Accelerated IMRT with concomitant boost after breast-conserving therapy (BCT): Preliminary clinical results and dose volume histogram (DVH) analysis

Abstract

outcomes based on type of boost used (electron vs. photon vs. implant). We evaluated 4 different photon beam arrangements for treatment of tumor beds too deep to boost with electrons to determine optimal approach in regards to conformity of dose distribution and exposure of normal tissue. Materials/Methods: Thirty patients with early stage breast cancer treated with segmental mastectomy and postoperative external beam irradiation whose tumor beds (CTV) measured 3.5 cm in depth were prospectively identifi ed for a1 6 Gy photon boost following standard whole breast radiation to 45 Gy. The lumpectomy scar was wired in all patients and the tumor bed was uniformly outlined from the planning CT. The CTV included the seroma beneath the scar down to the pectoralis muscle. Four photon boost plans were generated per patient using 2, 4, 5, and 6 fields. All field apertures were generated using a 2 cm margin around the CTV, with the exception of an appositional beam in the 5 field plan, which use da1c mmargin. All plans utilized noncoplanar beam arrangements with the exception of the 2 field plan. The dose distribution was evaluated using a conformity index defined as VCTV/V95, where V95 is the volume of breast covered by the 95% isodose line and VCTV is the volume of the CTV. The amount of normal breast tissue encompassed per plan (Vnt95) was calculated by subtracting VCTV from V95. Isodose distributions and DVHs were compared for ipsilateral lung and heart in all patients. Results: Mean tumor bed depth was 5.5 cm (range, 3.5‐8.9 cm). The mean and median CTV volume were 101 cc and 84 cc. When stratified by tumor bed volume ( vs. 100 cc), Vnt95 was significantly greater for patients with larger tumor beds (p 0.0001). The mean and median conformity indices were 0.31 and 0.29 (range 0.08 - 0.68). The conformity indices of the 4 and 6 field techniques were significantly superior to the 2 field technique (p 0.0003 and p 0.0001, respectively), but the difference between the 4 and 6 field techniques was not significant (p 0.27). Treatment with a 5 field technique provided the most conformal distribution of dose, with a mean conformity index of 0.41 as compared to 0.27, 0.29, and 0.28 for 2, 4 and 6 fields respectively, (p 0.0001). The Vnt95 was also significantly reduced with the 5 field beam arrangement (p 0.0001). This technique, however, exposed more normal tissue to a low dose of radiation with a mean heart and lung dose of 43 cGy (p 0.0001) and 67 cGy (p 0.0001), respectively. Treatment with either a 2, 4, or 6 field beam arrangement produced similar DVHs in the heart and lung, with mean doses of 17, 17, and 17 cGy (p 0.75) and 41, 42, and 42 cGy (p 0.21), respectively. Conclusions: For patients whose tumor beds are too deep to treat adequately with electrons, partial breast irradiation utilizing a4 field photon technique can provide conformal coverage while minimizing exposure of normal tissue. For patients with larger tumor beds, a 5 field beam arrangement may reduce the amount of normal breast tissue encompassed within the boost, with only minimal additional dose to the heart and ipsilateral lung.