Assessment of out-of-field absorbed dose and equivalent dose in proton fields

Abstract

Purpose: In proton therapy, as in other forms of radiation therapy, scattered and secondary particles produce undesired dose outside the target volume that may increase the risk of radiation-induced secondary cancer and interact with electronic devices in the treatment room. The authors implement a Monte Carlo model of this dose deposited outside passively scattered fields and compare it to measurements, determine the out-of-field equivalent dose, and estimate the change in the dose if the same target volumes were treated with an active beam scanning technique.

Methods: Measurements are done with a thimble ionization chamber and the Wellhofer MatriXX detector inside a Lucite phantom with field configurations based on the treatment of prostate cancer and medulloblastoma. The authors use a GEANT4 Monte Carlo simulation, demonstrated to agree well with measurements inside the primary field, to simulate fields delivered in the measurements. The partial contributions to the dose are separated in the simulation by particle type and origin.

Results: The agreement between experiment and simulation in the out-of-field absorbed dose is within 30% at 10-20 cm from the field edge and 90% of the data agrees within 2 standard deviations. In passive scattering, the neutron contribution to the total dose dominates in the region downstream of the Bragg peak (65%-80% due to internally produced neutrons) and inside the phantom at distances more than 10-15 cm from the field edge. The equivalent doses using 10 for the neutron weighting factor at the entrance to the phantom and at 20 cm from the field edge are 2.2 and 2.6 mSv/Gy for the prostate cancer and cranial medulloblastoma fields, respectively. The equivalent dose at 15-20 cm from the field edge decreases with depth in passive scattering and increases with depth in active scanning. Therefore, active scanning has smaller out-of-field equivalent dose by factors of 30-45 in the entrance region and this factor decreases with depth.

Conclusions: The dose deposited immediately downstream of the primary field, in these cases, is dominated by internally produced neutrons; therefore, scattered and scanned fields may have similar risk of second cancer in this region. The authors confirm that there is a reduction in the out-of-field dose in active scanning but the effect decreases with depth. GEANT4 is suitable for simulating the dose deposited outside the primary field. The agreement with measurements is comparable to or better than the agreement reported for other implementations of Monte Carlo models. Depending on the position, the absorbed dose outside the primary field is dominated by contributions from primary protons that may or may not have scattered in the brass collimating devices. This is noteworthy as the quality factor of the low LET protons is well known and the relative dose risk in this region can thus be assessed accurately.

Figure 1

Configuration of the nozzle and phantom and positions of the dosimeter in the horizontal plane. The dark shaded region shows the primary field in the phantom. The dosimeter was positioned at 5, 10, 20, and up to 60 cm lateral to the field edge (black, solid circles).

Figure 2

Simulated and experimental doses for (a) the passively scattered prostate cancer field and (b) the cranial medulloblastoma field at different WEDs. Data points are offset slightly for clarity. These data verify the suitability of the Monte Carlo model.

Figure 3

Simulated absorbed dose distributions for the prostate cancer field: (a) D_total,pass, (b) D_total,scan, (c) D_total,pass∕D_total,scan, (d) D_n,pass, (e) D_n,scan, and (f) D_n,pass∕D_n,scan. Contours in (a), (b), (d), and (e) show the contribution of neutrons to the data in Fig. 2a and have units of mGy∕Gy. Contours in (c) and (f) show the dose advantage in scanning for the prostate cancer field, which are factors of 10–30 in the entrance region. The beam enters the phantom at the upper left of each figure and primary fields are the high dose regions at the top of (a) and (b).

Figure 4

Neutron contributions to the absorbed dose in the simulation for the prostate cancer field. The lateral slices of Fig. 3d are shown in (a) and the lateral slices of Fig. 3e are shown in (b). Similar results for the medulloblastoma field are shown in (d)–(f). The neutron contribution to the absorbed dose is used to estimate the equivalent dose in Eq. 3, and one may use these results to reevaluate the equivalent dose if there is a new neutron weighting factor. Error bars represent 1 standard deviation of the mean from 20 independent simulations and data points are offset slightly for clarity.

Figure 5

Simulated partial contributions to the total absorbed dose from protons, neutrons, and photons. The neutron partial contributions are from Figs. 4a, 4b and the remaining contributions to the total dose are shown for protons and photons. The photon contribution is 5%–10% in passive scattering and 25%–60% in active scanning at 15–60 cm from the field edge. While the largest fraction of the dose deposited outside the field is due to neutrons, they do not deposit the entire dose even far from the field edge.

Figure 6

Simulated x-ray equivalent dose for the prostate cancer field (a) and the cranial medulloblastoma field (b) using 10 and 2 for the radiation weighting factors for neutrons and primary protons, respectively. The equivalent dose is determined from the dose distributions (Fig. 3) by 10×D_n+2×D_p+D_γ. Error bars represent 1 standard deviation of the mean from 20 independent simulations and data points are offset slightly for clarity.

Figure 7

Simulated average weighting factor for the prostate cancer field (a) and the cranial medulloblastoma field (b) using, alternately, 10 and 25 for the neutron radiation weighting factor, ω_n. Low LET components in the radiation field affect the average weighting factor even at large distances from the field edge for passive scattering and even more so for active scanning. The difference in the average weighting factor between the two modalities increases with distance from the field edge. Error bars represent 1 standard deviation of the mean from 20 independent simulations and data points are offset slightly for clarity.

Figure 8

Simulated absorbed dose (a) and equivalent dose using ω_n=10 (b) for active scanning with and without a range shifter. The medulloblastoma field is used for this comparison and the range shifter is Lucite with 7 cm water-equivalent thickness. At 10–20 cm from the field edge, the range shifter increases the out-of-field absorbed dose and equivalent dose by factors of 2–5. Error bars represent 1 standard deviation of the mean from 20 independent simulations and data points are offset slightly for clarity.

Figure 9

Simulated average weighting factor from this work (using ω_n=10) compared to the average quality factor using a silicon-on-insulator microdosimeter in Ref. . Although there are differences by up to 40% in this mixed radiation field, these results were obtained by different methods. These distributions have a similar tendency, the average weighting factor and quality factor increase with lateral distance from the field edge.