Dosimetric and radiobiological impact of dose fractionation on respiratory motion induced IMRT delivery errors: A volumetric dose measurement study

Abstract

Respiratory motion can introduce substantial dose errors during IMRT delivery. These errors are difficult to predict because of the nonsynchronous interplay between radiation beams and tissues. The present study investigates the impact of dose fractionation on respiratory motion induced dosimetric errors during IMRT delivery and their radiobiological implications by using measured 3D dose. We focused on IMRT delivery with dynamic multileaf collimation (DMLC-IMRT). IMRT plans using several beam arrangements were optimized for and delivered to a polystyrene phantom containing a simulated target and critical organs. The phantom was set in linear sinusoidal motion at a frequency of 15 cycles/min (0.25 Hz). The amplitude of the motion was +/- 0.75 cm in the longitudinal direction and +/- 0.25 cm in the lateral direction. Absolute doses were measured with a 0.125 cc ionization chamber while dose distributions were measured with transverse films spaced 6 mm apart. Measurements were performed for varying number of fractions with motion, with respiratory-gated motion, and without motion. A tumor control probability (TCP) model for an inhomogeneously irradiated tumor was used to calculate and compare TCPs for the measurements and the treatment plans. Equivalent uniform doses (EUD) were also computed. For individual fields, point measurements using an ionization chamber showed substantial dose deviations (-11.7% to 47.8%) for the moving phantom as compared to the stationary phantom. However, much smaller deviations (-1.7% to 3.5%) were observed for the composite dose of all fields. The dose distributions and DVHs of stationary and gated deliveries were in good agreement with those of treatment plans, while those of the nongated moving phantom showed substantial differences. Compared to the stationary phantom, the largest differences observed for the minimum and maximum target doses were -18.8% and +19.7%, respectively. Due to their random nature, these dose errors tended to average out over fractionated treatments. The results of five-fraction measurements showed significantly improved agreement between the moving and stationary phantom. The changes in TCP were less than 4.3% for a single fraction, and less than 2.3% for two or more fractions. Variation of average EUD per fraction was small (< 3.1 cGy for a fraction size of 200 cGy), even when the DVHs were noticeably different from that of the stationary tumor. In conclusion, IMRT treatment of sites affected by respiratory motion can introduce significant dose errors in individual field doses; however, these errors tend to cancel out between fields and average out over dose fractionation. 3D dose distributions, DVHs, TCPs, and EUDs for stationary and moving cases showed good agreement after two or more fractions, suggesting that tumors affected by respiration motion may be treated using IMRT without significant dosimetric and biological consequences.

Use of reduced dose rate when treating moving tumors using dynamic IMRT

“…( 4 , 5 ) We also found that the average of the doses calculated for each individual starting point in the respiratory phase gave a dose distribution that agreed with the expected dose distribution within 1%. However, Seco et al ( 1 ) showed that there are situations where although the average dose discrepancy will be small, the dose discrepancy can still be large for a minority of patients.…”

“…Similarly, several authors have shown that the dose discrepancy per fraction can be expected to be less than the dose discrepancy for individual fields. ( 5 ) Again, it can be expected that this depends on the combination of points in the respiratory cycle where the individual beams are turned on. The dose for an individual field for an individual fraction, as evaluated here, can be (very) conservatively considered as the worst‐case scenario.…”

Quality assurance device for four‐dimensional IMRT or SBRT and respiratory gating using patient‐specific intrafraction motion kernels

“…Second, the diode array is filmless, providing immediate dosimetric feedback (requirement 1). Third, each diode has an active detector area of 0.64 mm 2 in the beam's‐eye‐view [BEV (requirement 2)]. The detector density is minimally acceptable, with 221 diodes evenly spaced in the central 10 cm 2 area (requirement 3).…”

“…The first key component was a motion controller (NSC‐M Series) to provide power and independent signals to the multi‐axis steppers. The next critical components were two independent linear stages (NLS4), each with 0.13 μm resolution, maximum travel speed of 5.08 cm/s, maximum acceleration of 72 cm/s 2 , 0.5 μm bidirectional reproducibility, and ability to carry up to a 22.7‐kg horizontal load. The devices used in this study had a 6.35‐cm travel range in X and Y .…”

“…Previous studies of the dosimetry to a moving target and respiratory gating systems have not incorporated patient‐specific motion kernels. ( 2 – 4 ) A useful device would allow for assessment of fraction‐to‐fraction dose variation for a given ITV and intensity‐modulation technique;selection of the safest delivery option—step‐and‐shoot multileaf collimation (SMLC), dynamic multileaf collimation (DMLC), compensator‐based IMRT, and so on—for any given patient;determination of the optimal ITV;data‐driven decisions on whether gating is required, and the optimal duty cycle;patient‐specific verification and QA of delivery gating; andpre‐treatment dry runs to consider practical data such as delivery times, especially for hypofractionated treatment. …”