Optimising breast dosimetry: Improving homogeneity through the

application of angled IMRT fields

M. Squires (BMRS (RT) MA MIR JP MBA(cand.)) S. Cheers (BAppSc (MRS))

• Significant side effects associated with dose inhomogeneity

• Risk associated with low dose integral splay

• Options exist that maximise dose uniformity and minimise low dose

Background

• The angled segment technique offers two additional clinically feasible protocols

• To give the planner control over homogeneity (HI) and low dose conformity (CI)

Aim

• Twenty previously optimised tangent plans copied

• Single medially angled off IMRT beam appended to the existing beamset

• Plans (n=20) optimised and normalised (PTV V47.5 = 99.00%)

Method 1: Single medial segment

• Additional (laterally) angled off IMRT beam appended

• Plans (n=20) optimised and normalised (PTV V47.5 = 99.00%)

Method 2: Dual segments

• Statistically similar average absolute maximum dose (Dmax 54.55Gy vs. 54.71Gy, p=0.33)

• Reduced V107%(14.71cc vs. 23.17cc, p<0.01)

• Low dose (V1) integral splay volume was maintained (6410.04cc vs. 6402.45cc, p=0.44)

• But, reduced contralaterally (V1 splay over midline 6.60cm vs. 6.80cm, p=0.04)

• Ipsilateral mean lung dose slightly reduced (5.23Gy vs. 5.33Gy, p=0.04)

Results 1: Single medial segment

• Reduced average maximum dose(Dmax 53.79Gy vs. 54.71Gy, p=0.03)

• Reduced V107%(1.90cc vs. 23.17cc, p<0.01)

• Homogeneity improved (HI= 0.11 vs. 0.13, p=0.03)

• Ipsilateral mean lung dose unaffected (5.33Gy vs. 5.33Gy, p=0.48)

• Low dose (V1) integral splay increased by 1.5% (6501.14cc vs. 6402.45cc, p=0.04)

• Low dose (V1) appeared further contralaterally (8.40cm vs. 6.80cm over midline, p=0.02)

Results 2: Dual segments

Comparative results

• The addition of a medial segment serves to partially reduce high dose volumes whilst limiting the increase in low dose (win-win)

• Dual segments serve to fully reduce hotspot size at the expense of increased low dose contralateral splay

• Each technique provides greater scope to customise dosimetry to meet specific patient needs

Conclusions

References & Acknowledgements

ROC Gosford

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