TLD POSTAL DOSE QUALITY AUDIT FOR 6MV AND 15MV PHOTON BEAMS IN RADIOTHERAPY CLINICAL PRACTICE Sonja Petkovska 1, Margarita Ginovska 2, Hristina Spasevska 2, Nadica Dimitrovska 1, 1 University Clinic for Radiotherapy and Oncology, Skopje, RM 2 Faculty of Electrical Engineering and Information Technologies, Ss Ciril and Metodious University, Skopje, RM RAD 2012, April, Nish

Radiation therapy is the modality of choice for about half of the cancer patients at some stage during their treatment. Radiotherapy aim:  delivering as high dose as possible at the tumor volume  spearing the health tissue as much as possible. INTRODUCTION RAD 2012, April, Nish

INTRODUCTION  Delivered dose should provide the highest probability to cure the patient without inducing severe complications in normal tissues.  Delivered dose should be equal to prescribed one TCP – Tumor Control Probability NTCP – Normal Tissue Complication probability TSP – Probability for Succesfful Treatment RAD 2012, April, Nish

INTRODUCTION QA programme  Mechanical checks provide geometrical accuracy  Dosimetry checks provide dose distribution accuracy  Beam calibrations provide prescribed dose accuracy RAD 2012, April, Nish

INTRODUCTION External comparison with an independent dosimetry laboratory  Check the dose delivered by radiotherapy unit  Comparison in different beam parameters in clinical practice  Allows detecting dosimetric errors RAD 2012, April, Nish

ESTRO QUALity Assurance network (EQUAL) Compare beam parameters measured in the same conditions in different radiotherapy centers RAD 2012, April, Nish 2Gy

Thermoluminescent Dosimeters (TLDs) with lithium fluoride powder (LiF) MATERIALS AND MEASUREMENTS RAD 2012, April, Nish TLD holder Water phantom

MATERIALS AND MEASUREMENTS RAD 2012, April, Nish 4 TLDs per unit are irradiated one by one 2 for 6MV photon beam 2 for 15MV photon beam geometry setup for TLD irradiation

MATERIALS AND MEASUREMENTS RAD 2012, April, Nish D = 2Gy at reference depth 6MV photon beam299MU 15MV photon beam251MU

MATERIALS AND MEASUREMENTS RAD 2012, April, Nish Dcalc is TPS calculated dose Dm is measured dose with ionisation chamber beam (MV) Dcalc (Gy)Dm (Gy)  (%)

RESULTS RAD 2012, April, Nish The levels of deviation between measured and stated quantities (Qm/Qs) and the corresponding EQUAL actions are specified as follows: Qm/Qs < 3% optimal level 3% < Qm/Qs < 5% tolerance level 5% < Qm/Qs < 10% a level outside tolerance level Qm/Qs > 10% an emergency level

RESULTS RAD 2012, April, Nish beam (MV)  min (%)  max (%) Deviation for high energy photon beam s

CONCLUSION This evaluation shows significant absorbed dose reproducibility which confirmed the capability of the linear accelerator to deliver real treatments with an acceptable stability. RAD 2012, April, Nish The results from ML show that the relative deviation Qm/Qs for these analyzed 6MV and 15MV photon beams, do not exceed 3%.

Dose accuracy is just one segment of external quality procedures… CONCLUSION The purpose of this TLD audit is to check the dose delivered by radiotherapy unit, so this independent and trustworthy verification of dose delivery improve internal QA and help to detect some dosimetric errors. RAD 2012, April, Nish