EFFECTS OF TREATMENT SETUP VARIATION ON BEAMS EYE VIEW DOSIMETRY FOR RADIATION-THERAPY USING THE MULTILEAF COLLIMATOR VS THE CERROBEND BLOCK

Purpose: The purpose of this study is to quantify and compare retrospe ctively the effects of treatment setup variation on beam's eye view (B EV) dosimetry for radiation therapy using a multileaf collimator (MLC) vs. cerrobend block. Methods and Materials: A study was performed on a group of 18 patients with cancer of the head and neck, lung, and pel vis who were treated with irregularly shaped fields. The BEV dosimetry of the fields shaped with cerrobend blocks and the MLC was measured w ith films at the depth of dose prescription in a solid water phantom. A ''one-half-leaf'' insertion convention was used to shape the MLC. In addition, an average of 15 sequential daily port films was taken per patient during the course of radiotherapy. The port films were aligned with the prescription film for each patient, Systematic error and ran dom error of treatment setup for each patient were calculated. The eff ects of setup variation were incorporated by convolving the patient po rtal imaging data with the corresponding BEV film dosimetry. Two param eters were used to quantify the BEV dosimetry. First, the field penumb ra width was calculated, which represented the average of the normal s eparations between 20 and 80% isodose lines along the prescription out line. Second, the ratio of areas covered by the 90 and 20% isodose Lin es, A90/20, was determined. The BEV dosimetry was then characterized w ith and without the effects of treatment setup variation. In addition, the difference in BEV dosimetry between the cerrobend block and the M LC was used to estimate the corresponding changes in tumor control pro bability (TCP). These changes were also compared to the changes in TCP for the treatment with or without the effects of random setup variati on. Results: With or without daily setup variation, the use of cerrobe nd block was more favorable than the MLC in terms of the field penumbr a width and A90/20 for all treatment sites. In the absence of daily va riation, the MLC field penumbra width was on average 1.3 mm larger tha n that of the cerrobend block, and 0.9 mm larger in the presence of da ily setup variation. Similarly, the ratio A90/20 of the cerrobend bloc k was on average 0.03 larger than that of MLC without daily setup vari ation, and 0.02 with daily setup variation. The difference in field pe numbra width and A90/20 between the MLC and the cerrobend block was sl ightly reduced due to the effects of daily setup variation. For both t he cerrobend block and the MLC, daily setup variation produced a signi ficant increase in the held penumbra width, 2.3 mn for the cerrobend b lock and 1.9 mm for the MLC, and a decrease in the A90/20, 0.06 for th e former and 0.05 for the latter. The change due to the daily setup va riation was about a factor of 2 larger than the changes due to replaci ng the cerrobend block with the MLC. Using the TCP model, the change i n TCP due to the daily setup variation was more than a factor of 3 lar ger than the change in TCP due to replacing the cerrobend block with t he MLC. It was noted that the average changes in the penumbra, the A90 /20 and the TCP calculated for the patient population did not adequate ly describe the changes for the individual patient. Conclusion: Our re sults do not show significant dosimetric differences between the MLC a nd the cerrobend block in conventional radiation treatment, whether or not daily setup variation was taken into consideration. The effects o f daily setup variation alone produced a larger dosimetric change. The same results were obtained when the data were applied to calculate ch anges in TCP. For optimal radiation therapy, efforts should be concent rated on reducing daily setup variation. Our results also demonstrate the importance of frequent evaluation of MLC treatment using electroni c portal imaging devices.