TWO FIELD BREAST PLAN VS. OPTIMIZED CONFORMAL BREAST PLAN: COMPARISON OF PLAN PARAMETERS Authors: Borko Basarić, Ozren Čudić, Milan Teodorović, Borislava Petrović, Milutin Baucal, Laza Rutonjski, Ljubica Smiljanić Institute of Oncology Vojvodina, Radiotherapy department, Sremska Kamenica, Serbia

1. Introduction Breast cancer EBRT (Linac 6 MV, TPS, delineation - ICRU 50, planning, MLC..,) Basic 2 field tangential breast plan Optimized conformal breast plan (with additional small fields) OARs (Organs at risk) DVH (Dose-Volume Histogram)

Basic 2 field tangential breast plan

Optimized conformal breast plan (with additional small fields)

2. Materials and methods CT scans / 10 patients planned Restrictions: At least 95% of target volume covered with % of prescribed dose Small fields (5-10% of the prescribed dose) Planning >> Geometry, dose weighting, weight points, wedge angles, MLC, jaw positions, EDW DVH (comparison) results a) Plan comparison

2. Materials and methods b) Stability and reproducibility of accelerator output Dosimetric verification of small number of monitor units Farmer ionization chamber, water phantom and electrometer used

2. Materials and methods a) Farmer ionization chamber b) Electrometer (Scanditronix Wellhofer) c) Water phantom a) b) c)

2. Materials and methods Standard conditions 10 x 10 field size, SSD (Source-to-skin distance) of 100 cm and energy of 6MV (linear accelerator type: Varian 600 DBX

3. Results DVH comparison Average of 7-10% better dose coverage with additional small fields Better dose homogeneity with additional small fields No cold spots (with additional fields) Hot spots (with additional fields) up to 107% a) Plan comparison

DVH for basic plan (78.13% of CTV irradiated with 95% dose)

DVH for plan with additional small fields (97.18% of CTV irradiated with 95% dose)

3. Results MeasurementQ [nC] for (N x 5 MU); N=20 Q [nC] for (N x 10 MU); N=10 Q [nC] for (1 x 100 MU); N= Measured absolute charge for 100 MU in steps of 20 x 5,10 x 10 and 1 x 100 MU. Note that 400 MU equals 1 minute time of irradiation on linac. b) Stability and reproducibility of accelerator output

3. Results Dosimetric verification has shown that there were no significant differences in output for small fields and small number of MU (20x5 MU and 10x10 MU), in comparison to standard machine calibration output (1x100 MU).

4. Conclusion Study suggests that for the breast EBRT treatment, conformal optimized plan with additional fields has an advantage over basic two tangential field plan (dose reduction to organs at risk, increased target dose conformity, reduction of total number of MU in some cases and total body scatter radiation). It must be noted that additional fields are applied only if there are under-dosed regions in target volume that can not be irradiated when applying the basic two tangential field plan. It has been calculated that mean relative error of overall absolute charge measured in standard conditions for medical linear accelerator is less than 1% thus stability and reproducibility of accelerator dose output has been verified for this purpose.

References S. Webb - The Physics of Three Dimensional Radiation Therapy: Conformal Radiotherapy, Radiosurgery and Treatment Planning, 1997, Institute of Physics Pub F. M. Khan - The Physics of Radiation Therapy, 3rd edn, Lippincott, Williams and Wilkins, Baltimore, MD (2003). A. M. Gaya, R.F.U. Ashford – Cardiac complications of radiation therapy – Clinical Oncology, Volume 17, Issue 3, May 2005, pages ICRU (International Commission on Radiation Units and Measurements) report 50 – Prescribing, Recording and Reporting Photon Beam Therapy, ICRU (International Commission on Radiation Units and Measurements) report 62 – Prescribing, Recording and Reporting Photon Beam Therapy, IAEA TRS-398 – Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water, 2004.

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