An experimental investigation on intra-fractional organ motion effects in lung IMRT treatments

Abstract

Respiration-induced tumour motion can potentially compromise the use of intensity-modulated radiotherapy (IMRT) as a dose escalation tool for lung tumour treatment. We have experimentally investigated the intra-fractional organ motion effects in lung IMRT treatments delivered by multi-leaf collimator (MLC). An in-house made motor-driven platform, which moves sinusoidally with an amplitude of 1 cm and a period of 4 s, was used to mimic tumour motion. Tumour motion was simulated along cranial-caudal direction while MLC leaves moved across the patient from left to right, as in most clinical cases. The dose to a point near the centre of the tumour mass was measured according to geometric and dosimetric parameters from two five-field lung IMRT plans. For each field, measurement was done for two dose rates (300 and 500 MU min(-1)), three MLC delivery modes (sliding window, step-and-shoot with 10 and 20 intensity levels) and eight equally spaced starting phases of tumour motion. The dose to the measurement point delivered from all five fields was derived for both a single fraction and 30 fractions by randomly sampling from measured dose values of each field at different initial phases. It was found that the mean dose to a moving tumour differs slightly (<2-3%) from that to a static tumour. The variation in breathing phase at the start of dose delivery results in a maximum variation around the mean dose of greater than 30% for one field. The full width at half maximum for the probability distribution of the point dose is up to 8% for all five fields in a single fraction, but less than 1-2% after 30 fractions. In general, lower dose rate can reduce the motion-caused dose variation and therefore might be preferable for lung IMRT when no motion mitigation techniques are used. From the two IMRT cases we studied where tumour motion is perpendicular to MLC leaf motion, the dose variation was found to be insensitive to the MLC delivery mode.

Measurement of the interplay effect in lung IMRT treatment using EDR2 films

“…This is due to the canceling out of hot/cold spots when many fractions are combined. These results compare favorably with a previous experiment performed using an ion chamber for measurement (7) . In that study, the standard deviation was 0.19% and 0.35% for two different patients for 30 fractions.…”

“…The motion of the platform was set in the S‐I direction and the MLC leaves moving across the patient such that the two motions were always perpendicular. This was done to ensure consistency with a previously published study (7) . As a control, each of the five fields was measured individually, as well as a single film containing all five fields, with the platform in a static state.…”

“…( 1 – 4 ) Clinicians are hesitant to treat non‐small‐cell lung cancer patients with IMRT, even though clinical studies have demonstrated that an escalation in radiation dose results in significant improvement in the outcome of these patients. ( 5 , 6 ) The reluctance is due to organ motion effects, including dose blurring effect and interplay effect (7) …”

“…( 7 – 16 ) This effect is only an issue for dynamic delivery techniques (including step‐and‐shoot IMRT) and may cause hot/cold spots within the target as well as outside it. Previous experimental work used point measurements and/or a single beam only.…”

Robust optimization of VMAT for lung cancer: Dosimetric implications of motion compensation techniques

“…Several authors40, 41 have shown that extreme dose variations of up to 100% in IMRT field delivery due to the interplay effect are theoretically possible. Subsequent statistical analysis by Bortfeld42 found no significant difference over long course treatments (>10 fractions) however, comparisons were made against an introduced formalism rather than a CT dose distribution (of which variations of up to 20% were noted).…”