Bradley Kramer

A non-invasive immobilization system and related quality assurance for dynamic intensity modulated radiation therapy of intracranial and head and neck disease.

PURPOSE To develop and implement a non-invasive immobilization system guided by a dedicated quality assurance (QA) program for dynamic intensity-modulated radiotherapy (IMRT) of intracranial and head and neck disease, with IMRT delivered using the NOMOS Corporations Peacock System and MIMiC collimator. METHODS AND MATERIALS Thermoplastic face masks are combined with cradle-shaped polyurethane foaming agents and a dedicated quality assurance program to create a customized headholder system (CHS). Plastic shrinkage was studied to understand its effect on immobilization. Fiducial points for computerized tomography (CT) are obtained by placing multiple dabs of barium paste on mask surfaces at intersections of laser projections used for patient positioning. Fiducial lines are drawn on the cradle along laser projections aligned with nasal surfaces. Lateral CT topograms are annotated with a crosshair indicating the origin of the treatment planning and delivery coordinate system, and with lines delineating the projections of superior-inferior field borders of the linear accelerators secondary collimators, or with those of the fully open MIMiC. Port films exposed with and without the MIMIC are compared to annotated topograms to measure positional variance (PV) in superior-inferior (SI), right-left (RL), and anterior posterior (AP) directions. MIMiC vane patterns superposed on port films are applied to verify planned patterns. A 12-patient study of PV was performed by analyzing positions of 10 anatomic points on repeat CT topograms, plotting histograms of PV, and determining average PV. RESULTS AND DISCUSSION A 1.5+/-0.3 mm SD shrinkage per 70 cm of thermoplastic was observed over 24 h. Average PV of 1.0+/-0.8, 1.2+/-1.1, and 1.3+/-0.8 mm were measured in SI, AP, and RL directions, respectively. Lateral port films exposed with and without the MIMiC showed PV of 0.2+/-1.3 and 0.8+/-2.2 mm in AP and SI directions. Vane patterns superimposed on port films consistently verified the planned patterns. CONCLUSION The CHS provided adequately reproducible immobilization for dynamic IMRT, and may be applicable to decrease PV for other cranial and head and neck external beam radiation therapy.

Dosimetric comparison of stereotactic radiosurgery to intensity modulated radiotherapy

To compare the dosimetry achievable with an intensity modulated radiotherapy (IMR) system to that of stereotactic radiosurgery (SRS) for an irregularly shaped moderate size target. A treatment plan was selected from 109 single fraction SRS cases having had multiple non-coplanar arc therapy using a 6 MV linear accelerator fitted with circular tertiary collimators 1.00 to 4.00 cm in diameter at isocenter. The CT scan with delineated regions of interest was then entered into an IMR treatment planning system and optimized dose distributions, using a back projection technique for dynamic multileaf collimator delivery, were generated with a stimulated annealing algorithm. Dose volume histograms (DVH), homogeneity indices (HI), conformity indices (CI), minimum and maximum doses to surrounding highly sensitive intracranial structures, as well as the volume of tissue treated to > 80, 50, and 20% of the prescription dose from the IMR plan were then compared to those from the single isocenter SRS plan used and a hypothetical three isocenter SRS plan. For an irregularly shaped target, the IMR plan produced a HI of 1.08 and CI of 1.50 compared to 1.75 and 4.41, respectively, for the single isocenter SRS plan (SRS1) and 3.33 and 3.43 for the triple isocenter SRS plan (SRS3). The maximum and minimum doses to surrounding critical structures were less with the IMR plan in comparison to both SRS plans. However, the volume of non-target tissue treated to > 80, 50, and 20% of the prescription dose with the IMR plan was 137, 170, and 163%, respectively, of that treated with the SRS1 plan and 85, 100, and 123% of the volume when compared to SRS3 plan. The IMR system provided more conformal target doses than were provided by the single isocenter or three isocenter SRS plans. IMR delivered less dose to critical normal tissues and provided increased homogeneity within the target volume for a moderate size irregularly shaped target, at the cost of a larger penumbra.