193 Dosimetric aspects of dynamic multileaf collimation for intensity modulated radiotherapy

248

1. J. Radiation

Oncology

l

Biology

l

Physics

Volume

45, Number

3 Supplement

1999

formulated so that the following physical and clinical planning constraints are explicit in the beam as a function of exposure interval, 2) the number and size of the exposure intervals, 3) the collimation system, 3) off-focal radiation (e.g., “head scatter”), and 5) maximum leakage dose patient. Feasibility tests conducted on simple 21) problems confirm the usefulness of the dose optimization of IMRT leaf sequences.

weights: 1) the aperture leakage transmission that can be received model formalism for

193

DOSIMETRIC MODULATED

INTENSITY

Arnfield

MR’,‘,

Vii-ginia

Commonwealth

Wu Q’,*,

ASPECTS OF DYNAMIC RADIOTHERAPY Siebers University,

J’,‘, Mohan Richmond,

MULTILEAF

COLLIMATION

FOR

shape of the by the direct

R’,” VA, USA’;

McGuire

VA Medical

Center,

Richmond,

VA, USA”

Purpose: Leaf shape and transmission characteristics of the multileaf collimator (MLC) affect the dose delivered during intensity modulated radiotherapy (IMRT). The accuracy of dose calculation depends on correcting for the effects of the leaf shape on transmission and scatter. We investigated the magnitude and relative importance of these corrections. Materials and Methods: Studies were performed with an go-leaf. Varian Mark II multileaf collimator accelerator, for 6 MV. The projected width at isocenter of the tungsten leaves was 1 .O cm and the maximum 6.1 cm. MLC transmission was measured with film, for different field sizes defined by the jaws. For transmission was found by exposing two films: one with the MLC blocking the beam, the other with the MLC units (MU) were adjusted to achieve approximately the same optical density in both films. A single midline was scanned and the ratio of the (H&D calibrated) doses of the two films gave the leaf transmission. This the profile, spanning five leaves. The transmission through the curved leaf tip, in the direction of leaf travel, a ray-line method using the spectral energy fluence of our accelerator and the spectral attenuation coefficients spectral fluence of the 6 MV beam was calculated by the Monte Carlo technique, using the EGS4 code.

on a Clinac 2100C leaf thickness was each field size, the retracted. Monitor profile on each film was averaged over was calculated by of tungsten. The

Results: Measurements of leaf bank transmission versus field width gave an approximate extrapolated value of 1.5% direct transmission at zero field width. Since the measured apparent transmission was 2% for a 10 x 10 cm’ field, it was estimated that 0.5% of the total was due to leaf scatter. Integration of the calculated leaf tip transmission profile showed that transmission through the curved part of the leaf tip was equivalent to a shift of 0.1 cm in the leaf position. These results were incorporated into a program that was used to predict doses of fields that were created by sliding windows of different widths. Gaps of eight different widths between 0.5 cm and 10 cm were used to produce uniform fields with the same primary fluence. The measured doses for the fields increased with decreasing gap width. The doses for the fields created by the largest and smallest gaps differed by a factor of 1.9. This difference was due to increased leaf transmission and scatter. The ratio MUt/MUe of total MU (beam-on time) to effective MU (measured dose) was 1.4, 4.9 and 10.8 for the 10 cm, 2 cm and 0.5 cm gap-created fields respectively. The relative magnitudes of each component of dose depended on the MUt/MUe ratio. The percentage of total dose from leaf transmission (leakage) was estimated as 1.5%, 15% and 39% for the 10 cm, 2 cm and 0.5 cm gap fields respectively. The corresponding values for leaf scatter were 0.7%, 2.5% and 5.8%. Conclusion: These results show that the fraction of total dose due to leaf transmission and scatter increases substantially for large MUtlMUe ratios. A high MUt/MUe ratio implies a narrow average gap, which in clinical IMRT means a highly modulated field. The more modulated the field, the greater the percentage of dose is due to leaf transmission and scatter, and the more critical is the accurate modeling of leaf characteristics to ensure accurate dose calculations. The validity of this conclusion applies to the step-and-shoot as well as the sliding window techniques. Recently introduced leaf designs, which exhibit reduced leakage. will lessen the importance of leaf-dependent corrections.

194

THE POTENTIAL RECTALCANCER

BENEFIT

OF INTENSITY

MODULATED

RADIATION

THERAPY

(IMRT)

FOR

Robertson JM, Yan D, Girimonte PE, Kota K William Beaumont Hospital, Royal Oak, MI, USA Purpose: Small bowel toxicity due to treatment of rectal cancer is common. The INT 0114 study found grade 2 3 acute diarrhea in 28% of patients receiving adjuvant post-operative radiation therapy (RT) with 5.fluorouracil and leucovorin; leading to treatment breaks and/or early termination of therapy. Long-term bowel dysfunction, such as more frequent bowel movements and/or incontinence, has also been reported to be increased for both patients receiving adjuvant combined modality therapy as well as those treated with short course pre-operative RT alone. Additionally, future investigations in combined modality therapy may wish to include irinotecan, which has a very high rate of diarrhea when given alone and is a radiosensitizer. Any reduction in the volume of small bowel irradiated may improve the tolerance to therapy. Here we report our initial experience studying the potential use of IMRT to reduce the volume of small bowel irradiated during both post-operative and pre-operative RT for rectal cancer. Materials and Methods: Treatment planning CT scans of the abdomen and pelvis were obtained in seven patients with rectal cancer (2 treated after abdominoperineal resection (APR), 2 after low anterior resection (LAR), 2 treated pre-operatively (Pm-op) and 1 treated after wide local excision (WLE)). All patients were given oral contrast, placed prone on a rigid foam cradle with a cut-out area for small bowel exclusion, and instructed to have a full bladder. The clinical target volume (CTV) and small bowel volume were outlined and used to generate both a “conventional” three-field plan, with the fields shaped using a beam’s eye view, and an IMRT plan using a step and shoot approach with 5 coplanar beams and a 1 x 1 cm minimum beam resolution. The isocenter was specified to receive 100% of the dose, with the 95% isodose line encompassing the CTV + 3 mm and no more than plus 105% or 110% inhomogeneity within the target volume. Radiation dose distribution in the small bowel was determined for the three plans (conventional, IMRT 105%, IMRT 110%). Thirteen weekly treatment planning CT scans of the pelvis were obtained in 3 patients (2 with LAR, 1 with WLE) allowing comparison of the position of the small bowel over time versus the original CTV definition. Results:

In all cases IMRT

reduced

the volume

of small bowel

irradiated

(Table),

with the largest

reduction

for patients

with