Use of segmental boost fields in the irradiation of inguinal lymphatic nodes

Organ sparing by conformal avoidance intensity-modulated radiation therapy for anal cancer: dosimetric evaluation of coverage of pelvis and inguinal/femoral nodes

PURPOSE

To describe a novel and straightforward conformal avoidance intensity-modulated radiation therapy (IMRT) technique for coverage of pelvis and inguinal/femoral nodes and to compare the dosimetry of the new method with that of other traditional methods of radiation treatment.

METHODS AND MATERIALS

Data of 2 patients with anal cancer were used as example cases to illustrate details and advantages of conformal avoidance IMRT technique. Conventional photons with enface electrons design was created first, thereby providing "outermost boundaries" defined as planning target volume (PTV) for subsequent conformal avoidance IMRT design. Organs at risk (OARs), including femoral head and neck and external genitalia, were contoured as conformal avoidance structures. A step-and-shoot inverse IMRT planning was subsequently generated. For dosimetric comparison, a recently published technique by modified segmental boost was also generated. These treatment techniques were evaluated by dose-volume histogram (DVH) of PTV and OARs. Dose profiles at four different depths from each treatment planning were generated for comparison.

RESULTS

The DVH of PTV showed that coverage of the PTV was comparable among three treatment techniques. Percent volume of PTV receiving more than 90% prescription dose was in the range 94-98% for the three treatment techniques, and all had only 0-2% of PTV receiving more than 110% of prescription dose. The DVH of OARs confirmed that both femoral head and neck and external genitalia could be spared well by conformal avoidance IMRT as compared with the other two techniques. Although greater inhomogeneity of dose distribution within the PTV was noted by conformal avoidance IMRT technique, as shown by dose profiles at four different depths, the maximum doses at different depths were less than 115%, which was comparable to those planned by modified segmental boost technique. Planning by photons and enface electrons technique, however, showed a greater dose variation up to 134% of the prescription dose at 1.5 cm depth along photon-electron match-line.

CONCLUSIONS

To cover pelvis and inguinal/femoral nodes, conformal avoidance IMRT is technically simple to simulate, plan, and execute. Dosimetric study has demonstrated that it achieves comparable PTV coverage compared with other approaches while at the same time significantly sparing the surrounding OARs.

Effects on electron beam penumbra using the photon MLC to reduce bremsstrahlung leakage for an add-on electron MLC

Electron IMRT treatments have the potential to reduce the integral dose due to the limited range of the electrons. However, bremsstrahlung produced in the scattering foils could penetrate an added electron MLC (eMLC), thus producing an unmodulated dose contribution that could become unacceptable in electron IMRT treatments. To limit this bremsstrahlung contribution, the photon MLC (xMLC) was used to track the eMLC, but with a margin to avoid penumbra widening through partial screening of the effective electron source. The purpose of this work was to study the effect of the photon-electron MLC tracking on the electron beam penumbra for different treatment head designs. Both isocentric designs and designs where the eMLC is used close to the patient (proximity geometry) have been analysed using Monte Carlo simulations. At 22.5 MeV energy, a tracking margin of 1 cm was enough to avoid penumbra degradation for a helium-filled isocentric geometry, while air-filled geometries (including proximity geometries) require a 2-3 cm margin. Illustrated by an example of a chest wall treatment by electron IMRT, the use of 1 cm tracking margin will reduce the collimator leakage contribution by a factor of 36 as compared to using a static setting of the photon collimator.

Improved treatment of pelvis and inguinal nodes using modified segmental boost technique: dosimetric evaluation

PURPOSE

To describe a novel, yet simple, modified segmental boost technique (MSBT) and to compare the dosimetry of our method with that of other traditional methods of treatment for the pelvis and inguinal nodes.

METHODS AND MATERIALS

We developed a radiotherapy technique that uses linear accelerators with multileaf collimators to treat the pelvis and sequentially boost the inguinal regions, while minimizing "hot spots" across the match-line. This was achieved by angling the gantry for the inguinal fields so that their medial borders aligned with the divergence of the posterior pelvic field. Film dosimetry was performed to compare the MSBT with the traditional segmental boost technique, partial transmission block, and photon/electron combination techniques. These treatment techniques were scored on the basis of the dose homogeneity index, defined as the ratio of match-line maximum dose to the average dose at a given depth in the groin treatment area.

RESULTS

The values of the dose homogeneity index were the same (1.04) for MSBT and partial transmission block, and 1.21, 1.39, and 1.18 for the segmental boost technique, photon pelvis with electron tags, and photon pelvis with electron boost, respectively.

CONCLUSION

The MSBT proved to be technically simple while optimizing dose homogeneity compared with the other techniques and allows for maximum use of the features of modern linear accelerators.