oa Indications for Intensity Modulated Radiation Therapy (IMRT) for the Treatment of Large Left Breast Volumes

Purpose

The purpose of our study is to determine indications for the use of an IMRT technique for large volume size left breast cancers.

Materials/methods

We ran a retrospective review of Stage 0-III left breast cancers with large volumes treated with conservation therapy from April 2011 to January 2012 at the Radiation Oncology Department at the National Center for Cancer Care and Research in Doha, Qatar. Computer tomography simulation was used to design fields. Patients were treated supine and received 2 Gray (Gy) fractions to 50 Gy to the whole breast followed by an electron or 3D boost of 16 Gy using 6, 15 or mixed 6/15 megavoltage photons. A variety of techniques including electronic compensation (E-comp), field in field (FinF), and 3D with wedges (3DW) were compared. Dosimetric evaluations were made of the breast planning target volume (PTV), lung, heart and contralateral breast for each technique. RTOG skin toxicity grades, treatment data, and breast volumes were obtained by chart and treatment plan review. We further continued dose volume data evaluation using the following dosimetric parameters: uniformity index (UI), conformity index (CI) and homogeneity index (HI) parameters and radiobiological models to further assess our results.

Results

A total of 30 patients were treated, and all patients received chemotherapy. Volumes ranged from 718–3296 cc (ave.1483.33 cc). Separations ranged from 20–35 cm (ave. 23.87 cm). During treatment there were 63% Grade 1, 37% Grade 2, 0% grade 3/ 4 RTOG skin toxicity; no treatment breaks recorded. E-comp plans resulted in better coverage of the 95% volume (V95) coverage with improved dose homogeneity of the PTV. This was seen especially in breast volumes > 2400 cc. Also, there was a reduction in V110 and V115 in the FinF and Ecomp plans compared to conventional 3DW technique. In terms of dose to the contralateral breast Ecomp had a slight advantage (1%). No significant difference seen in the LT lung V20 and Heart V25 and Contralateral Breast V5 between Ecomp and FinF which was better than 3DW. The average dose coverage values of PTV with ecomp and FinF plans were comparable, and achieved an overall better target coverage than 3D plans. Forward planning IMRT gave higher conformity index values of 0.66 ± 0.07 and 0.67 ± 0.07 for ecomp and FinF plans respectively as compared to 0.65 ± 0.07 for 3D-CRT plans. The Homogeneity (HI) and Uniformity Index (UI) values favored ecomp and FinF values compared to 3DCRT with average HI values of 0.27 ± 0.16 for ecomp, 0.31 ± 0.14 for FinF, and 0.38 ± 0.33 for 3D-CRT. And uniformity index values of 1.11 ± 0.04, 1.13 ± 0.05 and 1.26 ± 0.39 for ecomp, FinF, and 3D respectively.

Conclusions

For women with large breast size Ecomp planning significantly improved dose homogeneity decreased acute skin toxicity and less hot spot value. These factors grow more important in women with large breasts, who may experience more Grade 3/4 skin toxicity and increased pain resulting in a lower quality of life with standard tangential fields. Therefore, we suggest using an Ecomp technique for left-sided breast cancer in the following situations: 1) breast volume >1500cc 2) separation of >25 cm or 4) combination of large volume/cup size with separation > 22 cm or vice versa, 5) the use of mixed beams. For patients with large breast volumes supine IMRT treatments can provide good dose homogeneity, spare dose to critical structures, and may be preferable to the prone breast irradiation. Conformity Index, Homogeneity Index and Uniformity Index are objective tools for evaluating dose coverage for breast planning. This study showed that a better target coverage for ecomp and FinF plans over 3D-CRT with a slight advantage of ecomp over FinF with large breast volumes. Furthermore, we will assess the potential advantage of IMRT over 3D planning using radiobilogical models.