Niemierko’s model. All these data were analyzed and evaluated by statistical method. Results: FIF produced less radiation dose to different OARs than IMRT did. The V5,V10,V20 and Dmean of heart was significantly lower in FIF cases. The overall dose received by right lung in IMRT were higher than that in FIF, with p-value <0.05. For left lung and whole lung, both the V5 and V10were significantly higher in IMRT cases, but the V20-V50 were significantly higher in FIF cases. The Dmax of spinal cord and the Dmean of contralateral breast were significantly higher in IMRT cases, while Dmax of contralateral breast were similar. No significant difference was found in Dmax of 2mm skin rind but Dmax of 3mm skin rind in IMRT was significantly greater than that in FIF. Yet, as the calculation uncertainty of skin dose from treatment planning system could be up to ±25%, no conclusion on the comparison of skin dose from both techniques could be made. Although the calculated generalized Equivalent Uniform Dose (gEUD) of heart was significantly greater in IMRT cases, the NTCP values of heart calculated by Niemierko’s gEUD-based NTCP model and LKB model in both IMRT and FIF groups were 0% .The NTCP of left lung was comparable in both two groups while those of right and whole lung were slightly higher in IMRT group. Conclusion: FIF techniques generally produced lower dose to various OARs and thus should produce less cardiac and pulmonary complication risk than IMRT. Yet, IMRT still has its role in treating breast cancer in our hospital. The choice of the technique for breast cancer should not only depend on dosimetric performance on OARs but also the patients‟ conditions and needs.