Surface dose prediction and verification for IMRT plans using line dose profiles † Ronald E. Berg, † Michael S. Gossman and ‡ Stephen J. Klash † Erlanger Medical Center, Department of Radiation Oncology, 975 East 3 rd Street, Chattanooga, TN and ‡ SJK Physics, Radiation Oncology, 2209 Ranch Road, Sachse, TX 75048

Abstract MOSFETs are used to verify the entrance dose for conventional radiation therapy patient treatments. For sliding window IMRT treatments a combination of fluence patterns are used to provide an integral dose to the target tissue. This renders predicting measurements at the skin surface more difficult because of substantial dose gradients in each beam. The objective was to find a method of predicting MOSFET measured entrance dose and test its applicability in the clinic.

Materials Helios CadPlan IMRT software Varian 2300C/D with MLC using 6 MV and 25 MV x-rays Thomson-Nielson Electronics Limited standard MOSFET system

This Research We report a planning methodology using line-dose profile for determining surface dose, operating with Helios CadPlan IMRT software Dose delivery to patient was confirmed, when surface dose from plan was compared to the MOSFET reading during sliding window IMRT actual treatment Response under energy specific bolus is indicated for 59 patients Both 6MV x-rays and 25MV x-rays were used in this data Reasonable agreement from MOSFETs validate approach to IMRT surface dose determination.

Other Published Research Mutic et.al. in 2000 reported under estimation to be 15 % in the dose to the near surface (3-6mm) during IMRT using PEACOCK 1.12 software (NOMOS Corp), x-ray energy was 6MV, TLDs used including 3mm bolus in a cylindrical phantom, serial tomotherapy delivery Dogan et.al. in 2003 reported that the convolution superposition algorithm in FOCUS inverse planning software (CMS) overestimated dose to surface of polystyrene phantom by 25% and to the first millimeter by 5%, pp-ionization chamber and TLDs comparable, 6MV x- rays used with single field IMRT comparisons

Other Published Research Jones et.al reported that by applying a max/min range to PTV detector structure volumes deep in a NOMOS phantom, using CORVUS software, that IMRT doses were within 7 % of that measured with MOSFETs. TLD response also compared were typically worse, IMRT plan delivery with 6MV x-rays only.

References Mutic S and Low DA. Superficial doses from serial tomotherapy delivery. Med Phys. 2000;27(1): Dogan N, Glasgow GP. Surface and build-up region dosimetry for obliquely incident intensity modulated radiotherapy 6 MV x rays. Med Phys. 2003;30(12): Jones AO and Kleinman MT. Patient setup and verification for intensity-modulated radiation therapy (IMRT). Med Dos 2003;28(3):

Methods Create structure on patient surface to resemble bolus Calculate dose for IMRT plan Use tools to draw a line from isocenter through bolus for a line-dose-profile Determine dose from profile to the surface beneath the bolus, where the MOSFET would lie Treat patient with the MOSFET and bolus in place and record the MOSFET reading Quantify differences between MOSFET reading and plan

Bolus creation on plan

Using line-dose-profile tool Stop point Start point

Line-dose-profile from plan Dose percentage is on Y axis Distance from start point to stop point (in mm) is on X axis Drastic decline

Extrapolation of surface dose Helios software does not do this for you. The extrapolation shown here represents the manual operation performed on plot printouts. Extrapolation to surface

In vivo measurement

Data (measurements 1-18)

Data (measurements 19-40)

Data (measurements 41-61)

Discussion Tabulated data was presented for 61 MOSFET measurements made on 59 IMRT patients. A total of 57 measurements (93%) were within 10 cGy of the predicted value. The remaining 4 measurements had large predicted values ranging from 157 cGy to 224 cGy, and still fell within 10 % of the expected value. The distribution of percentage difference between the measured values and planned values was approximately centered about zero.

Conclusion Previous research indicates the importance of reinterpreted surface doses in different types of currently used software for IMRT We show measurements made at skin surface agree reasonably well with Helios software predictions when bolus is included at the time of measurement The method of extrapolating a line-dose-profile to the skin surface is a useful technique for predicting in vivo MOSFET dose measurements. This serves as additional verification of the IMRT plan in addition to other necessary quality assurance prior to commencement of treatment.