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Is Non-Coplanar Tangential Beam IMRT Better Than the Field in Field Technique for Left Sided Whole Breast Radiotherapy?

H Sidhique¹*, M Madebo², T Kron³, J Cramb⁴, B Chua⁵, M Durai⁶, (1) Continental Hospitals, Hyderabad, India, (2)Dept of Physical Sciences, Peter MacCallum Cancer Centre, Melbourne,(3)Dept of Physical Sciences, Peter MacCallum Cancer Centre, Melbourne,(4)Dept of Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, ,(5) Dept of Radiation Oncology, Breast Services, Peter MacCallum Cancer Centre, Melbourne,6) Shidhivinayaka Cancer Centre, Miraj,

SU-E-T-621 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To study the comparison of 4 field non-coplanar tangential beam IMRT (nctbIMRT) with FIF technique for early stage left-sided, instead widely used comparison of coplanar IMRT with FIF.

Methods: Retrospective study was done on 10 patients left-sided, intact, whole breast RT. Followings are the 2 simple factors:d'and Δd, where d'-distance between skin and posterior border of PTV and Δd-distance between posterior border of PTV and heart at largest heart and breast CT- slice.Both are on line that passes though center of heart. With these, patients were classified into 4 classes based on PTV size and distance of heart from PTV as follows: Class 1: large PTV, close heart; 2: large PTV distant heart; 3 small PTV, close heart; 4: small PTV, very close heart.

Results:For patients with heart close to PTV, nctbIMRT showed better PTV coverage and better organs at risk sparing. The average D₉₅ of PTV and D₅ of heart for FIF were found to be 96.2% and 83.9%, 95.1% and 8.27%, 92.38% and 55.3% and 92.7% and 46.8% for class 1, 2, 3 and 4, respectively. For nctbIMRT, D₉₅ of PTV and D₅ of heart were 99.7% and 15.6%, 97.25% and 9.3%, 97.42% and 29.8% and 96.8% and 35% for class 1, 2, 3 and 4, respectively.

Conclusion: Unlike coplanar IMRT, nctbIMRT containing tangential beams, as shown in study, is advantageous in that it reduces volumes receiving low dose in critical structures.This method of classification is more realistic than making generalisation based on the average dosimetric parameters of all patients considered. While all patients in Class 1, 3 and 4 are beneficiaries of the nctbIMRT technique, patient in Class 4 presenting with heart adjacent to PTV benefited the most. On the other hand, FIF may be preferable for Class 2 patients with distant heart to the PTV.

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