N. Jornet

Thermoluminescence dosimetry applied to in vivo dose measurements for total body irradiation techniques

BACKGROUND AND PURPOSEnIn total body irradiation (TBI) treatments in vivo dosimetry is recommended because it makes it possible to ensure the accuracy and quality control of dose delivery. The aim of this work is to set up an in vivo thermoluminescence dosimetry (TLD) system to measure the dose distribution during the TBI technique used prior to bone marrow transplant. Some technical problems due to the presence of lung shielding blocks are discussed.nnnMATERIALS AND METHODSnIrradiations were performed in the Hospital de la Santa Creu i Sant Pau by means of a Varian Clinac-1800 linear accelerator with 18 MV X-ray beams. Different TLD calibration experiments were set up to optimize in vivo dose assessment and to analyze the influence on dose measurement of shielding blocks. An algorithm to estimate midplane doses from entrance and exit doses is proposed and the estimated dose in critical organs is compared to internal dose measurements performed in an Alderson anthropomorphic phantom.nnnRESULTSnThe predictions of the dose algorithm, even in heterogeneous zones of the body such as the lungs, are in good agreement with the experimental results obtained with and without shielding blocks. The differences between measured and predicted values are in all cases lower than 2%.nnnCONCLUSIONSnThe TLD system described in this work has been proven to be appropriate for in vivo dosimetry in TBI irradiations. The described calibration experiments point out the difficulty of calibrating an in vivo dosimetry system when lung shielding blocks are used.

Midplane dose determination during total body irradiation using in vivo dosimetry

BACKGROUND AND PURPOSEnDuring TBI techniques an accurate determination of the dose distribution is very difficult when using commercial treatment planning systems. In order to determine the midplane dose, an algorithm was developed based on the use of in vivo dosimetry.nnnMATERIALS AND METHODSnScanditronix EDP-30 diodes were placed at the entrance and the exit surface for in vivo dosimetry. The proposed algorithm was validated firstly in a regular and homogeneous phantom of different thickness with an ionization chamber and TL dosimeters and secondly in an Alderson anthropomorphic phantom with TL dosimeters. In this study, in vivo measurements were evaluated in 60 patients and furthermore, in 20 of them, the midplane dose calculated with this algorithm was compared with the method described by Rizzotti A, Compri C, Garusi GF. Dose evaluation to patients irradiated by 60Co beams, by means of direct measurement on the incident and on the exit surfaces. Radiother. Oncol. 1985;3:279-283.nnnRESULTSnNo differences were found between the two methods. The differences between dose values calculated with both methods and dose values measured with the ionization chamber and TL dosimeters were within +/-22% and +/-4%, respectively, in the regular and homogeneous phantom and within +/-2% in the Alderson phantom. The algorithm was useful in calculating the midplane dose when heterogeneities as lungs were present. Even when partial transmission blocks were used to reduce the dose to the lungs, the algorithm with modified correction factors gave a midplane lung dose in the Alderson phantom within 1.3% of the measurements with TL dosimeters. For 360 patients measurements in each A-P and P-A field, the relative deviations were analyzed between the measured and calculated entrance, exit dose and midplane dose and the prescribed dose, always applying the temperature correction factor. These deviations at the entrance dose were within +/-4%. Greater deviations were found for the exit dose measurements. Deviations larger than +/-10% corresponded in general to obese patients, with a thickness over 25 cm. The relative deviations between the total received and prescribed midplane doses in 60 patients were within +/-3%.nnnCONCLUSIONSnThe results indicate excellent correspondence between the total prescribed and calculated midplane doses using this algorithm while also no significant differences were found when the Rizzotti method was used. Comparison between doses measured with TL dosimeters in the core of Alderson phantom lungs and doses calculated from in vivo measurements showed that the proposed algorithm could be used in the presence of heterogeneities even when partial transmission blocks were used. The temperature correction factor must be applied in order to avoid a 2-3% dose overestimation.

Calibration of semiconductor detectors for dose assessment in total body irradiation

The aim of this paper is to discuss the measurements carried out to implement in vivo dosimetry with EDP-30 diodes in total body irradiation (TBI) techniques. Exit calibrations and calibrations behind cerrobend protection blocks showed the importance of calibrating diodes in all relevant clinical conditions. Special attention was given to calibration of diodes behind cerrobend blocks. Dependence of the calibration factors on the thickness of the shielding blocks was, therefore, studied. This dependence was again studied after adding a wax cap to the diode and when the ionisation chamber was placed at the same depth as the measuring point of the diode. Temperature dependence in diode sensitivity and dependence on accumulated dose for diodes response and for temperature correction factors were also examined.

Improving radiotherapy through medical physics developments

Quality assurance (QA) continues to be at the core of medical physicists work (as also reflected in the number of QA-related abstracts submitted to the 3rd ESTRO Forum). It is an essential component for safe, high quality treatments. Results from clinical trials show that poor quality correlates with poor treatment out- comes (19-21). In parallel with the continuing introduction and development of novel and increasingly complex techniques and technologies, the time needed for QA and dosimetry has increased (2). Clinical medical physicists necessarily spend a great deal of time on routine QA duties, with potential impact on the time available for other relevant areas such as clinical dosimetry, devel- opment and implementation of new techniques, management, and teaching and training (22-23). Time could be optimised if more of the QA was automated. In the 1950s, automation was seen as a new paradigm that would change societys way of working, min- imising routine tasks and freeing more time for creative and higher level work and providing improved work-life balance and quality of life. Unfortunately, more than half a century after the first pub- lications on automation, its application to QA in RT is still very lim- ited. It is also perceived that some of the proposed quality controls (QCs), metrics and tolerance limits proposed are out-dated by RT technology advances or insufficiently effective to detect at least some errors that may have a clinical impact (24-27). When new technology is being implemented, it is crucial to understand how systems and, in particular, treatment units behave in order to iden- tify failure modes and design quality controls capable of detecting any delivery error. Our principal aim is to ensure the patient receives the dose distribution as planned, and if significant differences are found, these should be reported and if possible re-addressed before the end of the treatment. We also need to widen the scope of the assessment of the results of our QC tests and the decisions taken on their basis, moving from a binary evaluation (pass or fail) to an evaluation of trends (temporary or systematic) using groups of data, applying approaches such as Statistical Process Control (28). In short, there is a clear need for innovative, efficient and effective QA methods with potential for automation. We have now reached a crossroads where we have to consider whether the original QA paradigms remain appropriate for the new technologies, techniques, priorities and resource availability. Routine implementation of beam intensity modulation with dynamic treatment techniques such as intensity-modulated RT (IMRT) and volumetric modulated arc therapy (VMAT) has had a considerable influence on QA procedures. International guidelines (29-31) propose specific QCs to ensure safe and accurate delivery

PO-0747: Revisiting guidelines for target definition after prostatectomy when taking MRI study into account

Results: Finally 550 patients with prostate cancer were included, with median age of 70 years old (47-85), Mean follow-up time was 136.8 months, between 5,6 and 245,8 months. D’Amico risk classification distribution was for low risk, mediun and high 20.4%, 36,5% and 43,1% respectively. RCI distribution categories was as follows 61,5%, 21,8 and 16,7%. Survival analysis showed significant differences (p<0.001) between RCI groups at 5 and 10 years. Survival probability was 98,2 and 88,5% ; 95% and 79,6% ; and 52,2% and 8,9% was respectively for each RCI category.

PO-0844: Feasibility of in vivo dosimetry using diodes in breast treatments delivered using a SIB-IMRT technique

Purpose/Objective: Entrance dose in vivo dosimetry (IVD) using diodes is an end-to-end QA procedure that is widely used in conventional radiotherapy treatments. However, in advanced delivery techniques, such as SIB-IMRT, the reliability of this QA procedure is controversial. We developed a reliable QA procedure for breast treatments delivered using a SIB-IMRT technique based on IVD using diodes. Materials and Methods: We calibrated 6-12 MV QED diodes (Sun Nuclear) using an emX electrometer (IBA) in terms of entrance dose at SSD = 90 cm and field size = 10 cm x 20 cm (average conditions for IMRT breast treatments). We measured correction factors for distance, field size, obliquity, and MLC transmission. We planned 14 SIB breast treatments using 6MV x-rays and 7-9 sliding window IMRT fields (TPS: Eclipse, algorithm: AAA v8.9; Varian). Determining entrance dose points is challenging due to fluence modulation, and also to patient contour irregularities, beam obliquity and tissue heterogeneities. To minimize these challenges, we calculated all treatment fields at 0o gantry angle, maintaining the SSD, on a regular water phantom. For each field, we recorded entrance dose (at maximum dose depth) at two points: on the central axis and on the region of highest fluence. Treatments were performed using a Clinac 2100C/D equipped with a Millenium 120 MLC and the RPM system (Varian) for respiratory motion management. We placed one diode at each of the aforementioned points on the patients skin. We assessed clinical uncertainties by placing the diodes at the same position but on a Plastic Water phantom (CIRS), and irradiating them under the conditions used for entrance dose calculations. To ensure positioning accuracy was lower than 0.5 cm, we attached a template which projected a 1 cm square grid at isocenter to the collimator using an add-on. We compared IVD measurements with the calculated entrance doses. Results: The table shows the average difference between measurements and calculations, excluding doses below fixed thresholds. Consistency improved by increasing the threshold because we excluded false positives due to non-significant dose contributions. Although agreement between in-phantom measurements and calculations was excellent, in patients we observed larger differences and variability due to positioning inaccuracies (figure). The table shows the two-fold tolerances which provided >95% of acceptable IVD assessments. Conclusions: IVD using diodes is a feasible QA procedure for breast treatments delivered using a SIB-IMRT technique. We propose using two diodes per field in high-fluence regions (entrance dose >0.2 Gy). We suggest a two-fold tolerance for IVD assessment (95% confidence limits): a dose difference of 15% for both diodes per field, and an average difference of 3.6% including all measurements per patient. This work was funded by grant Barcelona Board of the Spanish Association Against Cancer (AECC) 2012.

EP-1417: Is a decrease in dose rate in a kilovoltage X-ray radiotherapy unit directly linked to tube metallization?

Conclusions: Calibration of diodes for in vivo dosimetry in breast treatments delivered using a sliding window IMRT technique requires non-linear CFs. Setting the calibration geometry close to the average irradiation geometry minimizes the value and uncertainty of CFs. We recommend two or more detectors per field, placed on high-fluence regions, to avoid false-positive warnings due to variations of MLC transmission. This work was funded by a grant from the Barcelona Board of the Spanish Association Against Cancer (AECC) 2012.