Characterization of dose distribution in radiation therapy plans

A comparison of dose distributions in gross tumor volume between boron neutron capture therapy alone and combined boron neutron capture therapy plus intensity modulation radiation therapy for head and neck cancer

Nine patients with recurrent head and neck (H&N) cancer received boron neutron capture therapy (BNCT) in one fraction at the Tsing-Hua Open pool reactor (THOR) utilizing the THORplan treatment planning system (TPS). The aims of the present study were to evaluate the use of intensity modulated radiation therapy (IMRT) of 45 Gy in 20 fractions to compensate for the dose heterogeneity in gross tumor volume observed with single-fraction BNCT with mean prescription dose 19 Gy (w), and to evaluate planning quality indices of simulated BNCT+IMRT versus single-fraction BNCT alone. All IMRT plans were generated using the Eclipse TPS which employs the analytical anisotropic algorithm. The conformity index for the gross tumor volume (GTV) was better for the BNCT+IMRT plan than for the BNCT-alone plan (p = 0.003). In addition, the BNCT+IMRT plan provided significantly better homogeneity in the GTV (p = 0.03). The cold spots in inhomogeneous dose distribution in the BNCT plan may be a key factor for H&N cancer recurrence. Our results suggest that single-fraction BNCT combined with compensated multi-fraction IMRT improves treatment homogeneity and conformity than single-fraction BNCT alone, especially for tumor volumes >100 cm³, and possibly increases local tumor control.

The simultaneous integrated boost with proton beams in head and neck patients

The potential benefit of the simultaneous integrated boost (SIB) concept in proton therapy was investigated in a planning study. The proposed fractionation strategy consisted of a SIB treatment following a first phase of conventional fractionation to the elective volume (named SEQ/SIB). The novel method was compared to a conventional sequential fractionation and to a full SIB approach. Treatment plans were designed for five patients on the proton planning system developed for spot scanning at the Paul Scherrer Institute of Villigen (CH). Three to five beams were applied for all plans and fractionations. All effective dose distributions were corrected using biological models to take into account repopulation and time at repopulation onset. Corrected and uncorrected plans were compared on the basis of dosimetry criteria and dose-volume histograms. The results showed a dosimetric advantage for the SEQ/SIB approach in terms of target coverage, without significant disadvantages for risk structures and healthy tissue. Considering the high logistic impact and the limited availability of radiation facilities, the clinical exploitation of accelerated SIB treatment with protons appears promising.

Comparative dosimetric evaluation of the simultaneous integrated boost with photon intensity modulation in head and neck cancer patients

BACKGROUND AND PURPOSE

The objective of this study is to evaluate, at planning and dosimetric level, the potential benefits of the simultaneous integrated boost (SIB) concept with intensity-modulated radiation therapy (IMRT), using a comparative analysis on physical dose distributions corrected for radiobiological models. The concept of SIB at the end of the treatment has been analysed as an alternative acceleration scheme.

PATIENTS AND METHODS

Physical dose distributions were computed on a commercial planning system (Varian Cadplan-Helios) for five patients presenting with advanced head and neck carcinomas. Treatment plans were designed using five IMRT beams. Three fractionation strategies were compared in the study: the standard sequential irradiation SEQ of elective and boost volumes, the pure SIB, and a modified SIB (SEQ/SIB), where the actual SIB follows a first phase of conventional fractionation to the elective volume. All physical dose distributions were corrected using a linear quadratic biological model, taking into account also repopulation and time at repopulation onset. Objective quantities, derived from biological dose volume histograms, were used for the analysis.

RESULTS

Physical doses equivalent to 50 and 80 Gy (in fractions of 2 Gy) to elective volume and boost were calculated for the SIB and SEQ/SIB regimes. With SIB 54 and 72 Gy dose levels have to be delivered in 30 fractions, while in the SEQ/SIB scheme 36 Gy are delivered in 20 sessions to the elective volume, and further 18 and 35.5 Gy during the last 10 fractions are delivered to elective volume and boost, respectively (for a total physical dose of 71.5 Gy). The comparison showed: (1) the boost target homogeneity resulted in generally acceptable and comparable among sequential and modified SIB schemes, while it was statistically worse for the pure SIB approach; (2) the fraction of elective target volume not included in the boost volume was characterised by a higher level of dose heterogeneity; (3) the spinal cord never reached tolerance levels and maximum point dose was on average below 38 Gy (biologically corrected to 2 Gy/fraction); and (4) sparing of parotid glands strongly depends on their eventual inclusion in the target volumes: for glands not included or only partially included, it was possible on average to keep the dose to 2/3 of the volume below 29 Gy for all regimes (32 Gy as physical dose).

CONCLUSIONS

Feasibility of SIB techniques and in particular of the modified SIB appears to be dosimetrically proven and the results reported here justify the activation of a phase I protocol to verify clinically their impact using IMRT photon-based techniques.

Comparative analysis of intensity modulation inverse planning modules of three commercial treatment planning systems applied to head and neck tumour model

BACKGROUND AND PURPOSE

Three commercial treatment planning modules for intensity modulated radiation therapy (IMRT) Inverse Planning, MDS-Nordion Helax-TMS, Varian Cadplan-Helios, and CMS Focus, were compared in an attempt to determine potential application limits or dosimetric differences among various optimisation algorithms.

MATERIALS AND METHODS

A comparative analysis of intensity modulated dose distributions was conducted at planning level on a group of four patients presenting advanced head and neck cancers. In the study, we analysed primarily the static 'step and shoot' multileaf implementation of modulation realisation with some investigation, on the Cadplan-Helios implementation of the 'sliding window', the Varian dynamic approach to IMRT delivery. The whole study was carried out using the inverse planning tools implemented by vendors fully optimising each plan to obtain the best dosimetry given some general plan objectives. To achieve adequate target coverage, optimisation was carried out on Helax-TMS and CMS Focus adding extra margins of 5 or 6mm to the planning target volume (PTV). Beam arrangements were set with five and nine equally spaced fields. The study was conducted with two complexity levels. At the first level, dose-volume constraints were applied only to the target volume and to the spinal cord, while parotid glands were added at the second level. The relative values of dose distributions and dose-volume histograms were compared, together with an estimate of the biological implications in terms of Equivalent Uniform Dose to the target. In the Cadplan-Helios system also the dosimetric implications of the number of intensity levels selected for the discretisation of the fluence matrix were investigated.

RESULTS

With the application of common planning strategies and the proper consideration of treatment planning system (TPS) specific features (e.g. the PTV margin problem), no substantial differences among the three algorithms were demonstrated at the first level for PTV and spinal cord. At the second level of the study differences were outlined for Helax-TMS, where sub-optimal results were obtained with the 5-field geometry. Mainly due to the differences in optimisation volumes, Cadplan-Helios presented significant better sparing of healthy tissue around the PTV, in terms of mean dose to healthy tissue and Irradiated Volume at 50% dose level. Finally, to achieve dosimetrically acceptable and stable results on target, a minimum of eight intensity levels should be applied for the multileaf collimator (MLC) segmentation, giving an average of 1.5 segments per field and per intensity level.

CONCLUSIONS

Results obtained for the three IMRT TPS show in first instance that the optimisation algorithms analysed, as well as the conversion from computed fluences to multileaf sequences implemented in the planning systems can produce substantially equivalent dose plans (for target coverage and organs at risk sparing) if planning is performed with common strategies and once a strong understanding of each system feature is achieved. Secondly, a limited number of dose levels (about eight) is adequate at planning level.

The role of proton therapy in the treatment of large irradiation volumes: a comparative planning study of pancreatic and biliary tumors

PURPOSE

The purpose of this study was to examine the potential benefit of proton therapy for abdominal tumors. Extensive comparative planning was conducted investigating the most up-to-date photon and proton irradiation technologies.

METHODS AND MATERIALS

A number of rival plans were generated for four patients: two inoperable pancreatic tumors, one inoperable and one postoperative biliary duct tumor. The dose prescription goal for these large targets was 50 Gy, followed by a boost dose up to 20 Gy to a smaller planning target volume (PTV). Photon plans were developed using "forward" planning of coplanar and noncoplanar conformal fields and "inverse" planning of intensity-modulated (IM) fields. Proton planning was simulated as administered using the so called spot-scanning technique. Plans were evaluated on the basis of normal tissues' dose-volume constraints (Emami B, Lyman J, Brown A, et al. Tolerance of normal tissue to therapeutic irradiation. Int J Radiat Oncol Biol Phys 1990;21:109-122) and coverage of treatment volumes with prescribed doses.

RESULTS

For all cases, none of the forward calculated photon plans was able to deliver 50 Gy to large PTVs at the same time respecting the dose-volume constraints on all critical organs. Nine evenly spaced IM fields achieved or nearly achieved all maximum dose constraints to critical structures for two out of three inoperable patients. IM plans also obtained good results for the postoperative patient, even though the dose to the liver was very close to the maximum allowed. In all cases, photon irradiation of large PTV1s to 50 Gy followed by a 20 Gy boost entailed a risk very close to or higher than 5% for serious complications to the kidneys, liver, or bowel. Simple arrangements of 2, 3, and 4 proton fields obtained better dose conformation to the target, allowing the delivery of planned doses including the boost to all patients, without excessive risk of morbidity. Dose homogeneity inside the targets was also superior with protons.

CONCLUSION

For the irradiation of large PTVs located in the abdominal cavity, where multiple, parallel structured organs surround the target volumes, proton therapy, delivered with a sophisticated isocentric technique, has the potential to achieve superior dose distributions compared with state-of-the-art photon irradiation techniques. IM photon plans obtain better results in the postoperative case, because the reduced volume lessens the effect of the unavoidable increase of integral dose to surrounding tissues.

A treatment planning inter-comparison of proton and intensity modulated photon radiotherapy

PURPOSE

A comparative treatment planning study has been undertaken between standard photon delivery techniques,b intensity modulated photon methods and spot scanned protons in order to investigate the merits and limitations of each of these treatment approaches.

METHODS

Plans for each modality were performed using CT scans and planning information for nine patients with varying indications and lesion sites and the results have been analysed using a variety of dose and volume based parameters.

RESULTS

Over all cases, it is predicted that the use of protons could lead to a reduction of the total integral dose by a factor three compared to standard photon techniques and a factor two compared to IM photon plans. In addition, in all but one Organ at Risk (OAR) for one case, protons are predicted to reduce both mean OAR dose and the irradiated volume at the 50% mean target dose level compared to both photon methods. However, when considering the volume of an OAR irradiated to 70% or more of the target dose, little difference could be shown between proton and intensity modulated photon plans. On comparing the magnitude of dose hot spots in OARs resulting from the proton and IM photon plans, more variation was observed, and the ranking of the plans was then found to be case and OAR dependent.

CONCLUSIONS

The use of protons has been found to reduce the medium to low dose load (below about 70% of the target dose) to OARs and all non-target tissues compared to both standard and inversely planned photons, but that the use of intensity modulated photons can result in similar levels of high dose conformation to that afforded by protons. However, the introduction of inverse planning methods for protons is necessary before general conclusions on the relative efficacy of photons and protons can be drawn.