Intensity-modulated Proton Research Articles

Comparison of intensity modulated x-ray therapy and intensity modulated proton therapy for selective subvolume boosting: a phantom study

Selective subvolume boosting can theoretically improve tumour control probability while maintaining normal tissue complication probabilities similar to those of uniform dose distributions. In this work the abilities of intensity-modulated x-ray therapy (IMXT) and intensity-modulated proton therapy (IMPT) to deliver boosts to multiple subvolumes of varying size and proximities are compared in a thorough phantom study. IMXT plans were created using the step-and-shoot (IMXT-SAS) and helical tomotherapy (IMXT-HT) methods. IMPT plans were created with the spot scanning (IMPT-SS) and distal gradient tracking (IMPT-DGT) methods. IMPT-DGT is a generalization of the distal edge tracking method designed to reduce the number of proton beam spots required to deliver non-uniform dose distributions relative to IMPT-SS. The IMPT methods were delivered over both 180° and 360° arcs. The IMXT-SAS and IMPT-SS methods optimally satisfied the non-uniform dose prescriptions the least and the most, respectively. The IMPT delivery methods reduced the normal tissue integral dose by a factor of about 2 relative to the IMXT delivery methods, regardless of the delivery arc. The IMPT-DGT method reduced the number of proton beam spots by a factor of about 3 relative to the IMPT-SS method.For more information on this article, see medicalphysicsweb.org

High-Dose External Beam Radiation for Localized Prostate Cancer: Current Status and Future Challenges

Since the 1960s, external beam radiation has been one of the major curative treatment options for patients with clinically localized prostate cancer. Efforts to improve the efficacy of this modality have focused on delivering a higher dose, and several recent randomized trials have confirmed that this higher dose results in improved oncological outcomes, particularly for patients with intermediate-risk disease. Technological advancements over the past 2 decades have allowed highly conformal treatments that spare more normal tissue and reduce early and long-term treatment side effects. In a complementary fashion, methods have been developed for better real-time localization of the prostate such that radiation fields can be shifted before each treatment to match the daily shifts in the position of the target, leading to greater accuracy and allowing for smaller treatment margins that in turn will overlap with less normal tissue. With newer and more expensive technologies such as intensity-modulated radiation therapy and protons being used with increasing frequency for the treatment of prostate cancer, it becomes imperative to study the risks and benefits of each new modality so that informed cost-benefit decisions can be made. Similarly, there has been a growing interest in hypofractionation as a means of exploiting the supposed low alpha/beta ratio of prostate cancer to shorten overall treatment time and thereby improve convenience and lower costs. However, as with any new technology, it is necessary to proceed with caution in the arena of hypofractionation while we await the results of trials that will help us to determine the long-term risks and benefits of hypofractionation and whether biological assumptions about the underlying alpha/beta ratio can translate into a true clinical advantage.

TU‐D‐AUD‐03: Comparison Between the Lateral Penumbra of a Double Scattering Proton Beam and the Lateral Penumbra of a Scanning Beam

Purpose: Intensity modulated proton radiotherapy can reduce the dose to the critical structures by optimizing the distribution and intensity of the individual pencil beams. However, the finite size of a pencil beam and the scattering in air degrade the lateral penumbra in the patient, which could be inferior to the penumbra of a collimated scattered beam. In this study we compare the lateral penumbra of a pencil beam to the one of a collimated broad divergent beam, such as the one produced by a double scattering system, for different air gaps, range compensator (RC) thicknesses and pencil beam size (σ0). Method and Materials: The penumbra of the collimated broad divergent proton beam has been modeled using the generalized Fermi‐Eyges theory. The model was validated with measurements of the lateral profile in water. We analyzed 2400 treatment fields to establish typical clinical beam configurations such as air gap, RC‐thickness and collimator‐to‐patient distance (CSD), which we used to predict the penumbra (80% – 20%). Results: The model predicts the penumbra with accuracy of 0.3mm. The penumbra of a pencil beam at shallow depth is in general inferior to the penumbra of a collimated beam but is superior at larger depths. The penumbra equivalent depth (PED) exhibits a small dependence on the proton range, but it is strongly affected by the CSD. For CSD 10cm, RC‐thickness 6cm, and SSD 210cm, PED is 11cm for σ0=5mm. Conclusion: Given the source size and source position the model allows to predict the lateral penumbra of a double scattering system with good accuracy. Typically, for the configuration considered, for proton ranges larger than 11cm the pencil beam penumbra is superior to the collimated one. The inferiority at shallow depth can be compensated with a collimation of the scanning beam.

MO‐D‐M100J‐03: Reducing the Sensitivity of IMPT Treatment Plans to Setup Errors and Range Variations

Purpose: Intensity modulated proton therapy (IMPT) has the ability to deliver highly conformal dose distributions to tumors of complex shape. However, the accuracy of IMPT treatment plans is potentially compromised by various uncertainties, including setup errors and variations of the range of a proton beam in the patient. Method and Materials: We present two treatment planning concepts for IMPT which incorporate uncertainties into the optimization. The first approach minimizes the expectation value of the deviation of delivered dose and prescribed dose, assuming the delivered dose depends on several random variables that model the uncertainty. The second approach minimizes the maximal dose deviation that can occur. Results: It is shown that these methods can utilize the physical characteristics of the proton beam to make treatment plans relatively insensitive to a particular type of uncertainty.A treatment plan optimized while accounting for range uncertainties avoids placing a bragg peak directly in front of the OAR. Instead, the lateral fall‐off of the pencil beam is utilized in order to avoid the risk of overdosing the OAR. In addition, the dose distributions delivered by individual beams are relatively homogeneous in beam direction. This ensures tumor coverage and homogeneity.On the other hand, treatment plans optimized by accounting for setup errors (modeled as a rigid translation of the entire patient) utilize the distal fall‐off at a transition of OAR and tumor. In addition, the dose distributions delivered by individual beams avoid steep dose gradients perpendicular to the beam direction to ensure tumor homogeneity.Robustness of a treatment plan with respect to both setup error and range variations is possible to a limited degree only, since both types or uncertainty favor conflictive treatment plans. Conclusion: The presented methods are valuable to make IMPT treatment plans insensitive to various types of uncertainty.

MO‐D‐M100J‐01: Dose Painting With Intensity Modulated Proton Therapy and Intensity Modulated X‐Ray Therapy: A Comparison

Purpose: To compare intensity modulated proton therapy (IMPT) versus intensity modulated x‐ray therapy (IMXT) for the delivery of nonuniform dose prescriptions based on hypoxia‐imaging, so‐called dose painting. Materials and Methods: IMXT delivered with helical tomotherapy (HT) was compared to IMPT delivered with spot scanning (SS) and distal gradient tracking (DGT). The novel DGT method places beam spots where dose prescription gradients occur along the pencil beam axis. Fundamental dosimetric properties of each modality were assessed by creating optimized plans for 144 variations of a cylindrical phantom with six boost regions embedded inside a base tumor region. Clinical cases with biologically conformal dose prescriptions based on PET with the ‐ATSM hypoxia imaging radiopharmaceutical were planned. The effects on the nonuniform dose distribution of delivering IMPT on a 180° arc versus equi‐spaced beams spread over 360° were investigated. Results: Phantom studies showed that nonuniform dose plan quality for tomotherapy, SS, and DGT, was similar, but DGT plans were most sensitive to phantom size and boost region proximity. IMPT reduced normal tissue integral dose by a mean factor of around two relative to IMXT. Clinical dose deviations from the prescription were comparable for all modalities, but arc IMPT deliveries markedly reduced normal tissue dose and improved critical structure sparing without compromising the dose distribution in the tumor. Conclusions: In the target volume, IMXT and IMPT deliver comparable nonuniform dose distributions. IMPT offers improved integral normal tissue dose and sparing of critical structures over IMXT, as was the case for uniform dose deliveries. DGT reduces required beam spots by a factor of about three relative to SS. IMPT dose painting will require similar management of intrafractional patient motion as for IMXT, with the additional consideration of proton spot placement uncertainty.TR Mackie has a conflict of interest due to financial interest in TomoTherapy, Inc.

SU‐FF‐T‐45: A Simulation Study of Intra‐Fraction Motion Effects On the Proton Discrete Scanning Treatment

Purpose: Spot scanned beam delivery can be used to implement intensity modulated proton therapy (IMPT) to better conform dose distributions to the target volume. Unlike passive scattering beam delivery techniques, the interplay of the infra‐fraction organ motion and the discrete spots scanning can introduce ‘hot’ and ‘cold’ spots. We investigated the variation of computer simulated delivered dose, and the improvements for motion effects by scanning with multiple beam widths so that regions that needed higher resolution had smaller beam widths. Method and Materials: The simulation was done for one‐dimensional spots scanned with target motion. The motion pattern was recorded from an actual patient breathing normally. The analysis was made for a range of beam widths and number of repaintings. The beam widths were chosen to be narrow at the edge and wider in the center of the target in order to provide the best efficiency. Results: The variation in delivered dose can be 100% more than planned when there is no repainting. After 10 repaintings, the variation of dose distribution is reduced to 5% or 20% depending on beam width. Beam width modulation approaches high uniformity faster as the number of repaintings increases without the compensation of conformity. Conclusion: The dosimetric effect of the intra‐fraction target motion is a function of the beam width and number of repaintings. The simulation showed that the interplay produces large inhomogeneities. A wider beam can improve the uniformity, with the side effect of a larger penumbra. Multiple repainting and fractionation also smooth out these variations, with the cost of increased total treatment time. Beam width modulation has shown a good balance of these two factors and is expected to be a delivery technique to achieve both high uniformity and conformity as long as sufficient numbers of fractions or repainting.

SU-FF-T-133: Degeneracy and Robustness of IMPT Plans in the Treatment of Skull-Base Chordomas

Purpose: Intensity modulated proton therapy (IMPT) allows the delivery of highly conformal dose distributions through the application of multiple, simultaneously optimized fields. Bragg peaks are distributed for each field in 3D, therefore a large number of degrees of freedom are available in the optimization process (degeneracy of the solution). We tested the response of the clinically used optimization engine to changes in various parameters, in order to better understand the sensitivity of IMPT to such choices and particularly to find solutions less sensitive to range uncertainties. Method and Materials: For 4 patients with skull base chordomas, different plans were calculated by varying: i) number and angular configuration of fields; ii) planning parameters (i.e. dose-volume constraints) and iii) philosophy of treatment (e.g. IMPT for whole treatment or as boost). All plans were optimized to maximize target coverage while adhering to dose-limits of OARs. Robustness analysis was performed by modeling a systematic error of 3% in proton range. Comparisons were performed through visual and quantitative DVH analysis for targets and OARs. Results: 4-fields IMPT plans supply the best target coverage, with no improvement being observed using 6-fields. Changes in planning parameters inflence somewhat target coverages (differences in V95 of up to 10% in GTV). A treatment consisting of a homogeneus plan followed by a second series IMPT boost provides a higher D98 (up to 5% in the CTV) and, furthermore, provides a treatment less sensitive to range uncertainties. Conclusion: The quality of IMPT plans is relatively independent on the number of fields. V95 or D98 (but not both) can be optimised depending on whether IMPT is used for the whole treatment or as a boost and depending on the choice of planning parameters. However, a more robust treatment is achieved when IMPT plan is used as a boost only.

TH‐C‐230A‐10: A General Software Framework for Investigations in Radiation Therapy Planning

Purpose: To be able to experiment with various topics in advanced radiation therapy planning using a stand‐alone Windows‐based software. Method and Materials: Product code for IMRT planning was merged with research code for adaptive radiation therapy and dose computations. An architecture and graphical user interface tailor‐made for representing adaptive treatments was designed. Code for proton dose computation and treatment planning was developed and integrated into the system. Results: A software package that executes on a standard PC or advanced laptop has been developed. The system is capable of IMRT treatment planning using dose‐volume based functions, EUD, Poisson‐LQ and LKB biological models, both as objectives and hard constraints. Pencil beam and heterogeneity corrected collapsed cone dose computations can be used. The system can optimize every relevant combination of pencil beam weights, SMLC segments, gantry, couch and collimator angles. It is possible to perform intensity modulated proton therapy planning. Models for tumor repopulation and repair are included and irregularities in the fractionation scheme can be compensated for by allowing the dose to vary between fractions during optimization. The system exhibits a comprehensive GUI and functionality for simulation and evaluation of adaptive/IGRT treatments with algorithms for deformable dose accumulation based on information from portal imaging and 3D imaging modalities such as onboard CT scanners. Errors due to patient setup and organ motion can be counteracted and compensated for by couch shift and on‐ and offline IMRT replanning. Conclusion: A software environment suitable for studying various advanced topics in modern radiation therapy has been developed. The system has proven useful in research and development in IMRT optimization, biological models, proton therapy and adaptive radiation therapy. Conflict of Interest: The authors are employees and stock owners of the submitting company.

204 Intensity modulated photon (IMXT) and proton (IMPT) therapy for the treatment of head and neck tumors

204 Intensity modulated photon (IMXT) and proton (IMPT) therapy for the treatment of head and neck tumors

A treatment planning comparison of intensity modulated photon and proton therapy for paraspinal sarcomas

Purpose A comparative treatment planning study has been undertaken between intensity modulated (IM) photon therapy and IM proton therapy (IMPT) in paraspinal sarcomas, so as to assess the potential benefits and limitations of these treatment modalities. In the case of IM proton therapy, plans were compared also for two different sizes of the pencil beam. Finally, a 10% and 20% dose escalation with IM protons was planned, and the consequential organ at risk (OAR) irradiation was evaluated. Methods and materials Plans for 5 patients were computed for IM photons (7 coplanar fields) and protons (3 coplanar beams), using the KonRad inverse treatment planning system (developed at the German Cancer Research Center). IMPT planning was performed assuming 2 different sizes of the pencil beam: IMPT with a beam of full width at half-maximum of 20 mm, and IMPT with a “mini-beam” (IMPTM, full width at half-maximum = 12 mm). Prescribed dose was 77.4 Gy or cobalt Gray equivalent (CGE) for protons to the gross tumor volume (GTV). Surface and center spinal cord dose constraint for all techniques was 64 and 53 Gy/CGE, respectively. Tumor and OAR dose–volume histograms were calculated. Results were analyzed using dose–volume histogram parameters, inhomogeneity coefficient, and conformity index. Results Gross tumor volume coverage was optimal and equally homogeneous with both IM photon and IM proton plans. Compared to the IM photon plans, the use of IM proton beam therapy leads to a substantial reduction of the OAR total integral dose in the low-level to mid-dose level. Median heart, lung, kidney, stomach, and liver mean dose and dose at the 50% volume level were consistently reduced by a factor of 1.3 to 25. Tumor dose homogeneity in IMPTM plans was always better than with IMPT planning (median inhomogeneity coefficient, 0.19 vs. 0.25). IMPT dose escalation (to 92.9 CGE to the GTV) was possible in all patients without exceeding the normal-tissue dose limits. Conclusions These results suggest that the use of IM photon therapy, when compared to IM protons, can result in similar levels of tumor conformation. IM proton therapy, however, reduces the OAR integral dose substantially, compared to IM photon radiation therapy. As a result, tumor dose escalation was always possible with IM proton planning, within the maximal OAR dose constraints. In IM proton planning, reducing the size of the proton pencil beam (using the “mini-beam”) improved the dose homogeneity, but it did not have a significant effect on the dose conformity.

A treatment planning comparison of intensity modulated photon and proton therapy for paraspinal sarcomas

Purpose A comparative treatment planning study has been undertaken between intensity modulated (IM) photon therapy and IM proton therapy (IMPT) in paraspinal sarcomas, so as to assess the potential benefits and limitations of these treatment modalities. In the case of IM proton therapy, plans were compared also for two different sizes of the pencil beam. Finally, a 10% and 20% dose escalation with IM protons was planned, and the consequential organ at risk (OAR) irradiation was evaluated. Methods and materials Plans for 5 patients were computed for IM photons (7 coplanar fields) and protons (3 coplanar beams), using the KonRad inverse treatment planning system (developed at the German Cancer Research Center). IMPT planning was performed assuming 2 different sizes of the pencil beam: IMPT with a beam of full width at half-maximum of 20 mm, and IMPT with a “mini-beam” (IMPT M, full width at half-maximum = 12 mm). Prescribed dose was 77.4 Gy or cobalt Gray equivalent (CGE) for protons to the gross tumor volume (GTV). Surface and center spinal cord dose constraint for all techniques was 64 and 53 Gy/CGE, respectively. Tumor and OAR dose–volume histograms were calculated. Results were analyzed using dose–volume histogram parameters, inhomogeneity coefficient, and conformity index. Results Gross tumor volume coverage was optimal and equally homogeneous with both IM photon and IM proton plans. Compared to the IM photon plans, the use of IM proton beam therapy leads to a substantial reduction of the OAR total integral dose in the low-level to mid-dose level. Median heart, lung, kidney, stomach, and liver mean dose and dose at the 50% volume level were consistently reduced by a factor of 1.3 to 25. Tumor dose homogeneity in IMPT M plans was always better than with IMPT planning (median inhomogeneity coefficient, 0.19 vs. 0.25). IMPT dose escalation (to 92.9 CGE to the GTV) was possible in all patients without exceeding the normal-tissue dose limits. Conclusions These results suggest that the use of IM photon therapy, when compared to IM protons, can result in similar levels of tumor conformation. IM proton therapy, however, reduces the OAR integral dose substantially, compared to IM photon radiation therapy. As a result, tumor dose escalation was always possible with IM proton planning, within the maximal OAR dose constraints. In IM proton planning, reducing the size of the proton pencil beam (using the “mini-beam”) improved the dose homogeneity, but it did not have a significant effect on the dose conformity.

Advantage of protons compared to conventional X-ray or IMRT in the treatment of a pediatric patient with medulloblastoma

PurposeTo compare treatment plans from standard photon therapy to intensity modulated X-rays (IMRT) and protons for craniospinal axis irradiation and posterior fossa boost in a patient with medulloblastoma. MethodsProton planning was accomplished using an in-house 3D planning system. IMRT plans were developed using the KonRad treatment planning system with 6-MV photons. ResultsSubstantial normal-tissue dose sparing was realized with IMRT and proton treatment of the posterior fossa and spinal column. For example, the dose to 90% of the cochlea was reduced from 101.2% of the prescribed posterior fossa boost dose from conventional X-rays to 33.4% and 2.4% from IMRT and protons, respectively. Dose to 50% of the heart volume was reduced from 72.2% for conventional X-rays to 29.5% for IMRT and 0.5% for protons. Long-term toxicity with emphasis on hearing and endocrine and cardiac function should be substantially improved secondary to nontarget tissue sparing achieved with protons. ConclusionThe present study clearly demonstrates the advantage of conformal radiation methods for the treatment of posterior fossa and spinal column in children with medulloblastoma, when compared to conventional X-rays. Of the two conformal treatment methods evaluated, protons were found to be superior to IMRT.

A treatment planning comparison of intensity-modulated photon and proton therapy for paraspinal sarcomas

Purpose/Objective: A comparative treatment planning study has been undertaken between intensity-modulated (IM) photon and IM proton therapy (IMPT) in paraspinal sarcomas, as to assess the potential benefits and limitations of these treatment modalities. Proton therapy planning was performed assuming two different sizes of the pencil beam: IMPT with a beam of full width at half-maximum (FWHM) of 20 mm, and IMPT-M with a “mini-beam” (FWHM=12mm). Finally, a 10% and 20% dose-escalation with IM protons was planned and the consequential organ at risk (OAR) irradiation evaluated. Materials/Methods: Plans for five patients were created for IM photons (5 and 7 coplanar fields) and protons (3 coplanar beams), using KonRad inverse treatment planning system (developed at the German Cancer Research Center). Prescribed dose was 77.4 Gy-CGE to the gross tumor volume (GTV). Surface and center spinal cord dose-constraint for all techniques was 64 and 53 Gy-CGE, respectively. Tumor and organs at risk (OAR) dose-volume histograms (DVH) were calculated. Results were analyzed using the DVH parameters, inhomogeneity coefficient (IC) and conformity index (CI). Results: GTV coverage was optimal and equally homogeneous with both IM photon and IM proton plans. Compared to the IM photon plans, the use of the IM proton beam lead to a substantial reduction of the OAR total integral dose in the low- to mid-dose level. Heart, lung, kidney and stomach mean dose and median tumor dose at the 50% volume level were consistently reduced by a factor of 1.8–40. Tumor dose homogeneity in IMPT-M plans was always better than in IMPT plans (median IC, 0.19 vs. 0.25). IMPT dose escalation (to 85.1 and 92.9 Gy-CGE in GTV) was possible in all patients. Conclusions: These results suggest that the use of IM proton therapy, when compared to IM photons, can result in similar levels of tumor conformation. IM proton therapy, however, reduces the OAR integral dose substantially, compared to IM photon radiation therapy. As a result, tumor dose escalation was always possible with IM proton planning, within the specified OAR dose-constraints. In IM proton planning, reducing the size of the proton pencil beam (using the “mini-beam”) improved the dose homogeneity, but did not have a significant effect on the dose conformity.

Potential reduction of the incidence of radiation-induced second cancers by using proton beams in the treatment of pediatric tumors

Purpose : To assess the potential influence of improved dose distribution with proton beams compared to conventional or intensity-modulated (IM) X-ray beams on the incidence of treatment-induced secondary cancers in pediatric oncology. Methods and Materials : Two children, one with a parameningeal rhabdomyosarcoma (RMS) and a second with a medulloblastoma, were used as models for the purpose of this study. After defining the target and critical structures, treatment plans were calculated and optimized, four for the RMS case (conventional X-ray, IM X-rays, protons, and IM protons) and three for the irradiation of the spinal axis in medulloblastoma (conventional X-ray, IM X-rays, protons). Secondary cancer incidence was estimated using a model based on Publication No. 60 of the International Commission on Radiologic Protection. This model allowed estimation of absolute risks of secondary cancer for each treatment plan based on dose-volume distributions for the nontarget organs. Results : Proton beams reduced the expected incidence of radiation-induced secondary cancers for the RMS patient by a factor of ≥2 and for the medulloblastoma case by a factor of 8 to 15 when compared with either IM or conventional X-ray plans. Conclusions : The potential for a significant reduction in secondary cancers with pediatric cancers after using proton beams (forward planned or IM) in the treatment of RMS and MBD in children and adolescents represents an additional argument supporting the development of proton therapy for most radiotherapy indications in pediatric oncology.

Potential role of intensity modulated proton beams in prostate cancer radiotherapy

Purpose: The present study was undertaken to assess the potential benefit of intensity modulated (IM) proton beams in optimizing the dose distribution to safely escalate the tumor dose in prostate cancer radiotherapy. Methods and Materials: Four treatment plans were compared in a prostate cancer patient aiming to deliver 81 Gy to the target: 1) conformal 18 MV X-rays, 6-fields; 2) 214 MeV protons, 2-fields; 3) IM 15 MV X-rays, 5-fields; and 4) 177–200 Mev IM protons, 5-fields as in Plan 3. In addition, IM methods were used to further escalate the tumor dose to 99 Gy. Dose-volume histograms (DVH) were used to physically compare the treatment plans. DVH data were also used to obtain normal tissue complication probabilities (NTCP) for the rectum, bladder, femoral heads, and tumor control probabilities. Results: Although the planning target volume dose distribution was satisfactory with the four treatment plans, the homogeneity was slightly reduced in both X-ray plans (IM and standard) and the low-to-medium doses delivered to all organs at risk, and other normal tissues were significantly reduced by both proton plans. For a prescribed dose of 81 Gy, only the IM X-ray and IM proton plans both succeeded in predicting an acceptably low NTCP for the rectum (<5%, Grade 3). The integral nontarget dose was significantly reduced with IM proton beams (i.e., 3.1, 1.3, and 1.7 times less than Plans 1, 2, and 3, respectively). When escalating the dose to 99 Gy, no additional improvement between IM protons and IM X-ray beams was observed. Conclusions: Both IM X-ray and proton beams were able to optimize the dose distribution and comply with the goal of delivering the highest dose to the target while reducing the risk of severe morbidity to acceptable levels. The main advantage compared to IM X-rays was that IM protons succeeded in significantly reducing the low-to-medium dose to the nontarget tissues and achieved a small improvement in planning target volume (PTV) dose heterogeneity.

Optimizing radiotherapy of orbital and paraorbital tumors: intensity-modulated X-ray beams vs. intensity-modulated proton beams

Purpose: This study presents a dosimetric optimization effort aiming to compare intensity-modulated (IM) X-rays and IM protons in 4 different orbital and paraorbital tumors. These are most challenging targets for standard radiotherapy due to their close relationship with the eyes and related structures. Methods and Materials: A primary orbital lymphoma, an optic nerve meningioma, a sphenoidal ridge meningioma protruding into the orbit, and a pediatric parameningeal paraorbital rhabdomyosarcoma were selected for the purpose of this study. Planning target volumes (PTVs) and organs at risk (OAR) were defined in each patient CT data set for each tumor site. IM X-ray and IM proton three-dimensional treatment plans were implemented. The following total tumor doses were prescribed: 30 Gy for the orbital lymphoma, 54 Gy for both meningiomas, and 50.4 Gy for the rhabdomyosarcoma case. Dose-volume histograms (DVHs) were obtained for all targets and OAR with both treatment techniques. DVHs were used to predict normal tissue complication probabilities (NTCPs) for the OAR in the vicinity of the tumor. Results: The PTV coverage was optimal and equally homogeneous with both IM X-rays and IM proton plans in the 4 tumor sites. DVHs for most OAR were better with IM proton beams especially in the low- to mid-dose range region. The integral nontarget dose was lower with IM protons in every case (factor ranging from 1.5 to 1.9). However, predicted NTCPs (for severe late effects) were equally low for both treatment techniques in every tumor site. Conclusion: Although IM proton plans optimally decreased the dose to the OAR in all tumor sites, both optimized X-ray and proton beams equally succeeded to reduce severe-toxicity prediction risks to less than 5% while optimally treating the PTV.